Phanor Montoya

I would also like to thank my South-African colleagues and friends Dr Camilla Floros and Stuart Laing from ORI for their high quality assistance in fieldtrip operations and Scuba diving in South Africa. Special thanks also go to Alban Jamon for his assistance in fieldtrip in Mayotte, Karine Pothin and the Eco-guards team from the Reunion Marine Reserve (RNMR) for their time and assistance in fieldtrips in Reunion.

I greatly thank Dr Julius Francis (WIOMSA), Lilian Omolo (WIOMSA) and the whole WIOMSA/MASMA Board for their help, guidance and understanding all along the project.

Thanks are also due to Frederique Patche and Rebecca Pleignet (Reunion Region Council) for their valuable contributions for financing this project.

Last but not least, great thanks to my mother Pascale Hamard, my brother and sister Cécile and Jonathan Séré, my best friends Nicolas & Anna Guéniot, Laurent & Michael Maillot and Emilie Bonnet for their understanding and always being there when I needed.

Résumé Abstract

During the past two decades, the emergence and spread of infectious diseases have caused substantial declines in the biodiversity and abundance of reef-building corals. Despite their increased global prevalence and virulence, little is known about coral diseases on Indian Ocean coral reefs. This study aims to fill this gap in knowledge by identifying the main coral diseases and quantify their prevalence at three localities Reunion, South Africa and Mayotte, determining their spatial distribution and seasonal variation. Additionally this research aimed to describe and characterise at ecological, histological, molecular and microbiological levels of the following two unreported coral diseases on the WIO reefs: Porites white patch syndrome (PWPS) and Porites black patch syndrome (PBPS).

Principal findings of this study demonstrated the presence of six main coral diseases including black band disease (BBD), white syndromes (WS), pink line syndrome (PLS), growth anomalies (GA), skeleton eroding band (SEB) and Porites white patch syndrome (PWPS). The overall disease prevalence was higher in Reunion (7.5 ± 2.2%; mean ± SE) compared to South Africa (3.9 ± 0.8%; mean ± SE) and Mayotte (2.7 ± 0.3%; mean ± SE).

Acropora and Porites were the genera most vulnerable to disease. Spatial variability was detected in both Reunion and South Africa with BBD and WS more prevalent on shallow than deep reefs. There was also evidence of seasonality in two diseases: BBD and WS, their prevalence being higher in summer than winter.

Corals exhibiting signs of PWPS revealed extensive tissue fragmentation, generally associated with ovoid basophilic bodies resembling bacterial aggregates within the mesoglea of the body wall. Other organisms, including Cyanobacteria, Nematoda, Ciliata and endophytic algae, were also observed on diseased tissues and were generally associated with the dead epidermis and cell debris. Results of 16S rRNA sequence analysis revealed a high variability between bacterial communities associated with PWPS-infected and healthy tissues in Porites lutea. Several bacterial ribotypes affiliated to potential putative pathogens notably Shimia marina and Vibrio hepatarius were consistently found among the 16S rRNA sequences derived from the PWPS lesions, and absent and/or poorly represented in HT.

Primary pathogens involved in the PWPS were also investigated in this study using traditional culturing techniques and laboratory infection trials. Of the 14 isolates selected for the inoculation trials, only the bacterial strain P180R mostly phylogenetically closely related to Vibrio tubiashii with its closest known sister taxon, V. hepatarius, was shown to cause signs resembling those of PWPS and satisfied the four Henle-Koch's postulates. P180R displayed focalised and progressive tissue paling 12 h after inoculation and visible lesions of PWPS were observed 12 h thereafter. Signs of PWPS appeared on 90% of the exposed coral fragments (27 of 30) under controlled environmental conditions. Moreover, the virulence of this marine pathogen was tested and seemed to be strongly dependent on seawater temperature, resulting in significantly higher tissue loss at 30°C than 28°C and 26°C.

Finally, a multidisciplinary approach involving field surveys, gross lesion monitoring, histopathology and 454-pyrosequencing was investigated to characterize an atypical form of BBD named PBPS. Surveys conducted within two geomorphological zones over two consecutive summers and winters showed that it manifested spatial and seasonal variability, with more infected colonies observed on the reef flat and during the summer season.

Histology revealed cyanobacterial penetration of the compromised tissue as well as the presence of basophilic bodies resembling bacterial aggregates in the living tissue, adjacent to the bacterial mat. Bacterial 16S rRNA sequences yielded a broader diversity of bacterial taxa in PBPS-infected tissues than in healthy tissue, represented by the genus Vibrio (24.9%), followed by sulfate-reducers or sulfide-oxidizers such as Desulfovibrio (20%), Clostridium (12.9%) and Arcobacter (9.9%). PBPS appears to be a multi-stage disease triggered by cyanobacterial invasion and resulting in secondary infections by environmental bacteria that grow in mucus-like decomposing tissue. Analyses were performed on the mean prevalence of coral disease between regions, the most prevalent diseases (BBD, PWPS and WS) and the most susceptible coral genera including Table 11: Sequencing primers for 16S rRNA [START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF], pyrH, rpoA, recA, [START_REF] Thompson | Phylogeny and molecular identification of vibrios on the basis of multilocus sequence analysis[END_REF]

Table des matières

Liste des tableaux

Liste des figures

General introduction 1. Coral reefs: ecological goods and services

Coral reefs are among the most productive, diverse and complex ecosystems in the world [START_REF] Odum | Trophic structure and productivity of a windward coral reef community on Eniwetok Atoll[END_REF]) and support almost one third of the world's marine fish species [START_REF] Newton | Current and future sustainability of island coral reef fisheries[END_REF]. They provide many goods and services such as recreational activities and coastal protection [START_REF] Moberg | Ecological goods and services of coral reef ecosystems[END_REF]. Ecosystem services that reefs provide are estimated to be worth 173 billion of $US [START_REF] Martínez | The coasts of our world: Ecological, economic and social importance[END_REF]. They are also a valuable source of food for local communities [START_REF] Bruckner | New Threat to Coral Reefs: Trade in Coral Organisms[END_REF]. More than 100 countries, representing approximately 500 millions of people, depend on coral reefs for part of their livelihood or for part of their protein intake [START_REF] Salvat | Coral reefs-a challenging ecosystem for human societies[END_REF][START_REF] Bryant | Reefs at risk: A map-based indicator of threats to the world's coral reefs[END_REF]. Indeed, reef-related fisheries constitute approximately 2-5% of the total fish consumed by humans [START_REF] Smith | Coral-reef area and the contributions of reefs to processes and resources of the world's oceans[END_REF][START_REF] Pauly | Towards sustainability in world fisheries[END_REF]. In the Western Indian Ocean (WIO), approximately thirty million people depend directly or indirectly on the coastal environment for goods and services [START_REF] Mcclanahan | Conservation action in a changing climate[END_REF]. Moreover, coral reefs provide many nautical activities and represent a recreational area for the local population [START_REF] Piton | Océanographie physique des parages de l'île de la Réunion (Océan Indien)[END_REF]. Finally, coral reefs are also important in coastal protection from currents, waves and storms, are essential in maintaining biological diversity and constitute important spawning, nursery and breeding areas [START_REF] Moberg | Ecological goods and services of coral reef ecosystems[END_REF].

The scleractinian coral, a complex and diverse micro-ecosystem

Coral reefs frameworks are formed mainly by scleractinian corals (Fig. 1A) that secrete a calcium carbonate (CaCO3) hard skeletal structure. Briefly, scleractinian corals are composed of numerous polyps interconnected by the tissue called the coenenchyme (Fig. 1B). The polyp tissue consists in three distinct layers [START_REF] Veron | Aspects of the biogeography of hermatypic corals[END_REF][START_REF] Galloway | Antibacterial activity of marine and black band disease cyanobacteria against coral-associated bacteria[END_REF]; the epidermis, located either next to the external environment and the skeleton, contains several specific cells (Figs 1C, D) including the nematocytes (defence cells), calicoblastic cells (calcium carbonate secretion) and the mucocytes (mucus secretion). The mesoglea generally located between the epidermis and the gastrodermis is a connective tissue layer with some isolated cells e.g.

amoebocytes. The gastrodermis contains cells such as cnidocytes, amoebocytes, mucocytes but also the endosymbiotic algae called zooxanthellae [START_REF] Galloway | Antibacterial activity of marine and black band disease cyanobacteria against coral-associated bacteria[END_REF]). The whole is covered by a viscous and transparent mucus layer and attached to the calcium carbonate skeleton. Recent studies have shown that these coral structures/compartments (e.g. mucus layer, coral tissue and the skeleton) harbor a diverse, complex and abundant microbial population including endolithic algae [START_REF] Fine | Endolithic algae: an alternative source of photoassimilates during coral bleaching[END_REF], protozoa [START_REF] Toller | Genetic evidence for a protozoan (phylum Apicomplexa) associated with corals of the Montastraea annularis species complex[END_REF][START_REF] Sweet | Ciliate and bacterial communities associated with White Syndrome and Brown Band Disease in reef-building corals[END_REF], archaea [START_REF] Kellogg | Tropical Archaea: diversity associated with the surface microlayer of corals[END_REF][START_REF] Wegley | Coral-associated archaea[END_REF][START_REF] Siboni | Global distribution and diversity of coral-associated Archaea and their possible role in the coral holobiont nitrogen cycle[END_REF], fungi [START_REF] Bentis | Endolithic fungi in reef-building corals (Order: Scleractinia) are common, cosmopolitan, and potentially pathogenic[END_REF]Wegley et al. 2007a), bacteria [START_REF] Rohwer | Diversity and distribution of coral-associated bacteria[END_REF][START_REF] Knowlton | Multispecies microbial mutualisms on coral reefs: the host as a habitat[END_REF][START_REF] Bourne | Diversity of bacteria associated with the coral Pocillopora damicornis from the Great Barrier Reef[END_REF][START_REF] Ritchie | Regulation of microbial populations by coral surface mucus and mucus-associated bacteria[END_REF]Wegley et al. 2007b;[START_REF] Bourne | Microbial disease and the coral holobiont[END_REF][START_REF] Mouchka | Coral-associated bacterial assemblages: current knowledge and the potential for climate-driven impacts[END_REF][START_REF] Sweet | Bacterial assemblages differ between compartments within the coral holobiont[END_REF][START_REF] Barott | Microbial diversity associated with four functional groups of benthic reef algae and the reef-building coral Montastraea annularis[END_REF]) and viruses [START_REF] Marhaver | Viral communities associated with healthy and bleaching corals[END_REF][START_REF] Patten | Prevalence of virus-like particles within a staghorn scleractinian coral (Acropora muricata) from the Great Barrier Reef[END_REF][START_REF] Claverie | Mimivirus and Mimiviridae: giant viruses with an increasing number of potential hosts, including corals and sponges[END_REF][START_REF] Van Oppen | Coral-virus interactions: A double-edged sword?[END_REF]) providing diversified microhabitats [START_REF] Sunagawa | Threatened corals provide underexplored microbial habitats[END_REF]. This microbial-coral relationship termed "coral holobiont" is likely to play a key role in coral physiology and health [START_REF] Rosenberg | The role of microorganisms in coral health, disease and evolution[END_REF][START_REF] Bourne | Microbial disease and the coral holobiont[END_REF][START_REF] Ainsworth | The future of coral reefs: a microbial perspective[END_REF][START_REF] Kvennefors | Regulation of bacterial communities through antimicrobial activity by the coral holobiont[END_REF]. It has been suggested that microbial communities from the coral surface mucus layer (SML) may be used as a defense mechanism to protect the coral host against putative pathogens by displaying antimicrobial activity or though competition with non-resident bacteria for nutrition and space [START_REF] Rohwer | Diversity and distribution of coral-associated bacteria[END_REF][START_REF] Ritchie | Regulation of microbial populations by coral surface mucus and mucus-associated bacteria[END_REF][START_REF] Shnit-Orland | Coral mucus-associated bacteria: a possible first line of defense[END_REF][START_REF] Kvennefors | Regulation of bacterial communities through antimicrobial activity by the coral holobiont[END_REF]. For instance, Shnit-Orland and Kushmaro (2009) found that 25-70% of cultivable bacteria isolated from the mucus of six different coral genera displayed antibacterial activity. Other studies has demonstrated that some bacterial strains also retrieved from the coral SML were able to inhibit growth of bacterial pathogens such as Vibrio shiloi, V. coralliilyticus and Serratia marcescens implicated in several coral pathology [START_REF] Ritchie | Regulation of microbial populations by coral surface mucus and mucus-associated bacteria[END_REF][START_REF] Nissimov | Antimicrobial properties of resident coral mucus bacteria of Oculina patagonica[END_REF][START_REF] Rypien | Antagonistic interactions among coral-associated bacteria[END_REF]).

Infectious disease, an increasing threat to coral reef worldwide

Coral reefs are in serious decline worldwide with 20% of the world's coral reefs have already been degraded beyond the potential for recovery, 24% are under imminent risk of collapse and another 26% are under a longer term threat of collapse [START_REF] Wilkinson | Status of coral reefs of the world: 2004[END_REF]). These has been attributed to increasing anthropogenic pressure (e.g. mining for lime production, destructive fisheries, poaching, pollution, SCUBA diving etc.) and natural factors (e.g. storms, solar radiations, and warm temperature anomalies [START_REF] Bellwood | Confronting the coral reef crisis[END_REF]). These external factors acting individually or in synergy, could affect the resistance of corals by altering the complex microbial-coral associations and therefore stimulating the growth and severity of pathogenic organisms [START_REF] Green | The significance of coral disease epizootiology for coral reef conservation[END_REF][START_REF] Lesser | Nitrogen fixation by symbiotic cyanobacteria provides a source of nitrogen for the scleractinian coral Montastraea cavernosa[END_REF].

Over the past 40 years coral diseases have substantially contributed to the decline in the biodiversity and abundance of reef-building corals [START_REF] Peters | A survey of cellular reactions to environmental stress and disease in Caribbean scleractinian corals[END_REF][START_REF] Harvell | Emerging Marine Diseases-Climate Links and Anthropogenic Factors[END_REF][START_REF] Hughes | Climate change, human impacts, and the resilience of coral reefs[END_REF]. Beside to be a major contributor to coral mortality, they are also known to generate progressive tissue loss [START_REF] Bruckner | Spread of a Black-Band Disease Epizootic Through the Coral Reef System in St. Ann's Bay, Jamaica[END_REF], to affect the growth rate, reproductive capacity, recruitment and competitive ability of corals [START_REF] Petes | Pathogens compromise reproduction and induce melanization in Caribbean sea fans[END_REF]Bruno et al. 2007;[START_REF] Weil | Spatial variability in distribution and prevalence of Caribbean scleractinian coral and octocoral diseases-I Community-level analysis[END_REF][START_REF] Harvell | Climate change and wildlife diseases: when does the host matter the most?[END_REF][START_REF] Borger | The effects of a coral disease on the reproductive output of Montastraea faveolata (Scleractinia: Faviidae)[END_REF]. The increase of frequency and intensity of disease outbreaks have contributed to massive mortalities that reduced population of several scleractinian species and generated shifts in coral communities at local and regional scales. For instance, repeated epizootic events have been the most significant source of mortality for Caribbean populations of the two most important framework-building species Acropora palmata and A. cervicornis, over the past 20-25 years [START_REF] Aronson | White-band disease and the changing face of Caribbean coral reefs[END_REF][START_REF] Patterson | The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata[END_REF]. In the Florida Keys more than 70% of A.

palmata were decimated by white pox disease (WP) while 50% of sea fan tissue has been lost due to complete or partial disease-induced mortality [START_REF] Kim | The Rise and Fall of a Six-Year Coral-Fungal Epizootic[END_REF]. To date more than 30 distinct diseases of scleractinian corals, mainly identified according to their morphological aspect in the field (e.g. lesions or bands of tissue loss), have been reported with black band disease (BBD), white band disease (WBD), white plague disease (WPD), and growth anomalies found globally (Fig. 2) [START_REF] Green | The significance of coral disease epizootiology for coral reef conservation[END_REF][START_REF] Bruckner | Tracking the Trade in Ornamental Coral Reef Organisms: The Importance of CITES and its Limitations[END_REF][START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF][START_REF] Sutherland | Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals[END_REF][START_REF] Willis | Coral disease on the Great Barrier Reef[END_REF][START_REF] Weil | Status and progress in coral reef disease research[END_REF]). Among these diseases, only few putative agents including two ciliates [START_REF] Cróquer | First report of folliculinid ciliates affecting Caribbean scleractinian corals[END_REF][START_REF] Bourne | Identification of a ciliate (Oligohymenophorea: Scuticociliatia) associated with brown band disease on corals of the Great Barrier Reef[END_REF], one fungi (Sweet et al. 2013a) and six bacteria [START_REF] Israely | Growth, differentiation and death of Vibrio shiloi in coral tissue as a function of seawater temperature[END_REF]Ben-Haim et al. 2003b;[START_REF] Sussman | A single cyanobacterial ribotype is associated with both red and black bands on diseased corals from Palau[END_REF][START_REF] Arboleda | Vibrio sp. causing Porites ulcerative white spot disease[END_REF][START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF][START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF] were characterised or/and identified experimentally by fulfilling Henle Koch's postulates [START_REF] Koch | Ueber bakteriologische Forschung[END_REF].

Despite their increasing prevalence and virulence, little is known about the direct effects of biotic and abiotic factors on the infection, progression and transmission mechanisms of coral diseases. Nevertheless, several recent studies have found correlations mainly with temperature stress (Bruno and Selig 2007;[START_REF] Ward | Temperature affects coral disease resistance and pathogen growth[END_REF][START_REF] Croquer | Changes in Caribbean coral disease prevalence after the 2005 bleaching event[END_REF][START_REF] Sokolow | Effects of a changing climate on the dynamics of coral infectious disease: a review of the evidence[END_REF]; [START_REF] Kuehl | The roles of temperature and light in black band disease (BBD) progression on corals of the genus< i> Diploria</i> in Bermuda[END_REF][START_REF] Ruiz-Moreno | Global coral disease prevalence associated with sea temperature anomalies and local factors[END_REF]) and nutrient loading [START_REF] Bruno | Nutrient enrichment can increase the severity of coral diseases[END_REF]Garren et al. 2009;[START_REF] Haapkylä | Seasonal rainfall and runoff promote coral disease on an inshore reef[END_REF][START_REF] Kaczmarsky | Do elevated nutrients and organic carbon on Philippine reefs increase the prevalence of coral disease?[END_REF][START_REF] Vega-Thurber | Chronic nutrient enrichment increases prevalence and severity of coral disease and bleaching[END_REF].

For instance in the Caribbean several coral diseases including BBD [START_REF] Kuehl | The roles of temperature and light in black band disease (BBD) progression on corals of the genus< i> Diploria</i> in Bermuda[END_REF], WP [START_REF] Patterson | The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata[END_REF], and WPL [START_REF] Harvell | Emerging Marine Diseases-Climate Links and Anthropogenic Factors[END_REF] were more prevalent or spread across colonies more rapidly during summertime. Similarly, on the Great Barrier Reef (GBR) white syndrome (WS) and atrementous necrosis (AN) were more prevalent in summer than winter [START_REF] Jones | Dynamics of a temperature-related coral disease outbreak[END_REF][START_REF] Willis | Coral disease on the Great Barrier Reef[END_REF]). At microbial scale, anomalously high temperature could influence disease by altering basic biological and physiological properties of corals, particularly their resistance, impairing the balance between potential pathogen and host [START_REF] Rosenberg | Microbial diseases of corals and global warming[END_REF]. The Oculina patogonica-Vibrio shiloi is an interesting model to illustrate the effects of thermal stress on host-pathogen interactions. At high summer seawater, this bacterium "Vibrio shiloi" expresses a cell surface adhesion required to fix the coral tissue and produces toxins that inhibiting photosynthesis and lysing the symbiotic zooxanthellae [START_REF] Rosenberg | Microbial diseases of corals and global warming[END_REF][START_REF] Banin | Superoxide dismutase is a virulence factor produced by the coral bleaching pathogen Vibrio shiloi[END_REF].

Pollutions associated with organic matter enrichment from run-off or sewage discharge have also been found to increase coral disease prevalence [START_REF] Bruno | Nutrient enrichment can increase the severity of coral diseases[END_REF][START_REF] Kaczmarsky | Is there a relationship between proximity to sewage effluent and the prevalence of coral disease[END_REF][START_REF] Garren | Resilience of coral-associated bacterial communities exposed to fish farm effluent[END_REF][START_REF] Haapkylä | Seasonal rainfall and runoff promote coral disease on an inshore reef[END_REF][START_REF] Vega-Thurber | Chronic nutrient enrichment increases prevalence and severity of coral disease and bleaching[END_REF]. For example, relationships between exposure to sewage and high prevalence were found for WPL, BBD,

Porites ulcerative white spot disease (PUWSD), WP, Yellow blotch disease and AN [START_REF] Patterson | The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata[END_REF][START_REF] Bruno | Nutrient enrichment can increase the severity of coral diseases[END_REF][START_REF] Kaczmarsky | Is there a relationship between proximity to sewage effluent and the prevalence of coral disease[END_REF][START_REF] Voss | Nutrient enrichment enhances black band disease progression in corals[END_REF]. In addition, experimental evidence showed that nutrient enrichment can increase the prevalence and/or severity of the dark spot syndrome (DSS) on Siderastrea siderea (Vega- [START_REF] Vega-Thurber | Chronic nutrient enrichment increases prevalence and severity of coral disease and bleaching[END_REF]) and yellow band disease (YBD) on Montastrea annularis [START_REF] Haapkylä | Seasonal rainfall and runoff promote coral disease on an inshore reef[END_REF].

Status of coral diseases on the Western Indian Ocean reefs

The Western Indian Ocean (WIO) coral reefs range from, west to east, the coast of East Africa (Kenya to South Africa ), Pemba, Zanzibar, Comoros islands (Anjouan, Moheli, Mayotte), Eparse Islands (Mozambique channel), Madagascar, Mascarenes (Reunion, Mauritius, Rodrigues), and Seychelles (Fig. 3). Coral reefs are divided into four reef systems [START_REF] Obura | Resilience and climate change: lessons from coral reefs and bleaching in the Western Indian Ocean[END_REF][START_REF] Richmond | A field guide to the seashores of Eastern Africa and the Western Indian Ocean islands[END_REF]) including 1) the patch reefs mainly along the cost from South Africa to Mozambique, 2) fringing reefs which are the most common reef structure in the WIO (e.g. Reunion, Rodrigues, Mayotte, Zanzibar), 3) Barrier reefs in Mayotte and Mauritius, and 4) atolls such as Europa and Bassas de India. While coral bleaching have received intensive attention since the 1998 El Nino Southern Oscillation (ENSO) event [START_REF] Bigot | Coral bleaching in the Indian Ocean islands: Ecological consequences and recovery in Madagascar, Comoros, Mayotte and Reunion[END_REF][START_REF] Cole | Tropical Pacific forcing of decadal SST variability in the western Indian Ocean over the past two centuries[END_REF][START_REF] Goreau | Conservation of coral reefs after the 1998 global bleaching event[END_REF][START_REF] Mcclanahan | Bleaching damage and recovery potential of Maldivian coral reefs[END_REF][START_REF] Spencer | Coral bleaching in the southern Seychelles during the 1997-1998 Indian Ocean warm event[END_REF][START_REF] Celliers | Coral bleaching on high-latitude marginal reefs at Sodwana Bay, South Africa[END_REF][START_REF] Chabanet | Coral reef fish communities of Mayotte (western Indian Ocean) two years after the impact of the 1998 bleaching event[END_REF][START_REF] Conand | Bleaching in a western Indian Ocean Island, La Réunion: a multi-scale approach[END_REF]McClanahan et al. 2004a;[START_REF] Obura | Resilience and climate change: lessons from coral reefs and bleaching in the Western Indian Ocean[END_REF][START_REF] Mcclanahan | Western Indian Ocean coral communities: bleaching responses and susceptibility to extinction[END_REF] [START_REF] Harvell | Coral disease, environmental drivers, and the balance between coral and microbial associates[END_REF]).

Aims and objectives of this study

This PhD study aims at providing data on the prevalence and variability at local, regional and temporal scales, on three targets coral reef from the Western Indian Ocean. Moreover, this study focused on the characterization and identification of the agents of the pathogens combining with techniques from microbiology. The specific objectives are:

1. Identifying the main coral disease signs/ syndromes by systematically describing gross and microscopic lesions in scleractinian corals;

2. Investigating their impact on coral communities by quantifying their prevalence across three WIO reef ecosystems;

3. Determining the spatial distribution, and potential seasonal variations of coral disease outbreaks;

4. Characterizing microscopic morphology of lesions in corals;

5.

Describing and comparing the bacterial communities associated with both healthy and disease coral colonies;

6. Identifying the putative pathogenic agents associated/responsible to the selected disease signs/syndromes.

PhD outlines

This thesis, comprising four main chapters (Chapter 2-5), is the first quantitative and qualitative study assessing the ecology and aetiology of infectious coral diseases on the Western Indian Ocean coral reefs. Chapter 2 identifies the main coral diseases and quantifies their prevalence at these localities, determining their spatial distribution and seasonal There was also evidence of seasonality in two diseases: BBD and WS, their prevalence being higher in summer than winter. This was the first study to investigate the ecology of coral diseases in the WIO reefs and surveys should be expanded to confirm these patterns.

Keywords:

Coral diseases -Western Indian Ocean -Scleractinian corals -Seasonality -Spatial variability.

Introduction

The emergence and spread of infectious diseases have caused substantial declines in the biodiversity and abundance of reef-building corals during the last 40 years [START_REF] Garzón-Ferreira | Stony coral diseases observed in southwestern Caribbean reefs[END_REF][START_REF] Weil | Status and progress in coral reef disease research[END_REF]. For example, white syndromes (WS) including white band disease (WBD), white plague (WP) and white pox disease (WPD), have been the most significant cause of mortality in Caribbean populations of the two most important framework-building species over the past 20-25 years, Acropora palmata and A. cervicornis [START_REF] Patterson | The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata[END_REF][START_REF] Sutherland | Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals[END_REF][START_REF] Weil | Status and progress in coral reef disease research[END_REF]). To date, more than 30 distinct diseases, affecting at least 150 scleractinian corals, have been reported worldwide [START_REF] Sutherland | Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals[END_REF][START_REF] Weil | Status and progress in coral reef disease research[END_REF]. Most of them are known to generate tissue loss and subsequently affect growth rate, reproductive capacity, recruitment and the competitive ability of corals [START_REF] Petes | Pathogens compromise reproduction and induce melanization in Caribbean sea fans[END_REF][START_REF] Sutherland | Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals[END_REF]Bruno and Selig 2007;[START_REF] Weil | Spatial variability in distribution and prevalence of Caribbean scleractinian coral and octocoral diseases-I Community-level analysis[END_REF]. For instance, several studies have shown that black band disease (BBD) can generate the loss of up to 2 cm of tissue per day by producing high concentrations of sulphide that kill the coral tissue (Boyett et al. 2007;[START_REF] Haapkylä | Spatiotemporal coral disease dynamics in the Wakatobi Marine National Park, South-East Sulawesi, Indonesia[END_REF][START_REF] Sato | Dynamics of seasonal outbreaks of black band disease in an assemblage of Montipora species at Pelorus Island (Great Barrier Reef, Australia)[END_REF] African and Mayotte (fig. 1). The aims of the study were to identify the main coral diseases and quantify their prevalence at these localities, determining their spatial distribution and seasonal variation.

Methods

Study sites

Corals form fringing reefs at Reunion (21°07'S; 55°32'E; Fig. 4) and are 12 km² in area along 25 km of the coastline, mainly on the dry west coast. Three geomorphological zones are evident [START_REF] Montaggioni | Récifs coralliens des Mascareignes[END_REF]: 1) an outer reef slope (5-30 m) exposed to high turbulence and characterised by a basaltic substratum in alternating spurs and grooves, mostly covered by massive and encrusting corals, 2) a reef flat (0.5-2 m), generally composed of branching corals, and 3) an inner back-reef covered with sand and rubble (0.5-1 m). Due to their proximity to the coastline (± 500 m wide), these fringing reefs are subjected to high and increasing anthropogenic stressors such as water eutrophication from land-based pollution and overfishing and trampling.

South African reefs (1.9 km 2 ) are not typically accretive [START_REF] Schleyer | South African coral communities[END_REF]; the corals grow on late-Pleistocene beach rock, originating from submerged, fossilised coastal sand dunes [START_REF] Ramsay | 9000 years of sea-level change along the southern African coastline[END_REF]. In topography, the reefs consist of shallow pinnacles (8-10 m), extensive deep subtidal reef flats (14-18 m) and a sloping fore-reef edge (24-27 m; [START_REF] Celliers | Coral community structure and risk assessment of high-latitude reefs at Sodwana Bay, South Africa[END_REF]). The warm Agulhas Current, the prevailing regional current (maximum speed 1.5 m s -1 ), generates sub-tropical conditions in the area. All the reefs are located in marine protected areas and are in good condition.

Mayotte reefs consist of a large (15 km wide), deep (30-35 m) lagoon surrounded by a long barrier reef (150 km), which is 1.5 km wide in some areas and interrupted by 12 deep channels. Fringing reefs are also present along 210 km of the coastline of the island. A discontinuous, inner secondary barrier reef system (12 km long) is located on the south-west coast. Mayotte reefs are subjected to increasing pressures associated with human development. Abnormally high sedimentation rates due to mangrove deforestation constitute the main stressors but the reefs are also damaged by waste-water discharge and overfishing.

Survey methods

Surveys in South Africa were conducted on Two-mile Reef (TMR) in the central Maputaland reef complex at Sodwana-Bay in northern KwaZulu-Natal (27°24' S; 32°41'E). These were conducted at seven sites along a north-south gradient on TMR at two depth intervals: 8-10 m (shallow inshore region) and 12-16 m (deeper offshore region). On Mayotte, surveys were conducted at eight latitudinal sites on the barrier and fringing reef (Table 1 and Fig. 4). In Reunion, surveys were undertaken at four latitudinal sites (Table 1 and Fig. 4) on the outer reef slope and reef flat. Protocols were adapted to the different geomorphological zones. Five 10 × 2 m (1 m on each side of the transect line) transects were randomly laid parallel to depth prevalence of diseases was estimated as the number of diseased colonies divided by the total number of coral colonies >2 cm, identified to the genus level and counted within each transect in 1 × 1 m photoquadrats which covered the transect area (20-40 photo-quadrats per transect).

Finally, surveys in both Reunion and South Africa were conducted over two consecutive summers and winters to gain a measure of seasonality in the prevalence of the diseases. In Mayotte, coral diseases could only be monitored during the summer and winter of 2012.

Disease identification

Gross lesions observed during the surveys were photographed and identified using the Underwater Cards for Assessing Coral Health on Caribbean and Indo-Pacific Reefs [START_REF] Beeden | Coral Reef Targeted Research and Capacity Building for Management Program[END_REF]) and illustrations/descriptions available in the literature. It has been established

that similar gross lesions can be manifested by multiple microscopic pathologies and/or different causal agents [START_REF] Work | Characterizing lesions in corals from American Samoa[END_REF]. Therefore, to avoid subjective interpretations and verify field observations, samples of each coral disease was described according to the systematic nomenclature developed by [START_REF] Work | Systematically describing gross lesions in corals[END_REF]

Statistical analysis

Disease prevalence, calculated per transect, was averaged for each site in each region. Data were tested prior to analysis for homoscedasticity (Levene's test) and normality of variance (Kolmogorov-Smirnov and Lilliefors tests) and were then log transformed [log 10 (X)] for analysis of variance (ANOVA). Variations in the prevalence of coral disease between regions over the two survey years were tested in the consecutive summers and winters and across reef zones (shallow vs. deep) using two-way factorial ANOVA (STATISICA 8). Similarly, seasonality and spatial variations in the most prevalent diseases were examined using twoway factorial ANOVA. Finally, post hoc Fisher LSD tests were performed for multiple group comparisons.

Results

Description of the disease gross lesions in situ

A total of 76 coral disease surveys were conducted within the three regions at 22 sites between September 2010 and March 2012, covering an area of 7920 m 2 of reef. Photographs and samples taken from the reefs revealed the presence of six coral diseases manifesting discoloration, tissue loss and growth anomalies (Fig. 5). They included white syndromes (WS), black band disease (BBD), pink line syndrome (PLS), skeletal eroding band (SEB), growth anomalies (GA) and a newly identified coral disease, the Porites white patch syndrome (PWPS, [START_REF] Séré | Porites white patch syndrome: an unreported coral disease on Western Indian Ocean reefs[END_REF]). These are characterised in Table 2.

Coral disease prevalence and susceptibility

While the most prevalent coral disease recorded in Reunion was PLS followed by PWPS, BBD and WS, the most common disease on both South African and Mayotte reefs was WS followed by a much lower prevalence of any other (Table 3). The prevalence of all the coral diseases encountered varied significantly between the three regions (ANOVA; F= 7.72, p<0.01; Table 4); the lowest total mean prevalence was recorded in Mayotte with 2.7 ± 0.3 % (mean ± SE) between 2011 and 2012 and the highest in Reunion with diseases affecting 7.0 ± 1.2 % and 8.1 ± 0.2 % (mean ± SE) of all coral colonies between 2010-2011 and 2011-2012 respectively. In South Africa, the proportion of infected coral colonies was low but quite variable between the two survey periods, with the average disease prevalence higher (ANOVA; F= 4.72, p<0.01) in 2011-2012 (4.9 ± 0.9%; mean ± SE) than 2010-2011 (2.9 ± 0.8%; mean ± SE). (Fig. 5a-c)

Diffuse or distinct peripheral, basal or apical areas of bleached tissue, separating unaffected tissue from the intact but white skeleton or skeleton recently covered by green or brown algae.

Acropora spp., Astreopora spp., Platygyra daedalea and , Pocillopora verrucosa

Black band disease (BBD) (Fig. 5d,e)

Thin or wide undulating to serpiginous dark brown to black band, comprised predominantly of cyanobacteria. This band separates the unaffected tissue from a distinct area of tissue loss in which intact bare white skeleton is revealed (indicating acute to sub-acute tissue loss).

Astreopora, Coscinarea, Echninopora, Favia, Favites, Hydnophora, Pavona and, Platygyra Pink line syndrome (PLS) (Fig. 5j) Smooth to undulating band comprised of pink-coloured polyps, varying in width from a few millimetres to a few centimetres. This band separates the unaffected tissue from oblong to circular areas of tissue loss, the latter generally being diffused, centrally to peripherally situated and exposing skeleton covered by green or brown algae.

Porites spp.

Growth anomaly (GA) (Fig. 5g-i)

Focal, smooth to undulating surfaces, located principally at the surface of the colonies and containing partiallyformed, disorganized, exert calices.

Astreopora and Porites

Porites white patch syndrome

(PWPS) (Fig. 2k, l)
Diffuse, medium to large (50-300 mm diameter), circular to oblong tissue loss, surrounded by swollen white tissue. The older exposed skeleton is progressively colonized by endophytic algae and cyanobacteria.

Porites lutea and P. lobata

Skeletal eroding band (SEB) (Fig. 5f)

Thin, undulating to smooth black band, followed by a white band of bleached tissue, separating live tissue from a diffuse area of tissue loss. The exposed skeleton is generally colonised by endophytic algae. Acropora, Goniopora, Hydnophora and Porites seemed to be the most susceptible coral genera to disease on both the reef slope and reef flat in Reunion (Fig. 6). White syndromes (WS) were the most common disease affecting branching colonies of Acropora spp., only in the shallowest zone of the reef. Colonies of Goniopora sp., Hydnophora sp., and Porites spp.

Acropora muricata

Reunion South Africa Mayotte

Disease condition

were most susceptible to BBD, especially those on the reef-flat. Massive colonies of Porites spp. appeared to be the most vulnerable to disease, exhibiting multiple infections including PLS, PWPS, BBD, WS, GA and Nec. In South Africa, 11 coral genera were observed with signs of disease (Fig. 7). The most susceptible coral genera were Astreopora, Hydnophora, Pocillopora and Porites. GA, WS and Nec were more prevalent on encrusting and massive Astreopora spp. in summer and winter during both survey years. Both BBD and WS were the most prevalent diseases on Hydnophora sp. on shallow reefs. Colonies of Pocillopora spp.

exhibited high susceptibility to WS, whereas the massive corals Porites lutea and P. lobata, were more vulnerable to PWPS and PLS. Of the eight genera observed with diseases on both the barrier and fringing reefs of Mayotte (Fig. 8), Acropora, Astreopora and Porites seemed to be the most susceptible to disease. Colonies of Astreopora sp. were highly susceptible to GA and WS. Acropora spp. appeared to be particularly vulnerable to WS, whereas Porites spp, showed a particular susceptibility to GA, BB, PLS and Nec (Fig. 8).

Seasonal and spatial variations in coral disease prevalence

There was no evidence of seasonality in the overall prevalence (all diseases pooled) of coral diseases between winter and summer at the studied localities (ANOVA, p>0.05; Table 5).

However, coral disease prevalence differed significantly between reef zones in Reunion (ANOVA, p<0.01; Table 5) with a greater incidence on the reef flat than the reef slope (Fisher LSD, p<0.001). No difference was detected in South Africa and Mayotte between the shallower and deeper reefs (Fisher LSD, p>0.05). Of the seven coral disease states recorded in Reunion, the prevalence of only BBD seemed to vary between the reef zones and seasons (Table 5). Its prevalence was significantly higher on the reef flat (ANOVA, p<0.001; Table 5)

with the percentage of infected colonies being significantly higher in summer than winter (Fisher LSD,p<0.001). This spatial and seasonal pattern was observed particularly on Porites colonies which exhibited a higher mean BBD prevalence in summer (Fisher LSD, p<0.001;

Table 5; Fig. 6) and at the shallowest sites (Fisher LSD, p<0.05). Finally, WS varied significantly between reef zones (ANOVA, p>0.05, Table 5) but no significant difference was found between seasons (ANOVA, p>0.05). Among diseases recorded on South African reefs, WS was seasonal on Acropora (ANOVA; p<0.05; Table 5; Fig. 7), and Pocillopora (ANOVA; p<0.001), with a higher percentage of infected colonies in summer than winter. A similar, seasonal trend was evident for PWPS but the difference between summer and winter was not significant (ANOVA, p>0.05; Table 3). Finally, a significantly higher prevalence of WS was recorded on Pocillopora (ANOVA; p<0.01; Table 5; Fig. 7) in the shallow South African reef zones (Fisher LSD, p<0.001). At Mayotte, WS on Acropora spp. varied significantly between seasons (ANOVA, p<0.05; Table 5; Fig. 8), with a higher prevalence in winter than in summer (Fisher LSD, p<0.05). However, no significant difference was recorded between the fringing and barrier reef (ANOVA, p>0.05). Of the diseases found on shallow reefs in Reunion, BBD seemed particularly to manifest seasonality. For instance, when recorded on massive Porites spp., it was first observed during the summer 2010. It decreased significantly during the winter of 2011 and then reappeared the next summer. In South Africa, warmer water temperatures seemed to be linked to a higher prevalence of disease in the first year during summer. This was the case for WS-infected Acropora spp. and Pocillopora spp. However, no variations were observed for the same syndromes between the summer and winter of 2012. PWPS was also more prevalent in summer on both the reef slope and reef flat but the results were not statistically significant.

On Mayotte reefs, WS syndromes on Acropora spp. varied between seasons, with their prevalence being higher during the warmer months. Similar seasonal patterns have been reported for both BBD and WS in Australia [START_REF] Willis | Coral disease on the Great Barrier Reef[END_REF][START_REF] Boyett | Elevated temperature and light enhance progression and spread of black band disease on staghorn corals of the Great Barrier Reef[END_REF]Bruno and Selig 2007;[START_REF] Haapkylä | Spatiotemporal patterns of coral disease prevalence on Heron Island, Great Barrier Reef, Australia[END_REF]) and the Caribbean [START_REF] Bruckner | Spread of a Black-Band Disease Epizootic Through the Coral Reef System in St. Ann's Bay, Jamaica[END_REF]). These patterns may be due to impairment of the host's disease resistance under summer conditions, generating a shift in the natural bacterial communities in the coral holobionts towards opportunistic pathogens. Previous work has shown that elevated seawater temperatures

Abstract

The scleractinian coral Porites lutea, an important reef-building coral on western Indian

Ocean reefs (WIO), is affected by a newly-reported white syndrome (WS) the Porites white patch syndrome (PWPS). Histopathology and culture-independent molecular techniques were used to characterise the microbial communities associated with this emerging disease.

Microscopy showed extensive tissue fragmentation generally associated with ovoid basophilic bodies resembling bacterial aggregates. Results of 16S rRNA sequence analysis revealed a high variability between bacterial communities associated with PWPS-infected and healthy tissues in P. lutea, a pattern previously reported in other coral diseases such as black band disease (BBD), white band disease (WBD) and white plague diseases (WPD). Furthermore, substantial variations in bacterial communities were observed at the different sampling locations, suggesting that there is no strong bacterial association in Porites lutea on WIO reefs. Several sequences affiliated with potential pathogens belonging to the Vibrionaceae and Rhodobacteraceae were identified, mainly in PWPS-infected coral tissues. Among them, only two ribotypes affiliated to Shimia marina (NR043300.1) and Vibrio hepatarius (NR025575.1)

were consistently found in diseased tissues from the three geographically distant sampling localities. The role of these bacterial species in PWPS needs to be tested experimentally.

Keywords:

Coral disease -Western Indian Ocean -Scleractinian corals -Porites white patch syndrome -Bacterial communities -Causative agent.

Introduction

The scleractinian Porites lutea, commonly found on back reefs, lagoon and fringing reefs (Veron 2000), is an important reef-building coral in the western Indian Ocean (WIO) reefs.

Despite its widespread distribution, this hermatypic coral has shown a particular susceptibility to natural pressures such as predation [START_REF] Shafir | A Drupella cornus outbreak in the northern Gulf of Eilat and changes in coral prey[END_REF][START_REF] Cole | Coral-feeding wrasse scars massive Porites colonies[END_REF]) and infestation by parasites [START_REF] Cole | Diversity and functional importance of coral-feeding fishes on tropical coral reefs[END_REF][START_REF] Benzoni | Pink spots on Porites: not always a coral disease[END_REF]). Moreover, it seems to be more vulnerable to infectious disease than many other coral species [START_REF] Wilkinson | Status of coral reefs of the world: 2004[END_REF].

Of the 30 coral diseases described to date [START_REF] Sutherland | Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals[END_REF][START_REF] Weil | Status and progress in coral reef disease research[END_REF], eight are known to affect P. lutea worldwide. On Indo-Pacific reefs, colonies of P. lutea have been recorded with signs of black band disease (BBD), white plaque syndrome (WPL), growth anomalies (GA), yellow band disease (YBD) and pink line syndrome (PLS) [START_REF] Sutherland | Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals[END_REF]). Surveys conducted in the Gulf of Kutch [START_REF] Ravindran | Disease and stress-induced mortality of corals in Indian reefs and observations on bleaching of corals in the Andamans[END_REF], Papua New Guinea [START_REF] Frias-Lopez | Bacterial community associated with black band disease in corals[END_REF]) and Philippines have recorded BBD outbreaks in this scleractinian coral. In addition, a study performed on [START_REF] Myers | Molecular detection and ecological significance of the cyanobacterial genera Geitlerinema and Leptolyngbya in black band disease of corals[END_REF]) coral health and diseases in the northern Egyptian Red Sea has revealed two other syndromes: Porites ulcerative white spot (PUWS) and a white syndrome (WS) so far unreported on P. lutea [START_REF] Mohamed | Status of coral reef health in the northern Red Sea, Egypt[END_REF]. More recently, a white syndrome (WS) named Porites white patch syndrome (PWPS) was described on massive colonies of P. lutea on Western Indian Ocean (WIO) reefs [START_REF] Séré | Porites white patch syndrome: an unreported coral disease on Western Indian Ocean reefs[END_REF]. This syndrome was characterised by diffuse, medium to large (50-300 mm diameter), circular to oblong tissue loss, surrounded by swollen white tissue. The dead skeleton was progressively colonised by opportunistic algae and Cyanobacteria [START_REF] Séré | Porites white patch syndrome: an unreported coral disease on Western Indian Ocean reefs[END_REF]).

To date, nothing is known about the aetiology of PWPS. Previous studies on other white syndromes (white band disease (WBD), white plague disease (WP), progressive white syndromes (PWS), Australian subtropical white syndrome (ASWS), Acropora white syndrome in American Samoa (AWS), and Porites bleaching with tissue loss (PBTL)) have characterized organisms (bacteria, ciliates, helminths, fungi, algae) associated with both healthy and diseased coral colonies. These investigations have allowed identification of a number of putative pathogens [START_REF] Pantos | The bacterial ecology of a plague-like disease affecting the Caribbean coral Montastrea annularis[END_REF][START_REF] Pantos | Bacterial community structure associated with white band disease in the elkhorn coral Acropora palmata determined using culture-independent 16S rRNA techniques[END_REF][START_REF] Sunagawa | Bacterial diversity and White Plague Disease-associated community changes in the Caribbean coral Montastraea faveolata[END_REF][START_REF] Cárdenas | Shifts in bacterial communities of two caribbean reef-building coral species affected by white plague disease[END_REF][START_REF] Work | Pathology of tissue loss (white syndrome) in Acropora sp. corals from the Central Pacific[END_REF][START_REF] Godwin | The role of coral-associated bacterial communities in Australian subtropical white syndrome of Turbinaria mesenterina[END_REF][START_REF] Sudek | Histological observations in the Hawaiian reef coral, Porites compressa, affected by Porites bleaching with tissue loss[END_REF][START_REF] Sweet | Ciliate and bacterial communities associated with White Syndrome and Brown Band Disease in reef-building corals[END_REF][START_REF] Wilson | Bacterial communities associated with healthy and Acropora white syndrome-affected corals from American Samoa[END_REF]). Evidence of the involvement of bacteria as causative agents has been suggested in some studies on several of the WS observed on scleractinian corals [START_REF] Sussman | Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific[END_REF][START_REF] Arboleda | Vibrio sp. causing Porites ulcerative white spot disease[END_REF][START_REF] Luna | Vibrio harveyi as a causative agent of the White Syndrome in tropical stony corals[END_REF][START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF]. For example Serratia marcescens has been reported to be linked with white pox disease (WPD) in the Elkhorn coral Acropora palmata [START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF] and

Vibrio owensii to be the aetiological agent of Montipora white syndrome (MWS) in the Hawaiian coral Montipora capilata [START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF]). However, some of these potential causative agents have i) not been fully characterised in terms of fulfilling all Koch's postulates [START_REF] Bythell | White plague, white band, and other "white" diseases Coral Health and Disease[END_REF]) or ii) have been biased by the execution of infection trials with a specific pathogen rather than testing for multiple potential pathogens. Finally, no clear link between the proposed pathogens and signs of disease has been demonstrated at the gross and cellular level in aquaria or the field.

This study aimed to provide the first characterisation of bacterial communities associated with healthy and PWPS-affected massive colonies of P. lutea in three WIO regions: Mayotte, South Africa and Reunion. The sampling sites included a continental African reef and two oceanic islands separated by over 1500 km of mostly oceanic water masses. These sites were selected to highlight general microbial patterns associated with this disease and the host. The investigation presented here used both histopathology and culture-independent molecular (clone libraries) techniques. The main objectives were to i) describe the microscopic morphology of lesions in corals, and ii) identify the most represented bacteria associated with both healthy and infected coral tissues collected at the three localities.

Diseased tissues (DT) were sampled from the lesion boundary interface with visually healthy tissue (HT), and samples of HT were collected from visually healthy colonies with no signs of disease. Cores of both HT and DT (2.2 cm diameter to a depth of 0.5-1 cm) were collected using a sterile stainless steel core tube and placed individually in sterile disposable 50 ml polypropylene centrifuge tubes. Upon reaching the shore, the seawater within each tube was drained for 10 minutes, replaced with 100% ethanol and stored at -20˚C for subsequent DNA extraction and molecular analysis. Due to time and environmental constraints, corals exhibiting signs of PWPS could not be monitored for lesion progression.

Additional core samples were simultaneously collected at each location from five HT and five DT colonies using a hammer and chisel and fixed in 4% formalin for histopathology. Samples were protected in 1.5% (w/v) agarose to retain the spatial integrity of the tissues and microbial communities. They were then decalcified using 1% HCl and EDTA renewed every 12 h until the process was complete. Tissues from the decalcified fragments were dehydrated in a series of ethanol baths, cleared with xylene and embedded in paraffin wax. Sections of 6-8 μm were cut using a microtome, mounted on glass slides and stained with Harris haematoxylin and eosin with phloxine B to diagnose tissue fragmentation, necrosis and the presence of invasive organisms. Serial sections of the affected cells or tissues were examined under a light microscope at different magnifications (×200, ×400, ×1000) and photographed using NIS Element software (Nikon©). The number of colonies sampled per region for histology is presented in Table 8.

DNA extraction

Bacterial genomic DNA from both HT and DT were extracted using the NucleoSpin® Soil Kit (NucleoSpin Extract II, Macherey-Nagel, Düren, Germany). Refrigerated samples were dried at room temperature, after which approximately 150 mg of both tissue and skeleton were scraped from their surface using a sterile scalpel blade, placed in a 1.5 ml centrifuge tube with 700 µl of lysis buffer and crushed using a fresh disposable plastic rod. Samples were then placed in lysing matrix tubes for DNA extraction. The DNA was eluted with 50 µl sterile elution buffer, verified by electrophoresis in agarose gels (1.5% wt/vol) stained with GelRed TM (Biotium Inc., Hayward, California, USA) and finally stored at -20˚C. System 2700 thermal cycler. Amplification conditions for the PCR included an initial denaturing step of 4 min at 95.5˚C, followed by 30 cycles at 94˚C for 30 sec, 55˚C for 60 sec and 72˚C for 90 sec followed by a final extension step of 15 min at 72˚C.

Reunion South Africa Mayotte

Cloning and Sequencing

PCR was carried out using DNA templates prepared from each of the three individual DT or HT samples. PCR products were verified for quality, size and quantity by electrophoresis and spectrophotometry. Equimolar quantities of PCR products were then pooled for the three DT and HT samples at each locality (6 samples per locality) and each was subsequently separated by electrophoresis. Amplicons 1300-1500 bp long were excised from the gel under a UV transilluminator and the DNA was gel purified using NucleoSpin® Gel and a PCR Clean-up kit (Macherey-Nagel, Düren, Germany) for colony screening. The purified DNA was cloned into the pGEM-T vector system (Promega, Madison, WI) and ligation mixture was used to transform Escherichia coli JM109 competent cells. A total of 92-150 clones were randomly selected from each tissue category, spotted directly into 96-well plates and subjected to PCR with M13 forward and reverse primers (Inquaba Biotec TM , Pretoria, South Africa).

Amplification conditions for the PCR included an initial denaturing step at 94˚C for 5 min followed by 35 cycles of 94˚C for 1 min, 57˚C for 1 min and 72˚C for 1 min, with a final extension step at 72˚C for 7 min. PCR products were checked for quality, size and quantity by electrophoresis in agarose gels (1.5% wt/vol) as described above and sent to GENOSCREEN (Campus de l'Institut Pasteur, Lille, France) for Sanger sequencing.

Sequence analysis

The pair sequences obtained for each clone were edited and aligned using GENEIOUS TM Pro (V.5.6.3) sequencing software. All high quality consensus sequences (HQ> 65%) were submitted to BLAST at the National Centre for Biotechnology Information (NCBI, www.ncbi.nlm.nih.gov) to determine their percentage similarity with known 16Sr DNA sequences. Sequences matching at a similarity level of 1) 97-100% were considered as belonging to the same species level, 2) 93-96% were considered as belonging to the same genus level and 3) <93% were considered to fall below the similarity of the genus level [START_REF] Pantos | Bacterial community structure associated with white band disease in the elkhorn coral Acropora palmata determined using culture-independent 16S rRNA techniques[END_REF]. Bacterial rRNA sequences closely related to putative bacteria were aligned using Geneious alignment and rearranged manually. A phylogenetic tree was built using the Neighbor-Joining method of GENEIOUS TM Pro (V. 5.6.3). All 16r RNA gene sequences are accessible through the NCBI GeneBank database under accession numbers KF179641-KF180135.

Statistical analysis

Multidimentional Scale (MDS) analysis of bacterial communities associated with healthy and PWPS-affected tissues of Porites lutea collected at Reunion (R), South Africa (RSA) and Mayotte (M) was performed using PRIMER (V. 6.1.14). Data were square root-transformed and MDS analysis was carried out using the Bray-Curtis similarity coefficient. Finally, the Shannon-Weaver index (H) was calculated for each tissue category to characterise pooled bacterial diversities in healthy and diseased coral samples Of all sequences, 14.0% had no close relatives in the NCBI database and could only be classified as unknown bacterial clones. The bacterial diversity associated with HT-RSA (Table S1) was also dominated by sequences closely related to γ-proteobacteria (88.0%) followed by Cyanobacteria (5.0%), α-proteobacteria (2.0%), Spirochaetes (1.0%) and

Actinobacteria (1.0%). HT-R (Appendix 1) seemed to contain less group diversity, mainly dominated by γ-proteobacteria (67.0%), Cyanobacteria (11.0%) and Firmicutes (6.0%).

Unknown bacterial clones constituted 17.0% of all analysed sequences. The γ-proteobacteria retrieved from HT-M, HT-RSA and HT-R were dominated by bacterial species closely related to Endozoicomonas elysicola (accession no. NR041264), comprising 53.1%, 67.8 % and 81.8 % of the total γ-proteobacteria respectively. Sequences closely-related to Vibrio fortis (accession no. NR025575) were also common to bacterial communities associated with HT from all three sampling locations.

. Similar trends emerged for PWPS-RSA (Fig. 11), the bacterial classes being dominated by γproteobacteria (56.9%) and α-proteobacteria (14.6%), followed by Cyanobacteria (3.5%), Firmicutes (3.5%), Planctomycetes (2.1%), Bacteroidetes (0.7%), Cytophagia (0.7%), Deltaproteobacteria (0.7%), Flavobacteriia (3.0%) and Sphaerobacteridae (3.0%). Again, some marina (accession no. NR043300.1; n=3) and Silicibacter lacuscaerulensis (accession no.

NR029197; n=3). Among the bacterial strains retrieved from PWPS-RSA, the γproteobacteria, E. elysicola (accession no. NR041264; n=41) and Oceanospirullum beijerinckii (accession no. NR040784; n=5), the α-proteobacterium S. marina (accession no.

NR043300.1; n=3) and the Cyanobacterium Prochlorococcus marinus (accession no.

NR028762; n=5) were the most representative species.

Comparison of bacterial communities in healthy and diseased tissues

Distinctly partitioned ribotypes were detected among diseased and healthy tissues samples. In total, 31 (77.8%), 54 (90.0%) and 24 (92.3%) bacterial ribotypes were exclusively associated with PWPS-M, PWPS-RSA and PWPS-R respectively, while 28 (77.8 %), 17 (73.9%) and 10 (83.3%) were found only in HT-M, HT-RSA and HT-R respectively (Fig. 12).

Multidimentional scaling (MDS, Fig. 12B) analysis performed on the composition of bacterial 16S rRNA gene of each tissue categories revealed four distinct clusters representing four distinct bacterial communities. Similarities in bacterial composition in RDT and MDT were detected but SAHT, RHT, SADT and MHT samples exhibited more variability (Fig. 12). In addition, the bacterial diversity identified in PWPS tissues collected on the three WIO coral reefs was higher than in HT (Table 9). For instance, 39, 60 and 26 16S rRNA gene sequences affiliated to bacterial genera/species were identified in PWPS-M, PWPS-RSA and PWPS-R respectively, whereas only 36, 23 and 12 were obtained in HT-M, HT-RSA and HT-R respectively (Table 9). Among these, only six ribotypes were commonly detected in PWPS samples from the three sampling localities and were closely related to P. yeei (accession no.

NR025491), P. aquimaris (accession no. NR029038.1), S. marina (accession no.

NR043300.1), V. fortis (accession no. NR025575.1), V. hepatarius (accession no.

NR025575.1) and V. parahaemolyticus (accession no. NR041838.1). In HT, only 16S rRNA gene sequences affiliated to bacterial species E. elysicola (accession no. NR041264) and V.

fortis (accession no. NR025575.1) were common in samples collected at the three sampling localities.

Discussion

Histological observations

Corals exhibiting signs of PWPS revealed extensive tissue fragmentation, generally associated with ovoid basophilic bodies resembling bacterial aggregates within the mesoglea of the body wall. These aggregates were seen in 60% of all samples collected from corals with signs of PWPS. However, these aggregates could not be directly linked with the pathology as there was no clear evidence of inflammatory response or tissue lysis associated with these ovoid bodies. In addition, some clusters of basophilic bodies were also observed in sections of one healthy sample of P. lutea, preventing definitive conclusion that they constituted a bacterial infectious agent in the PWPS lesions. Similar observations on bacterial aggregates have been previously reported in several histopathological studies, in both healthy and WS-infected colonies of Acropora spp., Pocillopora meandrina and Porites compressa [START_REF] Galloway | Antibacterial activity of marine and black band disease cyanobacteria against coral-associated bacteria[END_REF][START_REF] Sudek | Histological observations in the Hawaiian reef coral, Porites compressa, affected by Porites bleaching with tissue loss[END_REF]. Direct identification from formalin-fixed, paraffinembedded coral tissue combined with descriptions of cellular changes over time may be a viable option to identify the role of these aggregates in PWPS. Other organisms, including Cyanobacteria, Nematoda, Ciliata and endophytic algae, were also observed on diseased tissues and were generally associated with the dead epidermis and cell debris. No obvious evidence of the direct physical ingress of these organisms into the tissue in cross-sections was observed suggesting that these were potentially opportunistic invaders.

Variability in bacterial diversity in PWPS and HT

The bacterial communities in both healthy and PWPS-infected tissues of P. lutea were dominated at all three sampling locations by members of the α-proteobacteria, γproteobacteria and Cyanobacteria (Fig. 11). However, diseased corals exhibited higher bacterial diversity compared to healthy ones (Table 9). These results are consistent with recent studies which have reported lower bacterial abundance and diversity in healthy corals than those displaying signs of WPD [START_REF] Sunagawa | Bacterial diversity and White Plague Disease-associated community changes in the Caribbean coral Montastraea faveolata[END_REF][START_REF] Cárdenas | Shifts in bacterial communities of two caribbean reef-building coral species affected by white plague disease[END_REF][START_REF] Roder | Bacterial profiling of White Plague Disease in a comparative coral species framework[END_REF]) and WBD [START_REF] Pantos | Bacterial community structure associated with white band disease in the elkhorn coral Acropora palmata determined using culture-independent 16S rRNA techniques[END_REF]. Examination of 16S rRNA gene sequences using cloning as a culture-independent molecular technique, revealed high variability between bacteria associated with PWPS-infected and healthy tissues of Porites lutea, with only a few ribotypes commonly found in both diseased and healthy tissues (Appendix 1). Among them, ribotypes similar to Pseudoalteromonas spp., Paracoccus yeei and Amphritea balenae have been previously identified in seawater [START_REF] Sweet | Bacterial assemblages differ between compartments within the coral holobiont[END_REF], soil [START_REF] Daneshvar | Paracoccus yeeii sp. nov.(formerly CDC group EO-2), a novel bacterial species associated with human infection[END_REF]) and sediment [START_REF] Miyazaki | Amphritea japonica sp. nov. and Amphritea balenae sp. nov., isolated from the sediment adjacent to sperm whale carcasses off Kagoshima, Japan[END_REF]) respectively, suggesting that these bacteria were present in the environment and opportunistically became resident in the coral mucus or associated with the healthy coral microbiota. Similar variations in the bacterial communities have been reported in several coral species affected by BBD [START_REF] Voss | Black band disease microbial community variation on corals in three regions of the wider Caribbean[END_REF], WPD [START_REF] Pantos | The bacterial ecology of a plague-like disease affecting the Caribbean coral Montastrea annularis[END_REF], WBD in the Caribbean [START_REF] Pantos | Bacterial community structure associated with white band disease in the elkhorn coral Acropora palmata determined using culture-independent 16S rRNA techniques[END_REF]) and other WS in Australia and American Samoa [START_REF] Kvennefors | Bacterial communities of two ubiquitous Great Barrier Reef corals reveals both site-and species-specificity of common bacterial associates[END_REF][START_REF] Godwin | The role of coral-associated bacterial communities in Australian subtropical white syndrome of Turbinaria mesenterina[END_REF][START_REF] Wilson | Bacterial communities associated with healthy and Acropora white syndrome-affected corals from American Samoa[END_REF]). The bacterial diversity found in PWPS was higher than in HT at all three localities. Our results

were similar to those reported for white syndromes including Acropora white syndrome (AWS) on American Samoan reefs [START_REF] Wilson | Bacterial communities associated with healthy and Acropora white syndrome-affected corals from American Samoa[END_REF]) and white plague disease in the Caribbean coral Montastrea annularis [START_REF] Pantos | The bacterial ecology of a plague-like disease affecting the Caribbean coral Montastrea annularis[END_REF]. This difference in the composition of bacterial communities may suggest that disease agents impair the structure of natural bacterial communities. It is likely that compromised or dead tissues represent a "micro-niche" that can be colonised by more competitive and opportunistic bacteria in the surrounding water and sediments or transmitted by other marine organisms [START_REF] Pantos | The bacterial ecology of a plague-like disease affecting the Caribbean coral Montastrea annularis[END_REF][START_REF] Pantos | Bacterial community structure associated with white band disease in the elkhorn coral Acropora palmata determined using culture-independent 16S rRNA techniques[END_REF][START_REF] Gil-Agudelo | Bacterial communities associated with the mucopolysaccharide layers of three coral species affected and unaffected with dark spots disease[END_REF][START_REF] Cárdenas | Shifts in bacterial communities of two caribbean reef-building coral species affected by white plague disease[END_REF][START_REF] Godwin | The role of coral-associated bacterial communities in Australian subtropical white syndrome of Turbinaria mesenterina[END_REF].

Interestingly, comparisons of bacterial communities associated with both PWPS-infected and healthy tissues also revealed distinct populations at the three sampling locations (Appendix 1, Fig. 12). This may suggest that no specific bacterial communities are associated with P. lutea on the WIO reefs. However some exceptions were recorded. For example, E. elysicola (accession no. NR041264) and V. fortis (accession no. NR025575.1) were found in both PWPS and HT collected at all the localities and seemed to be coral-specific. Another species,

V. rumoiensis (accession no. NR024680) seemed to develop the same specific bacterial-coral association but was only found in HT sampled on South Africa and Mayotte reefs and not those at Reunion. These bacterial strains, apparently ubiquitous in HT, may play an important role in coral health and growth [START_REF] Pantos | Bacterial community structure associated with white band disease in the elkhorn coral Acropora palmata determined using culture-independent 16S rRNA techniques[END_REF]. For instance the genus Endozoicomonas, found in several marine organisms [START_REF] Kurahashi | Endozoicomonas elysicola gen. nov., sp. nov., a γ-proteobacterium isolated from the sea slug Elysia ornata[END_REF][START_REF] Jensen | Intracellular Oceanospirillales bacteria inhabit gills of Acesta bivalves[END_REF][START_REF] Bayer | Bacteria of the genus Endozoicomonas dominate the microbiome of the Mediterranean gorgonian coral Eunicella cavolini[END_REF][START_REF] Nishijima | Endozoicomonas numazuensis sp. nov., a gammaproteobacterium isolated from marine sponges, and emended description of the genus Endozoicomonas Kurahashi and Yokota 2007[END_REF], seems to play an important role in corals, notably in the biogeochemical cycling of sulphur [START_REF] Raina | Coral-associated bacteria and their role in the biogeochemical cycling of sulfur[END_REF]). V. fortis was initially described as a probiotic that out-competes pathogen strains [START_REF] Thompson | Vibrio fortis sp. nov. and Vibrio hepatarius sp. nov., isolated from aquatic animals and the marine environment[END_REF][START_REF] Cunning | A survey of Vibrios associated with healthy and Yellow Band Diseased Montastraea faveolata[END_REF] or is involved in the recycling of dimethyl-sulfoniopropionate (DMSP), which may be detrimental to coral health [START_REF] Raina | Coral-associated bacteria and their role in the biogeochemical cycling of sulfur[END_REF]. However, further studies are needed to elucidate the ecological function of these genera in corals.

Potential pathogens associated with PWPS

In our study, several 16S rRNA gene sequences were closely related (97-100% similarity) to bacteria associated with coral diseases or known pathogens. Interestingly, one sequence was detected in PWPS from all three sampling localities but absent in healthy corals. Blast identification associated with phylogenetic analysis (Fig. 13) showed it to be closely related to the γ-proteobacteria V. hepatarius (accession no. NR025575.1), isolated for the first time from the healthy wild white shrimp Litopenaeus vannamei in Ecuador [START_REF] Thompson | Vibrio fortis sp. nov. and Vibrio hepatarius sp. nov., isolated from aquatic animals and the marine environment[END_REF]. Other 16S rRNA gene sequences affiliated to the family Vibrionaceae were associated with PWPS-infected tissues (Fig. 13). For instance, V. fortis was detected at all three sampling locations. This bacterium was first isolated from various marine organisms and has been reported to be pathogenic in corals [START_REF] Boyett | The ecology and microbiology of black band disease and brown band syndrome on the Great Barrier Reef[END_REF], fish and crustacean [START_REF] Austin | Pathogenicity of vibrios to rainbow trout (Oncorhynchus mykiss, Walbaum) and Artemia nauplii[END_REF], and is associated with several coral diseases including yellow band disease (YBD) in Montastrea faveolata [START_REF] Cunning | A survey of Vibrios associated with healthy and Yellow Band Diseased Montastraea faveolata[END_REF] and BBD in the Red sea [START_REF] Barneah | Characterization of black band disease in Red Sea stony corals[END_REF]). However, sequences affiliated with this species were also found in HT from all three localities, making this a less likely candidate for PWPS pathogenesis. In addition, ribotypes similar to V. parahaemolyticus, known to induce disease in humans [START_REF] Austin | Vibrios as causal agents of zoonoses[END_REF]) and many aquatic organisms [START_REF] Cho | Enteric bacteria isolated from acute diarrheal patients in the Republic of Korea between the year 2004 and 2006[END_REF], were identified as well as V rotiferanus associated with YBD in several Caribbean and Indo-Pacific scleractinian species [START_REF] Cervino | The Vibrio core group induces yellow band disease in Caribbean and Indo-Pacific reef-building corals[END_REF]). However, similar sequences were found in healthy coral tissues or were not represented in diseased tissues at all three localities. Finally, sequences affiliated to Shimia marina (accession no. NR043300.1) were recorded only in PWPS-infected corals at all three sampling localities. This roseobacter was previously reported in the coral Turbinaria mesenterina infected by ASWS [START_REF] Godwin | The role of coral-associated bacterial communities in Australian subtropical white syndrome of Turbinaria mesenterina[END_REF]) but no evidence of its pathogenicity has been established in previous studies. The potential pathogens related to the sequences obtained in this study thus need to be isolated, cultured and inoculated in laboratory corals to ascertain their ability to induce disease in corals.

Conclusions

This is the first study characterising bacterial communities associated with healthy and PWPS-infected Porites lutea coral colonies on WIO coral reefs. Microscopy revealed the inclusion of basophilic bodies like bacterial aggregates in the coral epidermis within the lesion area. We established that the structure of the microbial communities is different between diseased and healthy coral tissues, and between localities, by cloning the 16S rRNA gene as a culture-independent molecular method. Furthermore, higher bacterial diversity was observed in PWPS-infected tissues. This shift may be explained by a perturbation of the natural bacterial communities associated with coral holobionts which are progressively replaced by a succession of opportunistic bacteria including potential pathogens. Since the bacterial diversity at each of the three sites was assessed by analysing pooled samples, additional replicates including seasonal monitoring is needed to confirm the heterogeneity of bacterial species associated with PWPS in the areas studied. Several bacterial ribotypes affiliated to potential putative pathogens were consistently found among the 16S rRNA sequences derived from the PWPS lesions, and absent and/or poorly represented in HT.

Isolation, culture and subsequent infection trials to satisfy Henle-Koch's postulates would be needed to prove their pathogenicity.

Abstract

A novel white syndrome, the Porites white patch syndrome (PWPS), was recently reported and described on Western Indian Ocean coral reefs. Bacterial communities associated with this coral disease were characterised using culture-independent molecular techniques (clone libraries), yielding a number of putative bacterial pathogens potentially involved as causative agents of this pathology. A total of 14 bacterial strains retrieved from PWPS-infected colonies of Porites lutea were subsequently used in experimental infection trials. Coral fragments exposed to a bacterial strain (P180R) closely related (99-100 % sequence identity) to Vibrio tubiashii elicited similar evidence of disease to that observed in the field. Following experimental infection, this pathogen was successfully re-isolated from the diseased tissues and re-inoculated in healthy corals colonies. Signs of PWPS appeared on 90% of the exposed coral fragments (27 of 30) under controlled environmental conditions. Moreover, virulence of this newly-identified marine pathogen seemed to be strongly dependent on seawater temperature, resulting in significantly higher tissue loss at 30°C than 28°C and 26°C.

Nevertheless, further research is needed to determine if temperature is the primary factor that controls its virulence. Additional proof is needed to link P180R with PWPS at the gross and cellular level.

Keywords: Coral disease -Western Indian Ocean -Scleractinian corals -Porites white patch syndrome -Inoculation trials -Causative agent -Vibrio tubiashii

Introduction

Coral diseases are increasingly recognised as a significant threat to coral reefs (Bruno and Selig 2007;[START_REF] Harvell | Coral disease, environmental drivers, and the balance between coral and microbial associates[END_REF]) with more than 30 syndromes reported to date [START_REF] Weil | Status and progress in coral reef disease research[END_REF]). Among them, ten distinct white syndromes (WS), grossly characterized by advancing acute tissue loss [START_REF] Willis | Coral disease on the Great Barrier Reef[END_REF][START_REF] Sussman | Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific[END_REF] Thalassomonas loyana has been suggested to cause white plague-like disease on Eilat coral reefs [START_REF] Thompson | Thalassomonas loyana sp. nov., a causative agent of the white plague-like disease of corals on the Eilat coral reef[END_REF] and Aurentimona coralicida to be the aetiological agent of WP type II [START_REF] Denner | Aurantimonas coralicida gen. nov., sp. nov., the causative agent of white plague type II on Caribbean scleractinian corals[END_REF]. Several members of the genus Vibrio, e.g. V. corallilyticus (Ben-Haim and Rosenberg 2002), V. harveyi [START_REF] Luna | Vibrio harveyi as a causative agent of the White Syndrome in tropical stony corals[END_REF][START_REF] Godwin | The role of coral-associated bacterial communities in Australian subtropical white syndrome of Turbinaria mesenterina[END_REF]) and V. shilonii [START_REF] Israely | Growth, differentiation and death of Vibrio shiloi in coral tissue as a function of seawater temperature[END_REF][START_REF] Kushmaro | Vibrio shiloi sp. nov., the causative agent of bleaching of the coral Oculina patagonica[END_REF], have also been reported as putative pathogens in other WS. However, very few experimental studies involving inoculation trials have been undertaken on WS. To date, only six bacterial species have been identified as aetiological agents satisfying the following four Henle Koch's postulates [START_REF] Israely | Growth, differentiation and death of Vibrio shiloi in coral tissue as a function of seawater temperature[END_REF]Ben-Haim et al. 2003b;[START_REF] Sussman | Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific[END_REF][START_REF] Arboleda | Vibrio sp. causing Porites ulcerative white spot disease[END_REF][START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF][START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF]): a putative pathogen must be i) found only in diseased tissue, ii) isolated and grown in pure culture. The isolated and purified organisms must iii) cause disease when inoculated into a healthy organism and iv) be re-isolated from the infected host [START_REF] Koch | Robert Koch[END_REF].

For instance, the human pathogen Serratia marcescens, known to generate respiratory infections, pneumonia and meningitis [START_REF] Grimont | The genus Serratia[END_REF], has been found to generate WPD in the Elkhorn coral Acropora palmata [START_REF] Patterson | The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata[END_REF][START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF]. Additionally, the λ-proteobacterium Vibrio owensii, has been recently described as a pathogen of MWS in Montipora capitata [START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF]). However, despite having fulfilled all pathogenic criteria, no definitive proof linking some of these identified putative pathogens to the syndromes were demonstrated since inoculation trials were performed with one specific putative pathogen rather than multiple potential pathogens and/or strains of the same type.

Recently, a newly-identified WS, Porites white patch syndrome (PWPS), has been reported on massive Porites lutea, one of the most important reef-building coral on the Western Indian Ocean reefs [START_REF] Séré | Porites white patch syndrome: an unreported coral disease on Western Indian Ocean reefs[END_REF]). This WS is recognised by circular to oblong tissue loss (5.0-30.0 cm diameter) surrounded by a front of swollen and bleached tissues (1.0-20.0 cm width, Fig. 1). A recent molecular study on bacterial communities associated with PWPS-infected tissues in P. lutea conducted on three geographically distant sites of the western Indian Ocean (WIO), revealed the presence of several 16S rRNA gene sequences affiliated to potential causal agents [START_REF] Séré | Bacterial communities associated with Porites white patch syndrome (PWPS) on three Western Indian Ocean (WIO) coral reefs[END_REF]. Of the several identified potential pathogens, bacterial 16S rRNA sequence genes phylogenetically closely related to Shimia marina and Vibrio hepatarius were found only in diseased coral tissues collected at three distant sites of the WIO.

This study aimed to isolate and identify the primary pathogen(s) of PWPS by combining traditional culturing methods and molecular genotyping coupled with inoculation trials to test for Koch's postulates. Strain(s) inducing signs resembling PWPS in aquaria were further characterised using standard bacteriological tests. Finally, the impact of elevated seawater temperature, identified as an important abiotic factor enhancing the virulence several coral diseases, was also tested in aquarium experiments.

One fragment of 5 g (wet weight) of each sample was crushed using a sterile mortar and pestle and diluted in 10 ml of 0.22 µm filtered seawater. Crushed samples were immediately streaked (100 µl) in triplicate serial dilutions (1:1, 1:10 and 1:100) on Marine Agar (1.8%

Marine Broth, Difco-2216 USA, 0.9% NaCl, 1.8% Agar Bacto, Difco-214010, USA; MA) and thiosulfate citrate bile salts sucrose (TCBS) agar and incubated at 32°C for 24 h.

Cultivable bacterial strains exhibiting a unique colony were routinely subcultured to purification under the same growth conditions for their subsequent use in inoculation experiments. Amplification conditions for the PCR included an initial denaturing step of 4 min at 95.5˚C followed by 30 cycles at 94˚C for 30 sec, 55˚C for 60 sec, and 72˚C for 90 sec, with a final extension step of 15 min at 72˚C. Sequencing was carried out by Genoscreen (Lille, France).

DNA extraction, PCR amplification and 16S rDNA sequencing

Partial sequences (700-1100 bp) obtained for each bacterial isolate were examined for error and edited using GENEIOUS TM Pro (V.5.6.6) sequencing software. All partial sequences were then submitted to BLAST at the National Centre for Biotechnology Information (NCBI, www.ncbi.nlm.nih.gov) and compared with available sequences. The 16S rDNA consensus sequences of the bacterial isolates causing disease similar to PWPS were aligned with 15 reference rDNA gene sequences closely related to multiple strains of Vibrio species using the multiple sequence alignment method (93% identity). A phylogenetic tree based on the alignment of a region of 1584 bp was built by the neighbour-joining method using the NKY model of GENEIOUS TM Pro (V. 6.1.5). Bootstrap values were based on 1000 replicates.

Coral collection, maintenance and experimental design

Healthy colonies of Porites lutea were collected in South Africa at Sodwana Bay at 15 m using SCUBA and placed in zip-lock plastic bags containing fresh seawater, filled with oxygen and placed in a cool box for transport to the ORI Research Aquarium. Here they were acclimated in a 1000 l plastic tank for 3 months under the following controlled conditions: T:

24˚C, with a photoperiod 12:12 h light: dark. A total of 45 small P. lutea fragments were then cut from each colony and monitored for two weeks until tissue had grown on the exposed skeleton. All bleached and lysed colonies were discarded. After the healing period, five coral fragments were randomly assigned to each of nine individual 100 l pre-sterilised aquaria (Fig. 15). Each aquarium was continuously supplied with oligotrophic seawater (70 l/h) pumped from the sea and filtered through a sand filter, a bio-filter (live rock) and a protein skimmer (RK2 Systems, USA) designed to remove floatables, grease and oils. To eliminate any external contamination by parasites, microalgae and diseases, the filtered seawater was purified by passage through a UV sterilizer (55 W, Ao Aqua Medic, Germany). Corals were exposed to a constant irradiance of 249.2 µmol photons m -2 s -1 using metal halide lamps (HAILEA®LFHO LAMP, 4 x 54 W/ Extra Reef tubes blue and white) and were slowly acclimated to 28˚C by increasing the initial temperature (24˚C) by 0.5˚C per day using heaters (SICCE, SCUBA 100, Italy). A flow pump (320 SEIO TM PROP) was added to each aquarium to maintain aeration and water mixing. Experimental conditions, including pH, temperature, salinity, dissolved oxygen and light intensity, were kept constant and were assumed to be similar in each aquarium. The waste water was treated with another UV sterilizer (55 W, Ao Aqua Medic, Germany) before being discharged in a canal in order to avoid any risk of environmental contamination.

Phase 1: Screening of potential PWPS pathogens for infectivity

In total, 14 bacterial ribotypes retrieved from PWPS samples were selected for the inoculation experiments (Table 10). Only bacteria that were closely related (>95% identity) to identified pathogens found exclusively in PWPS-infected tissues at Reunion and Mayotte were selected.

Candidate strains for the infection trials were also selected according to 16S rDNA sequences found in the molecular-based study conducted by [START_REF] Séré | Bacterial communities associated with Porites white patch syndrome (PWPS) on three Western Indian Ocean (WIO) coral reefs[END_REF] using culture-independent molecular techniques. Each candidate strain was picked from pure cultures, incubated and grown at 28°C in sterile 15 ml PP Falcon tubes containing sterile Marine Broth (MB) until the end of the logarithmic phase (15-16h). The growth (CFU/ml) of each bacterial candidate was estimated by counting CFU on solid MA. Individual cultures were centrifuged at 3000 g for 10 min at 4°C and the pellets were transferred to sterile seawater (SSW) with a volume adjusted to contain a final concentration of ± 10 6 CFU ml -1 . Coral fragments were carefully removed from each aquarium and placed into a plastic container containing 2 l of sterile seawater for inoculation. Prior to bacterial inoculation, each fragment was lightly scratched with a sterile pipette tip to break the mucus barrier and thereby create an entry point for the potential pathogens, simulating a natural trauma. Inoculation experiments were conducted by smearing 0.5 ml of isolate (± 1x10 6 CFU ml -1 ) onto the surface of each of the five healthy fragments of P. lutea. Control aquaria (N=2), also containing five healthy coral fragments, were treated using the same protocol but without bacterial inoculation. The coral fragments were then monitored and photographed on a daily basis. The number of individuals that displayed evidence of PWPS (morbidity) or that died as a result of infection (mortality) were recorded.

Phase 2: Infection trials:

To fulfil Henle-Koch postulates three samples of fragments infected in phase 1 were crushed, diluted 1:10 in sterile and filtered seawater and plated in triplicate on both MA and TCBS.

Bacterial colonies that grew on both media overnight were subcultured to purification.

Bacterial strains with 100% 16S rRNA gene similarity to the strain that initiated disease resembling PWPS during phase 1 of the inoculation trials were then re-inoculated on healthy colony fragments. Prior to the phase 2, the aquarium system was carefully cleaned and sterilised to eliminate residual bacteria from contaminating and biasing the results. Batches of five fresh coral fragments (N= 45 coral fragments) from apparently healthy colonies of P.

lutea maintained in a 1000 l plastic tank were again re-distributed into each of the nine tanks.

Infection experiments including preparation of the inoculums and inoculation were repeated as described above. Three of the nine aquaria containing five coral fragments (N=15 fragments) were not inoculated as controls. The coral fragments were again monitored and photographed for signs of PWPS.

Effects of temperature on virulence

Temperature experiments were conducted using the experimental procedure described above.

Again, the aquarium system was carefully cleaned and sterilised prior to the experiment to prevent residual bacterial from contaminating and biasing the results. Fresh, apparently healthy coral fragments of P. lutea were divided into three treatments at three temperatures:

T1: 26°C, T2: 28°C and T3: 30°C with two replicates and one control for each treatment.

Each contained five coral fragments, randomly allocated to the nine 100 l tanks. Fragments were acclimatized for a week at their respective experimental temperatures and were monitored for apoptosis, bleaching and disease before commencing the temperature trials. The inoculum (± 1x10 6 CFU ml -1 ) was smeared onto the surface of each of healthy fragments as described in phases 1 and 2 and monitored and photographed. The mean proportions of infected colonies and virulence (mean number of dead colonies per tank) were calculated.

Finally, the rate of tissue loss (cm 2 ) was estimated for 170 h using image analysis software (AxioVison 4.8.2) to determine the progression rate of PWPS when exposed to different temperatures.

Multilocus sequence and phylogenetic analysis

A mutlilocus sequence typing (MLST) analysis using three additional housekeeping genes (pyrH, recA and rpoA) was performed to complete and confirm the 16S rRNA gene-based identification. The primers and PCR amplification conditions are presented in Table 11.

Partial sequences of 16S rRNA (1422 bp), pyrH (590 bp), recA (766 bp) and rpoA (802bp)

genes were examined and corrected manually and consensus sequences were determined using the two reads. The consensus sequences obtained for each gene were then aligned with reference sequences related to multiple bacterial strains selected from NCBI using GENEIOUS alignment method.

Additionally, sequences obtained for each gene were concatenated and aligned against concatenated sequences from 12 other species of Vibrio mainly identified as known pathogens [START_REF] Thompson | Phylogeny and molecular identification of vibrios on the basis of multilocus sequence analysis[END_REF]. Phylogenetic trees were built by the neighbour-joining method using the NKY model of GENEIOUS TM Pro (V. 6.1.5) with bootstrap values based on 1000 replicates.

Metabolic characterisation of putative pathogen(s)

The metabolic characterisation of the selected bacterial isolates was performed with API 20E

V4.1 strips following the manufacturer's instructions (Biomériux Durham, NC).

Susceptibility to antibiotics was performed using antibiotic diffusion disc (Oxoid) incubated for 24h at 30°C [START_REF] Thompson | Vibrio fortis sp. nov. and Vibrio hepatarius sp. nov., isolated from aquatic animals and the marine environment[END_REF]. The following antibiotic discs were tested: amikacin, amoxicillin, ampicillin, ceftiofur, cephalexin, chloramphenicol, ciprofloxacin, enrofloxacin baytril, metronidazole. The effect of temperature on growth was tested by incubating the bacterial cultures at 4°C, 20°C, 28°C and 37.5°C. Finally, the motility was assessed microscopically.

Statistical analysis

The proportion of infected coral fragments (virulence) and mortality in each tank were calculated for each aquarium. Means (± SE) of tissue loss rates for each infected fragments was calculated every 24 h at the three experimental temperatures. Data were log-transformed [log 10 (X)] to satisfy assumptions of normality and homogeneity of variance. Repeated

Measures ANOVA (STATISICA 8) was performed to test for differences in tissue loss between the temperature treatments and potential tank effects. When a significant difference was detected, a Fisher LSD post-hoc test was used to test for significance of the measured differences between the temperature treatments.

Results

Phase 1: Screening for potential PWPS pathogens (infectivity)

During phase 1 of the inoculation experiment, only one bacterial strain caused disease resembling PWPS, i.e. circular to oblong tissue loss surrounded by a bleached and swollen front of affected tissue (Fig. 16).This isolate (P180R) had close 16S rRNA gene genetic affiliation with the Gamaprotebacteria, Vibrio hepatarius (AJ345063, KF179862, and KF179731), and its closest known sister taxon, V. tubiashii (X74725, Fig. 17). In total, 80% (N=4 over 5 coral fragments) of the coral fragments of P. lutea exposed to this strain became infected and exhibited signs of disease after 12 h incubation. Among the 13 remaining isolates used in this study, two other strains, P6M and P17R, genetically closely related (>95% identity) to V. fortis (AJ514916) and V. rumoiensis (AB013297) respectively (Table 10), were positive after 24 h incubation, displaying a "light focalised" bleaching reaction on 60% and 20% of the inoculated colonies respectively. All coral fragments placed in control tanks remained unaffected during the experiment. Scratches made to remove the mucus and facilitate entry of pathogens healed completely (100%) 20 days after the first inoculation trials in control and trial fragments.

Phase 2: Infection trials

The strain P180R was re-isolated (P180R-ri) from the experimentally diseased corals exhibiting signs of PWPS during phase 1 of the study. Genetic analysis of 16S rRNA gene sequences confirmed 100% identity (>1400bp) with the strain used in phase 1 of the inoculations trials. Results from the replicated inoculations trials showed that 90% (N=27) of the fragments were positive (infective), exhibiting signs of PWPS resembling those observed during phase 1 of the experiment and in the field. All P. lutea fragments exposed to the pathogen developed signs of disease after 12 h incubation. No fragment exposed to P180R-ri

Temperature experiments

All colonies inoculated at the three temperature treatments had developed signs of PWPS 12h after inoculation (Fig. 5) but ANOVA analysis performed on means of average tissue loss revealed significant differences at the three temperature regimes (ANOVA: df= 2, F=39.77, P>0.01). The mean tissue loss at 30°C was significantly higher than at both 26°C (Fisher LSD post-hoc, p< 0.01) and 28°C (Fisher LSD post-hoc, p< 0.01). Fragments maintained in tanks at 28°C exhibited higher tissue loss than those at 26°C (Fisher LSD post-hoc, p< 0.01).

Values of the disease progression rate in fragments exposed to 26°C, 28°C and 30°C were 0.51 ± 0.50 cm 2 day -1 (mean ± SE), 0.73 ± 0.80 cm 2 day -1 (mean ± SE) and 1.45 ± 0.85 cm 2 day -1 (mean ± SE) respectively. The PWPS in fragments exposed to both 26°C and 28°C had ceased to spread 96 h after infection, whereas the lesions on corals exposed at 30°C decreased slightly but had not halted in their spread after 170 hr (Fig. 5). Of the P.lutea fragments maintained at 30°C, 40% died three weeks after the experiment was terminated. However, those exposed to both 26°C and 28°C started to heal 126 h post-inoculation and recovered completely within one month of commencement of the infection trials.

Discussion

This is the first study to identify a primary causative agent of PWPS under controlled conditions. Bacterial strains cultured and used in laboratory based-infection trials were consistent with those identified recently in PWPS-infected Porites colonies using culturedindependent molecular techniques [START_REF] Séré | Bacterial communities associated with Porites white patch syndrome (PWPS) on three Western Indian Ocean (WIO) coral reefs[END_REF]. Although marine bacterial culture is known to present challenges under laboratory conditions [START_REF] Hill | Extensive profiling of a complex microbial community by high-throughput sequencing[END_REF][START_REF] Harvell | Coral disease, environmental drivers, and the balance between coral and microbial associates[END_REF][START_REF] Bourne | Microbial disease and the coral holobiont[END_REF], 62.5 % of the 16S rRNA haplotypes identified using molecular methods and identified as putative pathogens [START_REF] Séré | Bacterial communities associated with Porites white patch syndrome (PWPS) on three Western Indian Ocean (WIO) coral reefs[END_REF]) could be successfully cultured and subsequently inoculated on P. lutea in this study.

Of the 14 isolates selected for the inoculation trials, only the bacterial strain P180R mostly phylogenetically closely related to V. tubiashii was shown to cause signs resembling those of PWPS [START_REF] Séré | Porites white patch syndrome: an unreported coral disease on Western Indian Ocean reefs[END_REF]. This strain exhibited physiological traits similar to those reported for other pathogenic Vibrio spp. such as motility, oxidase and catalase activities and also a tolerance to temperature variations [START_REF] Meron | Role of flagella in virulence of the coral pathogen Vibrio coralliilyticus[END_REF][START_REF] Arboleda | Vibrio sp. causing Porites ulcerative white spot disease[END_REF][START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF]. Interestingly, 16S rRNA gene sequences previously found in PWPSinfected tissues at three geographically distant WIO coral reefs [START_REF] Séré | Bacterial communities associated with Porites white patch syndrome (PWPS) on three Western Indian Ocean (WIO) coral reefs[END_REF] were closely related to P180R, suggesting its role as a primary pathogen of PWPS. V. tubiashii was initially recognised as a causative agent of a disease called vibriosis considered as a significant pathogen affecting early life stages of several species of bivalves [START_REF] Hada | Vibrio tubiashii sp. nov., a pathogen of bivalve mollusks[END_REF][START_REF] Elston | Re-emergence of Vibrio tubiashii in bivalve shellfish aquaculture: severity, environmental drivers, geographic extent and management[END_REF][START_REF] Hasegawa | The extracellular metalloprotease of Vibrio tubiashii is a major virulence factor for pacific oyster (Crassostrea gigas) larvae[END_REF]) and fish [START_REF] Austin | Pathogenicity of vibrios to rainbow trout (Oncorhynchus mykiss, Walbaum) and Artemia nauplii[END_REF]. This bacterial species is known to secrete and release toxins including haemolysin, cytolysin and a metalloprotease that are suspected to be pathogenicity factors inducing rapid onset of symptoms such as reduction in larval mobility and tissue necrosis [START_REF] Kothary | Purification and characterization of a vulnificolysin-like cytolysin produced by Vibrio tubiashii[END_REF][START_REF] Elston | Re-emergence of Vibrio tubiashii in bivalve shellfish aquaculture: severity, environmental drivers, geographic extent and management[END_REF].

Time sequence photographs revealed a rapid response by fragments of P. lutea to the putative pathogen after inoculation, compared to other coral diseases. Indeed, coral tissues exposed to P180R displayed focalised and progressive tissue paling 12 h after inoculation and visible lesions of PWPS were observed 12 h thereafter. In contrast, previous experimental studies have reported that healthy Acropora palmata fragments infected by Serratia marcescens exhibited signs of WPD only 4-23 days after inoculation [START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF].

Additionally, visual signs of PUWS in Porites cylindrical were visible only 15 days postinoculation with a Vibrio sp., whereas V. owensii generated signs of WS within 30 days postincubation under laboratory conditions [START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF]. In this study, the infectivity rate of P180R was high (90%) compared to those reported for PUWS generated by a Vibrio sp. phylogenetically related to V. natriegens and V. parahaemolyticus [START_REF] Arboleda | Vibrio sp. causing Porites ulcerative white spot disease[END_REF], and to WPD generated by S. marcescens [START_REF] Patterson | The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata[END_REF], but was relatively consistent with those reported for WS caused by the known pathogens V.

coralliilyticus [START_REF] Sussman | Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific[END_REF]) and V. harveyi [START_REF] Luna | Vibrio harveyi as a causative agent of the White Syndrome in tropical stony corals[END_REF]). Nevertheless, no coral individuals died during the experiments and the infected tissues started to recover 20-29 days post-inoculation under laboratory conditions. This low pathogenicity may be related to i) the immune response in P.lutea which was not impaired under the experimental conditions (T°= 27.8 ± 0.05°C, pH= 8.27 ± 0.02, light = 249.2 µmol photons m -2 s -1 ), and it was therefore able to fight infection; and/or ii) environmental conditions which did not allow full expression of the virulence of P180R [START_REF] Brown | Transcriptional activation of c3 and hsp70 as part of the immune response of Acropora millepora to bacterial challenges[END_REF].

Seawater temperature has been previously identified as an important abiotic factor impairing coral resistance to disease and enhancing bacterial virulence [START_REF] Cervino | Relationship of Vibrio species infection and elevated temperatures to Yellow Blotch/Band Disease in Caribbean corals[END_REF][START_REF] Barash | Bacterial Strain BA-3 and a filterable factor cause a white plague-like disease in corals from the Eilat coral reef[END_REF][START_REF] Bally | Thermodependent bacterial pathogens and mass mortalities in temperate benthic communities: a new case of emerging disease linked to climate change[END_REF][START_REF] Weil | Spatial variability in distribution and prevalence of Caribbean scleractinian coral and octocoral diseases-I Community-level analysis[END_REF]. For example, tissue loss in Pocillopora damicornis (Ben-Haim et al. 2003b) and bleaching in Oculina patagonica [START_REF] Kushmaro | Bleaching of the coral Oculina patagonica by Vibrio AK-1[END_REF][START_REF] Kushmaro | Vibrio shiloi sp. nov., the causative agent of bleaching of the coral Oculina patagonica[END_REF]) resulting from infections by V. corralliilyticus and V. shiloi respectively have been reported to be temperature-dependant. Kushmaro et al. [START_REF] Kushmaro | Vibrio shiloi sp. nov., the causative agent of bleaching of the coral Oculina patagonica[END_REF] showed that V. shiloi caused bleaching at 25-26°C but not at 16°C.

Furthermore, it has been demonstrated that P. damicornis inoculated with V. coralliilyticus generated no signs of infection at 22°C but bleached at temperatures of 24-26°C and developed rapid tissue lysis when exposed to temperatures of 27-29°C (Ben-Haim et al. 2003b). More recently, a study investigating the potential relationship between Vibrio infection and temperature in Yellow blotch disease (YBD) also identified temperature as an important driver of infection [START_REF] Cervino | Relationship of Vibrio species infection and elevated temperatures to Yellow Blotch/Band Disease in Caribbean corals[END_REF]. In this same study, coral colonies inoculated with a consortium of Vibrio species and exposed to 31°C resulted in 80% mortality, whereas those infected at 20°C still exhibited signs of YBD but without mortality [START_REF] Cervino | Relationship of Vibrio species infection and elevated temperatures to Yellow Blotch/Band Disease in Caribbean corals[END_REF]).

The potential effects of temperature on the virulence of PWPS were also tested in this study.

The results yielded text-book model temperature dependencies with the virulence of P180R increasing progressively with increasing incubation temperatures. Additionally, all fragments of P. lutea that were exposed to 26-28 °C displayed signs of PWPS 24 h post-inoculation but the progression of infections diminished concomitantly 96 h after artificial infection. In contrast, PWPS infection of corals incubated at 30°C persisted for up to 170 h after commencement of the infection assays and resulted in 40% mortality several weeks after the end of the experiments. These responses to increased temperatures may be explained in different ways. First of all, high temperatures may compromise defence mechanisms in corals by impairing their immune system and thereby their ability to control pathogens [START_REF] Harvell | Coral disease, environmental drivers, and the balance between coral and microbial associates[END_REF][START_REF] Palmer | Evidence of an inflammatory-like response in non-normally pigmented tissues of two scleractinian corals[END_REF][START_REF] Brown | Transcriptional activation of c3 and hsp70 as part of the immune response of Acropora millepora to bacterial challenges[END_REF]. Healthy bacterial communities associated with corals are also known to provide important benefits [START_REF] Rohwer | Diversity and distribution of coral-associated bacteria[END_REF][START_REF] Lesser | Discovery of symbiotic nitrogen-fixing cyanobacteria in corals[END_REF][START_REF] Thurber | Metagenomic analysis of stressed coral holobionts[END_REF][START_REF] Littman | Responses of coral-associated bacterial communities to heat stress differ with Symbiodinium type on the same coral host[END_REF][START_REF] Rypien | Antagonistic interactions among coral-associated bacteria[END_REF], notably antimicrobial compounds that protect the coral host against pathogens (Shnit-Orland and Kushmaro 2009).

In this regard, Alker et al. [START_REF] Alker | Characterization of Aspergillus sydowii (Thom et Church), a fungal pathogen of Caribbean sea fan corals[END_REF]) have demonstrated that anomalously high temperatures reduced the efficiency of antifungal compounds produced by the sea fan Gorgonian ventalina. Thermal stress may also result in shifts in the natural bacterial communities associated with P. lutea, allowing the settlement and proliferation of opportunistic putative pathogens [START_REF] Cervino | Relationship of Vibrio species infection and elevated temperatures to Yellow Blotch/Band Disease in Caribbean corals[END_REF]). This might be investigated through deep sequencing, comparing complete bacterial communities between diseased and healthy corals over time when exposed to increasing temperatures. Finally, the virulence of pathogens may be enhanced at elevated temperatures, causing them to attain a toxic state and accelerating their growth rate [START_REF] Lipp | Preliminary evidence for human fecal contamination in corals of the Florida Keys, USA[END_REF]. For instance, Elston et al. [START_REF] Elston | Re-emergence of Vibrio tubiashii in bivalve shellfish aquaculture: severity, environmental drivers, geographic extent and management[END_REF] showed that virulence of V. tubiashii in shellfish hatcheries was strongly correlated with increasing sea surface temperature (SST).

Conclusions

This study has shown the relevance of combining culture techniques and laboratory-based infection trials to identify putative coral pathogens. The role of P180R as a primary pathogen of PWPS has been demonstrated by satisfying Henle-Koch's postulates. Indeed, the strain used here induced the development of disease resembling that of PWPS during inoculation 1 and it was successfully re-isolated from inoculated corals that displayed the disease. This reisolated strain was closely related to P180R (100% identity), again produced signs of disease in healthy P. lutea during phase 2 of the inoculation trials. Therefore, our results add another marine pathogen belonging to the Vibrionacea. Additionally, PWPS has been identified as a thermo-dependent coral disease as its severity increased at elevated seawater temperature.

Nevertheless, additional proofs, using complementary approaches such as Transmission Electron Microscopy (TEM) or fluorescence in situ hybridization (FISH), are needed to link P180R with the disease signs at the gross and cellular level. Finally, further research should be conducted to establish whether thermal stress i) affects the coral immunity response, ii) plays a primary role in the virulence mechanisms of V. tubiashii (toxin production) and compromises coral health, and/or iii) affects the coral-associated communities. al. 2006;[START_REF] Myers | Molecular detection and ecological significance of the cyanobacterial genera Geitlerinema and Leptolyngbya in black band disease of corals[END_REF]). Among these groups, a few bacteria have been suspected to be primary pathogens, including Desulfovibrio spp [START_REF] Viehman | Culture and identification of Desulfovibrio spp. from corals infected by black band disease on Dominican and Florida Keys reefs[END_REF]) and Vibrio coralliilyticus [START_REF] Arotsker | Vibrio sp. as a potentially important member of the Black Band Disease (BBD) consortium in Favia sp. corals[END_REF]); however, none of these species were tested experimentally and/or satisfied Henle Koch's postulates. In addition, variations have been detected in bacterial communities associated with BBD across geographic regions and between sympatric coral species [START_REF] Voss | Black band disease microbial community variation on corals in three regions of the wider Caribbean[END_REF]. For instance, the presence of 16S rDNA sequences similar to Trichodesmium and Oscillatoria were reported in BBD-infected samples from Papua New-Guinea [START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF], whereas members of the genera Geitlerinema, Leptolyngbya, Lyngbya, Oscillatoria and Phormidium were detected in BBD from the Caribbean and Philippines [START_REF] Sekar | Microbial Communities in the Surface Mucopolysaccharide Layer and the Black Band Microbial Mat of Black Band-Diseased Siderastrea siderea[END_REF][START_REF] Myers | Molecular detection and ecological significance of the cyanobacterial genera Geitlerinema and Leptolyngbya in black band disease of corals[END_REF]). More recently, a transitional stage of BBD has been identified with different microbial communities and named "cyanobacterial patch (CP)". Molecular analysis revealed shifts in cynobacterial populations from sequences similar to Bennothrix sp. in CP to those related to Oscillatoria sp.

in BBD [START_REF] Sato | Dynamics of seasonal outbreaks of black band disease in an assemblage of Montipora species at Pelorus Island (Great Barrier Reef, Australia)[END_REF][START_REF] Sato | Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida[END_REF]). The high variability in BBD bacterial communities found between localities and infected host species may indicate that BBD actually derives from an earlier infectious stage which, after impairing the host's resistance, favours the infection and subsequent proliferation of opportunistic microorganisms such as cyanobacteria. However, beyond this highly speculative assumption and despite being intensively studied worldwide, the mechanisms of BBD development have remained unclear and no primary pathogens have been clearly identified.

Recent surveys, conducted on western Indian Ocean (WIO) coral reefs over two consecutive summers and winters between 2010 and 2012 (Séré et al. submitted), revealed an atypical form of black syndrome on two of the main framework building corals, Porites lobata and P. lutea, and termed here "Porites black patch syndrome" (PBPS). Following standardized terminology [START_REF] Work | Systematically describing gross lesions in corals[END_REF], PBPS is characterized by a diffuse, centrally to peripherally situated, medium to large black patch (1-10 cm in width), leaving behind it the dead skeleton generally covered by a whitish filamentous film (Fig. 20). The older exposed skeleton is progressively colonized by endophytic algae. Our study aimed at providing a comprehensive characterization of PBPS using a multidisciplinary approach involving field surveys, gross lesion monitoring, histopathology and molecular techniques. Here we have used 16S barcoding by 454-pyrosequencing to investigate bacterial community compositions in the i) diseased tissue, ii) apparently healthy tissue adjacent to the diseased region, and iii) healthy tissue taken from a separate healthy Porites colony.

Material and methods

Field surveys and progression rate

Surveys were conducted in Reunion over two consecutive summers and winters, within four latitudinal sites and within two cross-shelf locations (Reef slope and Reef flat). Details of the sites are given in Table 13. structure. DT was sampled from the lesion boundary interface with visually healthy tissue (VHT) and healthy tissue (HT) was cored from an uninfected colony as a control. Fresh PBPS mat was collected from four infected Porites lutea colonies using needless and sterile syringe to identify the dominant cyanobacterial strains. Samples were placed in 10 ml centrifuge tubes with seawater and held in darkness at 20°C until return to the laboratory. Finally, two samples of DT, two samples of visually healthy tissue (VHT) and one sample of HT were collected using 10 mm core tubes to characterise the bacterial communities associated with PBPS.

Samples were placed in 15 ml polypropylene centrifuge tubes and kept under low light conditions at 2°C in a cool box. Immediately upon the return to the laboratory, the seawater within each tube was decanted and the coral samples were immersed in absolute ethanol and stored at -80°C for molecular analysis.

Histopathology

Samples were protected in 1.5% (w/v) agarose to retain the spatial integrity of the tissues and microbial communities. They were then decalcified using 1% HCl and EDTA renewed every 12 h until the process was complete. Decalcified tissues were then dehydrated in a gradient of ethanol baths, cleared with xylene and embedded in paraffin wax. Cross sections of 6-8 μm thick were cut using a microtome, mounted on glass slides and stained with Harris haematoxylin and eosin (with phloxine B). These stains are used to diagnose tissue fragmentation, necrosis and identify invasive organisms [START_REF] Work | Pathology of tissue loss (white syndrome) in Acropora sp. corals from the Central Pacific[END_REF][START_REF] Sudek | Histological observations in the Hawaiian reef coral, Porites compressa, affected by Porites bleaching with tissue loss[END_REF]. Serial sections were examined under a light microscope and photographed using NIS Element software (Nikon©).

Cyanobacterial culturing, isolation, and identification

Cyanobacterial filaments (one when possible) were isolated from the raw samples under a light microscope and transferred to agar plate containing Z8 medium [START_REF] Kotai | Instructions for preparation of modified nutrient solution Z8 for algae[END_REF] enriched with NaHCO3, (NH 4 ) 2 SO 4 and Vitamin B12. Inoculated plates were incubated at 27°C with a 12h light:dark photoperiod and constant irradiance of 20 µmol photons m -2 s -1 . Bacterial strains were routinely passaged to purification until their use in molecular analysis. Genomic

DNA was then extracted from each cyanobacterial isolate by boil lysing in 100 μL of 5mM

Tris/HCl at 100°C for 5 minutes. PCR amplification was carried out in a volume of 25 µl GoTaq®Hot Start Green Master Mix (Promega, Madison, WI), 0.5mM of 16S rRNA gene cyanobacterial primers CYA781R/CYA106F [START_REF] Nübel | PCR primers to amplify 16S rRNA genes from cyanobacteria[END_REF][START_REF] Sussman | A single cyanobacterial ribotype is associated with both red and black bands on diseased corals from Palau[END_REF][START_REF] Rasoulouniriana | Pseudoscillatoria coralii gen. nov., sp, nov., a cyanobacterium associated with coral black band disease (BBD)[END_REF], and 10 ng of template DNA. Amplification conditions for the PCR included an initial denaturing step of 5 min at 94˚C, followed by 35 cycles at 94˚C for 60 sec, 60˚C for 60 sec, 72˚C for 60 sec, and a final extension step of 5 min at 72˚C. Sequences obtained for each cyanobacterial strain were examined for error and edited using GENEIOUS TM Pro (V.5.6.6) sequencing software [START_REF] Kearse | Geneious Basic: an integrated and extendable desktop software platform for the organization and analysis of sequence data[END_REF]. All consensus sequences were submitted to BLAST at the National Centre for Biotechnology Information (NCBI, www.ncbi.nlm.nih.gov) and compared with published sequences. The 16S rDNA sequences were aligned with reference rRNA gene sequences closely related to known cyanobacterial strains. A phylogenetic tree was built by the neighbour-joining method of GENEIOUS TM Pro (V. 5.6.6). Bootstrap values were based on 1000 replicates.

Metagenomic profile of bacterial 16S rRNA Genes

DNA extraction

Bacterial genomic DNA was extracted from HT, VHT and DT using the NucleoSpin® Soil Kit (NucleoSpin Extract II, Macherey-Nagel, Düren, Germany). Approximately 150 mg of both tissue and skeleton were scratched from the core surface using a sterile scalpel blade, placed in a 1.5 ml centrifuge tube with 700 µl of lysis buffer and crushed using a fresh disposable plastic rod. Samples were then placed in lysing matrix tubes for DNA extraction.

The DNA was eluted with 50 µl sterile elution buffer, verified by electrophoresis in agarose gels (1.5% wt/vol) stained with GelRed TM (Biotium Inc., Hayward, California, USA) and finally stored at -20˚C.

PCR and 454 pyrosequencing

Bacterial 16S metabarcode sequencing of the bacterial communities associated with HT, VHT and DT samples was performed using 454-pyrosequencing technology (Roche, Nutley, NJ, USA) at GENOSCREEN (Campus de l'Institut Pasteur de Lille, France). In our design, 16S

rRNA variant regions V3 and V4 were amplified using forward (TACGGRAGGCAGCAG)

and reverse (GGACTACCAGGGTATCTAAT) primers. These primers were linked to 5' with MID tags, a GsFLX key and GsFLX adapters. Each sample was amplified independently twice with distinct MID tags, allowing the identification of each gene pool. Quality control was performed using the Agilent DNA 100 (Agilent Technologies). The quantity of each PCR product was measured with Picogreen and all products were mixed together in equimolar concentrations prior to 454 GsFLX sequencing.

Sequences analyses

All 454 GSFLX sequences (reads) were sorted by MID identification using GENEIOUS TM Pro (V. 5.6.3). Raw sequences were first trimmed using the RDP Pyrosequencing Pipeline (available online at http://pyro.cme.msu.edu/index.jsp) using the following filter criteria: forward primer max edit distance, 2; reverse primer max edit distance, 1; max number of "Ns", 1 and min sequence length, 200. Trimmed reads were then compared against the NCBI reference 16S database (www.ncbi.nlm.nih.gov) using nucleotide BLAST, returning 50 hits coefficient to compare the bacterial community structures of the different tissue categories and the Simpson's and Shannon's diversity indices were calculated. Finally comparisons of average values of bacterial communities associated with different tissue categories were performed using the t-test in STATISTICA.

Results

PBPS Prevalence and virulence

A total of 3520 m 2 of reef and 5363 massive colonies of Porites lutea were surveyed between September 2010 and January 2012 (Table 13). PBPS varied seasonally and between the two geomorphologic reef zones. For instance, colonies of Porites exhibited significantly higher PBPS prevalence on the reef flat (ANOVA: F = 1.18, p < 0.01), affecting an average of 4.1 ± 2.0 % (mean ± SE) colonies compared to those observed at the deeper sites (0.2 ± 0.4; mean ± SE). The percentage of infected colonies recorded on both the reef flat and reef slope was significantly higher during summers than winters (ANOVA winter 1 vs. summer 1 : F = 3.89, p < 0.01; ANOVA winter 2 vs. summer 2 : F = 0.6, p < 0.05). The rate of tissue mortality measured on five massive colonies of P. lutea between 2011 and 2012 was 4.4 ± 0.12 (mean ± SE) mm day -1 . Among the monitored colonies, two died approximately 13 months after the beginning of the study.

Microscopic characterisation

Comparison of cross sections of PBPS-infected P. lutea colonies (Fig. 21A) revealed the presence of three distinct tissue regions; the first one being the oldest area of infection comprising dead and degraded tissue associated with cell debris, endophytic algae and other organisms such as cyanobacteria and ciliates (Fig. 21C). The second region was characterised by a dense mat of microorganisms visually dominated by filamentous cyanobacteria (Fig. 21C) which were mechanically perforating the compromised and dead tissue (Fig. 21E andF).

Finally, the last portion of the lesion constituted viable (living) tissue with granular pigment cells in both the epidermis and gastrodermis (Fig. 21B). Basophilic bodies like bacterial aggregates were also observed in this tissue region and were regularly surrounded by the same granular cells.

Identification of the dominant cyanobacterial strain

Cyanobacterial strains isolated from four individual PBPS-infected Porites lutea were characterised by dense clumps of brown filaments that were able to colonize the entire petri dish surface (Z8) in a single week (Fig. 22). The four strains were motile and appeared morphologically similar with pointed, arrow-like calyptra. The isolates were genetically close to each other (>98% identity) and phylogenetically affiliated to the cyanobacterium Pseudoscillatoria coralii (FJ210722) and Roseofilum reptotaenium (HM048872).

Bacterial community structure

Following Roche 454-pyrosequencing, a total of 52 257, 38 778 and 21 309 sequence reads were obtained from samples of PBPS (DT1A-DT1B and DT2A-DT2B), apparently healthy tissue (HT1A-HT1B and HT2A-HT2B) and healthy tissue (HTA and HTB) respectively (Table 14). In the same order, 23 763 (45.47%), 30 602 (78.91%) and 13 806 (64.78%) sequences reads remained after trimmed analysis, with a minimum length of 250 bp. The community structure in each subsample of the different tissue categories was compared with both Cluster and PCoA analysis in MEGAN (V5.0.77) based on Bray-Curtis similarity matrices (Fig. 23). Results revealed three clearly separated groups, including a Cluster 1 grouping all diseased samples, a Cluster 2 grouping only the HT1A and HTA subsamples and a Cluster 3 with the remaining healthy and visually healthy subsamples (Fig. 23A andB).

Rarefaction curve analysis (Fig. 23 C) assessessing species richness showed almost all the samples nearly paralleled the x axis, indicating that the overall bacterial diversity was well represented. Only VHT2A and B did not level off, even with a respective sequence reads number of 10 687 and 8176 (Table 14). Bacterial diversity estimated with Simpson's and Shannon's diversity indices showed that DT had higher bacterial diversity than both VHT and HT (Table 14).

Bacterial community composition in DT

Bacterial communities associated with PBPS tissues were represented by eight classes, including Alphaproteobacteria, Clostridia, Cytophagia, Cyanobacteria, Deltaproteobacteria, Epsilonproteobacteria, Gammaproteobacteria and Spirochaeta (Fig. 24). In DT1, the most representative class was the Alphaproteobacteria (35.9%) followed by the Gammaproteobacteria (24.4%), Deltaproteobacteria (18.9%), Cytophagia (11.5%), Firmicute (5.1%), Epsilonproteobacteria (3.0%), Spirochaeta (1.0%) and Cyanobacteria

Comparative analysis of bacterial communities associated with DT, VHT and HT

Sequence reads from all samples and subsamples of each tissue category were combined (DT, VHT and HT) and compared statistically. Before analysis, all read counts were normalized to avoid bias due to datasets with different size [START_REF] Mitra | Visual and statistical comparison of metagenomes[END_REF].The average number of taxonomic reads affiliated to bacterial genera in DT was significantly different from those in VHT (t-test: df=2 p<0.0001) and HT (t-test: df= 2 p= 0.0001). However no significant difference was obtained between VHT and HT (t-test: df= 2 p=0.001). Among the genera found in our study, only Vibrio and Pseudomonas were common between DT, VDT and HT (Figs 25 and 26).

Discussion

This study constitutes the first characterisation of Porites black patch disease (PBPS), an atypical form of BBD found on Reunion coral reefs. Surveys conducted on two geomorphological reef zones revealed spatial variability, with more infected colonies on the reef flat (0.5-1m) than the reef slope (10-20 m). Similar patterns were previously reported in Florida Keys [START_REF] Kuta | Ecological aspects of black band disease of corals: relationships between disease incidence and environmental factors[END_REF], the Republic of Maldives [START_REF] Montano | First report of coral diseases in the Republic of Maldives[END_REF] and southern India [START_REF] Thinesh | Coral disease prevalence in Mandapam group of islands, Gulf of Mannar, Southeastern India[END_REF][START_REF] Thinesh | Coral disease prevalence in the Palk Bay, Southeastern India-With special emphasis to black band[END_REF] where BBD is more abundant at shallow than deep sites. This may be due to the proximity of the Reunion reef to the coastline (± 500 m wide), where it is constantly exposed to high and increasing anthropogenic stress from sewage discharges, land-based pollution and eutrophication with compounds such as nitrates, ammonium, and phosphate [START_REF] Naim | Seasonal responses of a fringing-reef community to eutrophication (Reunion-Island, Western Indian-Ocean)[END_REF][START_REF] Chazottes | The effects of eutrophication-related alterations to coral reef communities on agents and rates of bioerosion (Reunion Island, Indian Ocean)[END_REF].Several studies suggested that nutrient enrichment, sewage discharge and runoff may facilitate and increase disease outbreaks by enhancing the pathogen virulence and/or impairing host resistance [START_REF] Voss | Nutrient enrichment enhances black band disease progression in corals[END_REF][START_REF] Rodriguez | Dynamics of Black Band Disease in a Diploria strigosa population subjected to annual upwelling on the northeastern coast of Venezuela[END_REF][START_REF] Haapkylä | Seasonal rainfall and runoff promote coral disease on an inshore reef[END_REF]).

Seasonal variations were also observed between 2010 and 2012: the average prevalence of PBPS recorded on the reef flat was at its highest in summer 2010 (8.7 ± 2.8%; mean ± SE), declined to low levels in winter 2011 (1.4 ± 1.0%; mean ± SE) and then increased the following summer (7.7 ± 2.5%; mean ± SE). Our results are consistent with those previously found during BBD surveys on the Great Barrier Reef in Australia [START_REF] Boyett | Elevated temperature and light enhance progression and spread of black band disease on staghorn corals of the Great Barrier Reef[END_REF]Sato et al. 2009) and in Venezuela [START_REF] Rodriguez | Dynamics of Black Band Disease in a Diploria strigosa population subjected to annual upwelling on the northeastern coast of Venezuela[END_REF], the Red Sea [START_REF] Zvuloni | Spatio-temporal transmission patterns of black-band disease in a coral community[END_REF] and on Caribbean reefs [START_REF] Edmunds | Extent and effect of black band disease on a Caribbean reef[END_REF]. Seasonal fluctuations in BBD prevalence have been generally assumed to be driven by high summer sea temperatures, which may reduce host resistance or/and increased pathogen virulence [START_REF] Rützler | The black band disease of Atlantic reef corals. 111. Distribution, ecology, and development[END_REF][START_REF] Edmunds | Extent and effect of black band disease on a Caribbean reef[END_REF][START_REF] Kuta | Ecological aspects of black band disease of corals: relationships between disease incidence and environmental factors[END_REF][START_REF] Rodriguez | Dynamics of Black Band Disease in a Diploria strigosa population subjected to annual upwelling on the northeastern coast of Venezuela[END_REF][START_REF] Sato | Dynamics of seasonal outbreaks of black band disease in an assemblage of Montipora species at Pelorus Island (Great Barrier Reef, Australia)[END_REF][START_REF] Sato | Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida[END_REF]). However, the hot season (December to March) in Reunion is also associated with heavy rainfall, leading to high levels of ground water infiltration and surface water runoff, and an increased level of pollutants such as pesticide, fertiliser, sewage from septic systems and waste water in lagoon waters [START_REF] Naim | Seasonal responses of a fringing-reef community to eutrophication (Reunion-Island, Western Indian-Ocean)[END_REF][START_REF] Chazottes | The effects of eutrophication-related alterations to coral reef communities on agents and rates of bioerosion (Reunion Island, Indian Ocean)[END_REF].

Field monitoring performed on tagged colonies confirmed the virulence of PBPS with a mean tissue mortality rate reaching 4.4 ± 0.12 mm day -1 (mean ± SE). This value was higher than rates of BBD progression previously reported on other scleractininan corals in the Florida Keys [START_REF] Kuta | Black band disease and the fate of diseased coral colonies in the Florida Keys[END_REF], in Australia [START_REF] Boyett | Elevated temperature and light enhance progression and spread of black band disease on staghorn corals of the Great Barrier Reef[END_REF][START_REF] Sato | Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida[END_REF], Indonesia [START_REF] Haapkylä | Spatiotemporal coral disease dynamics in the Wakatobi Marine National Park, South-East Sulawesi, Indonesia[END_REF]) and India [START_REF] Borger | The spatial and temporal dynamics of coral diseases in Dominica, West Indies[END_REF][START_REF] Thinesh | Coral disease prevalence in Mandapam group of islands, Gulf of Mannar, Southeastern India[END_REF]). However, regional comparisons may be confounded by differing innate immune responses between coral species [START_REF] Mydlarz | Cellular responses in sea fan corals: granular amoebocytes react to pathogen and climate stressors[END_REF][START_REF] Mydlarz | What are the physiological and immunological responses of coral to climate warming and disease?[END_REF]). Nevertheless, our results

still pinpoint the high destructive potential of this PBPS on the Reunion reefs, especially considering that 40% of the tagged colonies died after one year of monitoring.

Cross sections performed on PBPS-infected tissues revealed a dense microbial mat visually dominated by filamentous cyanobacteria adjacent to the living tissue. Similar cyanobacterial aggregates have been previously observed in several studies on typical BBD [START_REF] Bythell | Histopathological methods for the investigation of microbial communities associated with disease lesions in reef corals[END_REF][START_REF] Ainsworth | Coral Disease Diagnostics: What's between a plague and a band?[END_REF][START_REF] Barneah | Characterization of black band disease in Red Sea stony corals[END_REF][START_REF] Sato | Dynamics of seasonal outbreaks of black band disease in an assemblage of Montipora species at Pelorus Island (Great Barrier Reef, Australia)[END_REF]). 16S rRNA gene sequences obtained from fresh and purified cyanobacteria strains isolated from the same infected colonies were closely related (99.8% identity) to two newly reported BBD cyanobacterial species. These were Pseudoscillatoria coralii [START_REF] Rasoulouniriana | Pseudoscillatoria coralii gen. nov., sp, nov., a cyanobacterium associated with coral black band disease (BBD)[END_REF]) and Roseofilum reptotaenium [START_REF] Casamatta | Characterization of Roseofilum reptotaenium (Oscillatoriales, Cyanobacteria) gen. et sp. nov. isolated from Caribbean black band disease[END_REF], isolated on different coral hosts from the northern Red Sea and the Caribbean. The sequences were also 99.8% homologous to the BBD cyanobacterium found on infected colonies in the central Great Barrier Reef [START_REF] Sato | Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida[END_REF].

Morphologically, the isolated cyanobacterial strain possessed pointed terminal cells, or "calyptra", that may play a role in the tissue invasion observed in the histological crosssections, and are probably involved in the secretion of toxins or other compounds [START_REF] Whitton | Cyanobacterial diversity in relation to the environment Algal Toxins: Nature, Occurrence, Effect and Detection[END_REF]. Previous studies have indeed suggested that cyanobacteria may facilitate their penetration by secreting cyanotoxins [START_REF] Richardson | Sulfide, microcystin, and the etiology of black band disease[END_REF][START_REF] Stanić | Microcystin production and ecological physiology of Caribbean black band disease cyanobacteria[END_REF]. The ability of cyanobacteria to penetrate coral tissue has been demonstrated in other studies of BBD [START_REF] Barneah | Characterization of black band disease in Red Sea stony corals[END_REF][START_REF] Sato | Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida[END_REF]), highlighting their destructive potential in corals.

Although these cyanobacteria clearly play a role in PBPS development, it remains unclear whether or not healthy corals would be infected by such bacteria in the absence of other aggravating factors including the potential role of other infectious bacteria [START_REF] Sato | Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida[END_REF].

The precise mechanisms of coral invasion by cyanobacteria are thus unknown and require further investigation.

Granular and pigmented cells, resembling melanin deposits described by [START_REF] Palmer | Evidence of an inflammatory-like response in non-normally pigmented tissues of two scleractinian corals[END_REF] in Porites sp., were found in high densities in PBPS-infected tissue of P. lutea. Such inflammations, found mainly in DT, are likely to be the result of an immune response, suggesting that healthy corals are capable of mounting a defence against invading organisms [START_REF] Palmer | Evidence of an inflammatory-like response in non-normally pigmented tissues of two scleractinian corals[END_REF][START_REF] Mydlarz | What are the physiological and immunological responses of coral to climate warming and disease?[END_REF]). However, basophilic bodies, resembling bacterial aggregates similar to those observed in PWPS [START_REF] Séré | Bacterial communities associated with Porites white patch syndrome (PWPS) on three Western Indian Ocean (WIO) coral reefs[END_REF], were found in the living tissue adjacent to the bacterial mat but were not observed in HT. Thus, it is possible that other bacteria may promote PBPS by initiating a primary infection that impairs the immune processes in corals. However, no evidence of direct physical destruction resulting from these basophilic bodies could be detected in the histological sections.

Bacterial community analysis via V3-V4, 16S metabarcoding suggested that the number of bacterial taxa identified in this study was higher than in other metagenomic analyses of bacterial communities associated with scleractinian corals (Wegley et al. 2007a;[START_REF] Littman | Metagenomic analysis of the coral holobiont during a natural bleaching event on the Great Barrier Reef[END_REF]. A higher diversity of bacterial taxa was found in PBPS-infected tissues than in healthy tissues. While VHT and HT mostly contained Gammaproteobacteria, DT samples yielded bacterial sequences affiliated to Alphaproteobacteria, Clostridia, Cytophagia, Cyanobacteria, Deltaproteobacteria, Epsilonproteobacteria, Gammaproteobacteria and Spirochaeta. This result is consistent with patterns observed in other studies that have characterised microbial communities from other black syndromes [START_REF] Cooney | Characterization of the bacterial consortium associated with black band disease in coral using molecular microbiological techniques[END_REF][START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF][START_REF] Sekar | Microbial Communities in the Surface Mucopolysaccharide Layer and the Black Band Microbial Mat of Black Band-Diseased Siderastrea siderea[END_REF]). However, the taxonomic complement of bacteria in both the HT and VHT was low compared to previous work that has focused on the bacterial community in the same

Porites species using traditional methods [START_REF] Mckew | Characterization of Geographically Distinct Bacterial Communities Associated with Coral Mucus Produced by Acropora spp. and Porites spp[END_REF][START_REF] Li | Highly Heterogeneous Bacterial Communities Associated with the South China Sea Reef Corals Porites lutea, Galaxea fascicularis and Acropora millepora[END_REF].

Although variations were detected between DT samples, the major bacterial classes found in this study were also observed in several previous works on typical BBD from distant locations and in different coral species [START_REF] Cooney | Characterization of the bacterial consortium associated with black band disease in coral using molecular microbiological techniques[END_REF][START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF][START_REF] Sekar | Microbial Communities in the Surface Mucopolysaccharide Layer and the Black Band Microbial Mat of Black Band-Diseased Siderastrea siderea[END_REF][START_REF] Barneah | Characterization of black band disease in Red Sea stony corals[END_REF][START_REF] Sekar | Microbial community composition of black band disease on the coral host Siderastrea siderea from three regions of the wider Caribbean[END_REF][START_REF] Sato | Successional changes in bacterial communities during the development of black band disease on the reef coral, Montipora hispida[END_REF][START_REF] Miller | A meta-analysis of 16S rRNA gene clone libraries from the polymicrobial black band disease of corals[END_REF]). Among them, Vibrio (Gammaproteobacteria) was the dominant genus in PBPS-infected tissues representing 24.9% of the overall sequence reads. Several members of this genus have been identified as pathogens of corals, their virulence being attributed to enzyme secretions that initiate tissue penetration and degradation [START_REF] Ben-Haim | A novel Vibrio sp. pathogen of the coral Pocillopora damicornis[END_REF]Ben-Haim et al. 2003b;[START_REF] Rosenberg | The Vibrio shiloi/Oculina patagonica model system of coral bleaching[END_REF]. In the present study, phylogenetic analyses revealed the presence of sequences affiliated to Vibrio species that are known to be highly proteolytic (Appendix 2). These were found only in infected tissues and included the human pathogen V.

cholerae [START_REF] Halpern | Dependent population dynamics between chironomids (nonbiting midges) and Vibrio cholerae[END_REF], the coral pathogen V. coralliilyticus [START_REF] Ben-Haim | A novel Vibrio sp. pathogen of the coral Pocillopora damicornis[END_REF]Ben-Haim et al. 2003b) and two other sequences groups related to V.

proteoliticus and V. rumoiensis. The latter have previously been detected in BBD on Netherlands Antilles reefs [START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF]. Our results suggest that Vibrio spp. may play an important role in the pathogenicity of PBPS by creating an "entry point" that may permit the settlement and the progression of the black patch microbial community. However, since vibrionic sequences were also abundant in non-infected tissues, their role in PBPS in

Porites lutea needs to be individually assessed using multidisciplinary approaches combining bacterial culturing and inoculation/infection trials (Henle-Koch's postulates).

The second most represented bacteria in PBPS samples was Desulfovibrio, accounting for up to 20% of the overall 16S rRNA sequence genes. It has previously been found in BBDinfected corals from different locations and seems to be an important component in BBD aetiology [START_REF] Viehman | Culture and identification of Desulfovibrio spp. from corals infected by black band disease on Dominican and Florida Keys reefs[END_REF]). However, none of sequences were related to known pathogens but the majority were closely related to D. aespoeensis, a sulfate-reducing bacterium from natural deep groundwater [START_REF] Motamedi | Note Desulfovibrio aespoeensis sp. nov., a mesophilic sulfate-reducing bacterium from deep groundwater at äspö hard rock laboratory, Sweden[END_REF]. Additionally, we did not find any sequences related to sulfide-reducing bacteria in either VHT or HT, suggesting that they were opportunistic microorganisms that occupy an anoxic microenvironment [START_REF] Cooney | Characterization of the bacterial consortium associated with black band disease in coral using molecular microbiological techniques[END_REF][START_REF] Viehman | Culture and identification of Desulfovibrio spp. from corals infected by black band disease on Dominican and Florida Keys reefs[END_REF][START_REF] Sekar | Microbial community composition of black band disease on the coral host Siderastrea siderea from three regions of the wider Caribbean[END_REF]) rich in carbon compounds derived from cell debris and other organic nutrients produced during the lysis of coral tissue [START_REF] Viehman | Culture and identification of Desulfovibrio spp. from corals infected by black band disease on Dominican and Florida Keys reefs[END_REF]. Other predominant genera found only in DT were Clostridium, followed by Arcobacter, which represented 12.9% and 9.9% of the total bacterial sequences respectively. Although their roles in BBD aetiology were not clearly identified, these genera have also been consistently found in BBD on several scleractinian corals at distant locations [START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF][START_REF] Sekar | Microbial Communities in the Surface Mucopolysaccharide Layer and the Black Band Microbial Mat of Black Band-Diseased Siderastrea siderea[END_REF][START_REF] Sato | Pyrosequencing-based profiling of archaeal and bacterial 16S rRNA genes identifies a novel archaeon associated with black band disease in corals[END_REF]. Arcobacter is regularly found in sulfide rich environments similar to those found in BBD lesions [START_REF] Glas | Biogeochemical conditions determine virulence of black band disease in corals[END_REF]. In this study, the majority of sequences affiliated to it were closely related to A.

sulfidicus, an obligate marine microaerophile that oxidizes sulfides [START_REF] Wirsen | Characterization of an autotrophic sulfide-oxidizing marine Arcobacter sp. that produces filamentous sulfur[END_REF]. Other sequences affiliated to Flexibacter, Cytophaga, Oceanospirillum, and Spirochaeta were also present in PBPS samples. As with Desulfovibrio, Clostridium and Arcobacter, these genera have been detected in the BBD microbial mat at different locations [START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF][START_REF] Sato | Pyrosequencing-based profiling of archaeal and bacterial 16S rRNA genes identifies a novel archaeon associated with black band disease in corals[END_REF]) and seem to be involved in the production of sulfated compounds suspected to exacerbate BBD virulence [START_REF] Sato | Dynamics of seasonal outbreaks of black band disease in an assemblage of Montipora species at Pelorus Island (Great Barrier Reef, Australia)[END_REF]). However, their implications in PBPS aetiology warrant further investigations since none of them were phylogenetically closely related to known pathogens.

In summary, PBPS was found on two of the main framework building corals, Porites lobata and P. lutea. It manifested spatial and seasonal variability with more infected colonies observed on the reef flat and during the summer season. Histopathology performed on this patch-like form of BBD revealed a dense microbial mat visually dominated by a filamentous cyanobacterium genetically related to both Pseudoscillatoria coralii and Roseofilum reptotaenium which seemed to mechanically penetrate the coral tissue. Additionally, basophilic bodies were found only in the living tissue adjacent to the bacterial mat suggesting a potential implication of bacteria that initiate a primary infection. However, no direct evidence linking these basophilic bodies to the lesions could be highlighted using histopathology. This study also characterized the structural patterns of bacterial assemblage associated with PBPS using 16S barcoding by 454-pyrosequencing. We demonstrated that diseased tissue yielded a broader diversity of bacterial taxa than healthy tissue, dominated by the genus Vibrio followed by sulfate-reducers and sulfide-oxidizer found only in PBPSinfected tissue and which seem ubiquitous to BBD worldwide.

The Western Indian Ocean coral reefs (WIO) are also affected by coral diseases

This study represents the first quantitative and qualitative coral diseases assessment on the WIO coral reefs investigating the prevalence and variability of coral diseases at temporal and spatial scales (Chapter 2). While it was assumed that coral diseases were not as serious as they

WIO coral reefs, reservoirs of unreported coral pathology.

This is the first study to describe and characterise at ecological, histological, molecular and microbiological levels of two previously unreported coral diseases on the WIO reefs. The first one, Porites white patch syndrome (PWPS) was recorded on Reunion, South African and Mayotte reefs with prevalence (2010-2012) estimated around 2.3 ± 0.6%, 0.2 ± 0.3% and 1.0 ± 1.4% (mean ± SE) respectively (Chapter 2). PWPS affects massive Porites lobata and P.

lutea by generating diffuse, medium to large (5.0-30.0 cm diameter), circular to oblong tissue loss, surrounded by a 1.0-20.0 cm width zone of swollen, paler tissues [START_REF] Séré | Porites white patch syndrome: an unreported coral disease on Western Indian Ocean reefs[END_REF]. season. Histopathology performed on this patch-like form of BBD revealed a dense microbial mat visually dominated by a filamentous cyanobacterium genetically related to both Pseudoscillatoria coralii and Roseofilum reptotaenium which seemed to mechanically penetrate the coral tissue. Additionally, basophilic bodies resembling those observed in PWPS (chapter 3) were found only in the living tissue adjacent to the bacterial mat suggesting a potential implication of bacteria that initiate a primary infection. However, no direct evidence linking these basophilic bodies to the lesions could be highlighted using histopathology. This study also characterized the structural patterns of bacterial assemblage associated with PBPS using 16S barcoding by 454-pyrosequencing. We demonstrated that diseased tissue yielded a broader diversity of bacterial taxa than healthy tissue, dominated by the genus Vibrio followed by sulfate-reducers and sulfide-oxidizer found only in PBPS-infected tissue and which seem ubiquitous to BBD worldwide.

PBPS: a primary or secondary infection?

During the course of this study two independent field observations revealed the development of PBPS from an original PWPS on Porites corals. From these observations we suggested that cyanobacteria likely form a secondary infection in Porites corals, already infected by other microbial organisms. It is possible that a succession of infections by different pathogens may increase, in a synergic way, the progression of coral diseases with black syndromes such as PBPS. Histological analysis revealed that PBPS shares some of the characteristics of PWPS.

For instance basophilic bodies, resembling bacterial aggregates similar to those observed in PWPS [START_REF] Séré | Bacterial communities associated with Porites white patch syndrome (PWPS) on three Western Indian Ocean (WIO) coral reefs[END_REF], were found in the living tissue adjacent to the bacterial mat but were not observed in healthy tissues (HT). Thus, it is possible that other bacteria may promote PBPS by initiating a primarily infection impairing the immune processes in corals. This highly speculative assumption is consistent with results published by [START_REF] Sato | Dynamics of seasonal outbreaks of black band disease in an assemblage of Montipora species at Pelorus Island (Great Barrier Reef, Australia)[END_REF] reporting morphological changes from a brown or green cyanobacterial-infected lesions named cyanobacterial patches (CP) to typical BBD lesions associated with changes in microbial communities from CP dominated by sequences closely related to Blennothrix sp. directly upon the PBPS-infected area. This suggests that coral-feeding organisms inflicting these scars may act as transmission vectors of disease. These observations are consistent with previous studies which have provided interesting insights into the likely contribution of marine corallivores in the initiation and dissemination of coral disease [START_REF] Sussman | The marine fireworm Hermodice carunculata is a winter reservoir and spring-summer vector for the coral-bleaching pathogen Vibrio shiloi[END_REF][START_REF] Raymundo | Functionally diverse reef-fish communities ameliorate coral disease[END_REF][START_REF] Chong-Seng | Selective feeding by coral reef fishes on coral lesions associated with brown band and black band disease[END_REF][START_REF] Nicolet | The corallivorous invertebrate Drupella aids in transmission of brown band disease on the Great Barrier Reef[END_REF]. For example, on the Great Barrier Reef (GBR) six species of coral-feeding fishes were observed to feed intensively on BBD [START_REF] Cole | Coral-feeding wrasse scars massive Porites colonies[END_REF] During surveys some marine invertebrates such as nudibranchs were also observed to feed upon the PBPS-infected area (Fig. 28D). Previous researches have reported several cases of predation by corallivorous invertebrates and demonstrated their potential role in transmission of disease [START_REF] Sussman | The marine fireworm Hermodice carunculata is a winter reservoir and spring-summer vector for the coral-bleaching pathogen Vibrio shiloi[END_REF][START_REF] Dalton | Progressive coral tissue mortality following predation by a corallivorous nudibranch (Phestilla sp.)[END_REF][START_REF] Nugues | Brown-band syndrome on feeding scars of the crown-of-thorn starfish Acanthaster planci[END_REF][START_REF] Nicolet | The corallivorous invertebrate Drupella aids in transmission of brown band disease on the Great Barrier Reef[END_REF]. For example, [START_REF] Sussman | The marine fireworm Hermodice carunculata is a winter reservoir and spring-summer vector for the coral-bleaching pathogen Vibrio shiloi[END_REF] showed that the fireworm Hermodice carunculata act as a winter reservoir and vector of Vibrio shiloi the causative agent of bleaching of the coral Oculina patagonica. Alternatively, benthic and endolytic algae and therefore may also act as vector of coral diseases (Sweet et al. 2013b).

Future directions

This study fills a gap in knowledge about coral disease prevalence worldwide and represents a baseline survey and a frame for future research programs in the area. It is also the first study to describe and characterise two unreported diseases PWPS and PBPS, using a multidisciplinary approach. However, this is a preliminary work based on surveys conducted only at three geographically distant localities and during two consecutive years. More investigations are needed notably on their drivers and transmission dynamics to improve our understanding of coral diseases and our ability to mitigate (prevent or reduce) their impacts at local and regional scales. Therefore, long-term disease surveys should be integrated and normalised to the existing monitoring programs regularly conducted on the WIO reefs such as the Global Coral Reef Monitoring Network (GCRMN) and Reef Check in order to expend surveys area and share valuable and consistent data. Finally, training should be provided to managers and non-expert marine scientist from WIO regions in order to optimize the quality and viability of the data collection and also enlarge the surveillance network.

Although temperature has been identified as the most likely environmental parameter influencing coral disease, our understanding on how other potential factors (acting individually or in synergy) could affect the resistance of corals and stimulate the growth and severity of pathogenic organisms remain unclear. Long term monitoring programs conducted on permanent transects set-up on different sentinel areas selected according to their state of disturbance could be an efficient tool to measure potential effects of biotic and abiotic factors on coral disease incidence and virulence. Thus, regular measures of physical and chemical Another important challenging area of research is the control of coral diseases. The best approach would always be the complete rehabilitation of the coral reef ecosystem notably by applying efficient control of reef fisheries and improving the watersheds and coastal management (main sources of water pollution). However this would be only possible with an intensive and interoperable effort among governments, municipalities and researcher.

Alternatively, the best line of defence seems to be the "biological control" of coral disease.

While actions plans such as aspirating or covering the affected area of diseased corals, vaccinations and antibiotic treatments seem unrealistic in term of logistic, expenses and ethic, the "biological control" is likely the more natural and realistic approach to mitigate coral diseases. Therefore, more investigations are needed to:

a. Understand interactions among coral associated bacteria and their links with the host.

b. Identify the role and functions of associated microorganisms in coral health.

c. Indentify, isolate and test on diseased corals potential "probiotic" candidates exhibiting specific properties such as biocides or antibiotics production that inhibit virulence of the pathogens and super competitors.

Finally, more research is needed to assess the potential role of corallivorous organisms as reservoirs and vectors of coral diseases. Investigations on the identification of local vertebrate and invertebrate corallivores and their feeding behaviour coupled with descriptions of their foraging modes is required to ascertain their potential implications in spread and progression of diseases. Additionally, screening of microorganisms associated with corallivores may provide interesting insight into potential reservoirs which can serve as vector for the transmission of putative pathogens.

Appendices

Appendix 1 : Bacterial 16S rRNA gene sequences from samples of apparently healthy (HT) and PWPS-diseased (DT) Porites lutea tissues collected at Mayotte (M), Reunion (R) and South Africa (SA).

Group/Subdivision affiliation

  Remerciements ......................................................................................................................... 1. The Western Indian Ocean coral reefs (WIO) are also affected by coral diseases ............ 2. WIO coral reefs, reservoirs of unreported coral pathology. ............................................... 3. PBPS: a primary or secondary infection? .......................................................................... 4. Potential sources of ubiquitous bacteria associated with both PWPS and PBPS ............... 5. Future directions ................................................................................................................. Appendices ............................................................................................................................ References .............................................................................................................................

Table 1 :Table 2 :Table 3 :Table 4 :Table 5 :

 12345 Geographic coordinates and details of the survey sites. ............................................ 37 Description and characterisation of coral diseases observed on Reunion, Mayotte and South African reefs. ........................................................................................................... 41 Overall occurrence (number of diseased coral colonies) and prevalence (the number of diseased coral colonies divided by the total number of colonies identified to the genus level within each transect; ± SE) of the main coral diseases in coral genera on Reunion (n= 23562 coral colonies), South African (n= 17140 coral colonies) and Mayotte (n= 19426 coral colonies) reefs between 2010 and 2012. .................................................................................. 41 Mean coral disease prevalence (± SE) in three Western Indian Ocean regions during successive winters and summers in 2010-2012. ....................................................................... 42 Summary of factorial ANOVA testing of seasonal and spatial variations in coral diseases on Reunion (REU), South African (RSA) and Mayotte (MAY) reefs across reef zones (Shallow=S; Deep=D) over two consecutive summers (S1-2) and winters (W1-2).

Table 6 :

 6 Acropora (Acr), Pocillopora (Poc) and Porites (Por). *: Significant difference (p<0.05); **: Highly significant difference (p< 0.01); ns: no significant difference (p>0.05). ..................... 48 Overall coral disease prevalence (± SE) and diseases recorded on Indian Ocean coral reefs. WS: white syndrome; BBD: black band disease; PWPS: Porites white patch syndrome; PLS: pink line syndrome; SED: skeleton eroding band; Nec: necrosis; GA: growth anomaly; BrB: brown band disease. WBD: white band disease; WP: white plague disease; YB: yellow band disease; PS: pink spot; UWS: ulcerative white spot; PDDr: Porites dark discoloration response. ................................................................................................................................... 50 Table 7: Disease conditions recorded worldwide on the genus Porites. WS: white syndrome; BBD: black band disease; PWPS: Porites white patch syndrome; PLS: pink line syndrome; SED: skeleton eroding band; Nec: necrosis; GA: growth anomaly; WBD: white band disease; WP: white plague disease; YBD: yellow band disease; PS: pink spot; UWS: ulcerative white spot; PDDr: Porites dark discoloration response; PR: pigmentation response; Trem: trematodiasis; TLS: tissue loss syndrome; BND: brown necrotizing disease; Por bl w/TL: Porites bleaching with tissue loss; PorDTTS: Porites discolored tissue thinning syndrome. . 52 Table 8: Number of samples collected for histopathology, sections analysed and diagnosis partitioned by tissue category and region. HT = healthy tissue. DT = disease tissue. ............. 62 Table 9: Number of samples collected, clones retrieved by tissue category and region and diversity index (Shannon-Weaver). HT = healthy tissue. DT = disease tissue. ....................... 66 Table 10: List of strains retrieved from samples of PWPS collected on Mayotte and Reunion reefs and used during phase 1 inoculation trials. ..................................................................... 87

  genes and PCR amplification conditions used for multilocus sequence typing (MLST) analysis. .......................................................................................................... 89 Table 12: Biochemical and susceptibility tests on strains P180R and P180R-ri. -= nonsusceptible, + = weakly susceptible, ++ = moderately susceptible, +++ = highly susceptible.95 Table 13: Location and depth of the reef sites and stations selected for this study. .............. 107 Table 14: Total sequences read before and after sequence trimming, number of bacterial classes and genera and diversity indices for each sample and subsample of PBPS (DT1A-DT1B and DT2A-DT2B), apparently healthy tissue (HT1A-HT1B and HT2A-HT2B) and healthy tissue (HTA and HTB). ............................................................................................. 116

Figure 1 :Figure 2 :Figure 3 :Figure 4 :Figure 5 :

 12345 Figure 1: Simplified anatomy of scheratinian corals. A. Colony of Pocillopora sp. B. Colony of expended polyps of Pocillopora sp. C. Diagram of gross anatomy of a polyp. D. Three dimensional diagram representing the different tissue layers of the surface body wall of a scleractinian polyp (C and D taken from Galloway et al. (2007)). .......................................... 23

Figure 6 :

 6 Figure 6: Prevalence (%) of the main coral diseases in nine scleractinian genera in Reunion: Porites white patch syndrome (PWPS), white syndromes (WS), pink line syndrome (PLS), black band disease, growth anomalies (GA) and skeleton eroding band (SEB). Prevalence is calculated relative to the total number of colonies in the respective taxa, per coral genus and per reef zone (reef flat and reef slope) for two consecutive summers and winters. Note that necrosis (Nec) is included in the analysis. ............................................................................... 44

Figure 7 :Figure 8 :Figure 9 :Figure 10 :Figure 11 :Figure 12 :Figure 13 :Figure 14 :Figure 15 :Figure 16 :Figure 17 :Figure 18 :Figure 19 :Figure 21 :Figure 22 :Figure 23 :Figure 24 :Figure 25 :Figure 26 :Figure 27 :Figure 28 :

 789101112131415161718192122232425262728 Figure 7: Prevalence (%) of the main coral diseases in 11 scleractinian genera at Sodwana-Bay, South Africa: Porites white patch syndrome (PWPS), white syndromes (WS), pink line syndrome (PLS), black band disease (BBD) and growth anomalies (GA). Prevalence is calculated relative to the total number of colonies in the respective taxa, per reef zone for two consecutive summers and winters. Note that bleaching (Ble) and necrosis (Nec) are included in the analysis. .......................................................................................................................... 45

Figure 1 :

 1 Figure 1: Simplified anatomy of scheratinian corals. A. Colony of Pocillopora sp. B. Colony of expended polyps of Pocillopora sp. C. Diagram of gross anatomy of a polyp. D. Three dimensional diagram representing the different tissue layers of the surface body wall of a scleractinian polyp (C and D taken from Galloway et al. (2007)).

Figure 2 :

 2 Figure 2: Coral diseases reported worldwide with (A) black band disease (BBD) on Pavona sp., (B) white band disease (WBD) on Acropora sp., white plague (WP) on Montastraea annularis (Bruckner 2002) and, growth anomaly (GA) on Astreopora sp.

Figure 3 :

 3 Figure 3: Map of the Western Indian Ocean (Adapted from Mangubhai (2007)). The main coral reefs are in red on the map.

Figure 4 :

 4 Figure 4: Map showing the study locations in the Western Indian Ocean

Figure 5 :

 5 Figure 5: Principal coral diseases observed on the WIO reefs. White syndrome (WS) on (a) Porites lutea (b) Montipora sp. and (c) Acropora sp. Active black band disease (BBD) on (d) Goniopora djiboutinensis and (e) Hydnophora sp. Skeleton eroding band (SEB) on (f) Acropora sp. growth anomalies (GA) on (g) Astreopora sp., (h) Porites lobata and (i) Acropora sp. Pink line syndrome (PLS) on (j) Porites lobata. White patch syndrome (PWPS) on (k and l) Porites lutea.

Figure 6 :

 6 Figure 6: Prevalence (%) of the main coral diseases in nine scleractinian genera in Reunion: Porites white patch syndrome (PWPS), white syndromes (WS), pink line syndrome (PLS), black band disease, growth anomalies (GA) and skeleton eroding band (SEB). Prevalence is calculated relative to the total number of colonies in the respective taxa, per coral genus and per reef zone (reef flat and reef slope) for two consecutive summers and winters. Note that necrosis (Nec) is included in the analysis.

Figure 7 :

 7 Figure 7: Prevalence (%) of the main coral diseases in 11 scleractinian genera at Sodwana-Bay, South Africa: Porites white patch syndrome (PWPS), white syndromes (WS), pink line syndrome (PLS), black band disease (BBD) and growth anomalies (GA). Prevalence is calculated relative to the total number of colonies in the respective taxa, per reef zone for two consecutive summers and winters. Note that bleaching (Ble) and necrosis (Nec) are included in the analysis.

Figure 8 :

 8 Figure 8: Prevalence of the main coral diseases in eight scleractinian genera in Mayotte: Porites white patch syndrome (PWPS), white syndromes (WS), pink line syndrome (PLS), black band disease (BBD) and growth anomalies (GA).Prevalence is calculated relative to the total number of colonies in the respective taxa per coral genus for one consecutive summer (March 2012) and winter (August 2011). Note that necrosis (Nec) are included in the analysis.

  were amplified from DNA extracts by PCR using universal primers 28F and 1492R[START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF]. PCR reactions were carried out in a final volume of 25 µl containing GoTaq®Hot Start Green Master Mix (Promega, Madison, WI), 0.5 mM of each primer and 10 ng of template DNA. Reaction mixtures were incubated in a GeneAmp®PCR

Figure 10 :

 10 Figure 10: Photomicrographs of diseased Porites lutea coral tissues: Porites white patch syndrome (PWPS). A) Cross-section showing the well-defined boundary between fragmented (FT) and diseased tissue (DT); Cd = cell debris; Ci = Ciliata. B) P. lutea with PWPS. Note ovoid basophilic bodies (bb) like bacterial aggregates; Ft = fragmented tissue. C) Cross section of P.lutea affected by PWPS showing dead tissue full of cell debris (Cd) and Cyanobacteria (Cy). D) Close-up of an ovoid basophilic body (Bb). E) Nematoda (Ne) in the tissue debris. F) Endophytic algae (ea) in dead tissues.

Figure 11 :

 11 Figure 11: Relative abundance (%) of bacterial phyla retrieved from three diseased (DT) and three healthy (HT) samples of Porites lutea collected in Mayotte (MAY), South Africa (RSA) and Reunion (REU).

Figure 12 :

 12 Figure 12: Multidimensional scaling (MDS) ordination of bacterial communities associated with healthy (HT) and PWPS-affected tissues (DT) of Porites lutea collected at Reunion (R), South Africa (RSA) and Mayotte (M).

Figure 13 :

 13 Figure 13: Neighbour-joining phylogenetic tree for the 16SrRNA gene sequences that were closely related to known and putative pathogens found in both healthy (HT) and Porites white patch syndrome (PWPS)-infected tissues (DT) of Porites lutea from Mayotte (MAY), South African (RSA) and Reunion (REU) corals. Numbers at each node are bootstraps values (%) obtained after 1000 iterations.

A

  total of 278 pure cultures were retrieved from diseased samples collected at Reunion and Mayotte. Genomic DNA was extracted from each bacterial colony by boil lysing in 100 μl of 5mM Tris/HCl at 100°C for 5 minutes. PCR amplification was carried out in a volume of 25 µl containing GoTaq ® Hot Start Green Master Mix (Promega, Madison, WI), 0Frias-Lopez et al. 2002), and 10 ng of template DNA.

Figure 15 :

 15 Figure 15: Experimental set-up of inoculation trials showing the nine 100 l tanks individually supplied with oligotrophic seawater and exposed to constant, artificial light.

Figure 17 :

 17 Figure 17: Neighbour-joining phylogenetic tree for the 16S rRNA gene showing the relatedness of the strain P180R with reference Vibrio strains. Numbers at each node are bootstraps values (%) obtained after 1000 iterations. The scale bar corresponds to the number of subdivisions per nucleotide position.

Figure 18 :

 18 Figure 18: Mean tissue loss (cm 2 ± SE) of Porites lutea inoculated with P180R and held at 26°C, 28°C and 30°C.

Figure 19 :

 19 Figure 19: Neighbour-joining phylogenetic tree using the concatenated partial sequences (3580bp) of 16S rRNA, pyrH, recA and rpoA showing the relatedness of the strain P180R with reference concatenated Vibrio strains. Numbers at each node are bootstraps values (%) obtained after 1000 iterations. The scale bar corresponds to the number of subdivisions per nucleotide position.

Figure 20 :

 20 Figure 20: A) Map of the study sites, B) and C) Porites black patch disease on the massive coral Porites lutea. Note the whitish filamentous film that forms patches.

  The surveys were conducted in five 10 m x 2 m belt transects perpendicular or parallel to the depth contours on the reef slope, and three 20 m x 2 m belt transects parallel to the depth contours on the reef flat. All Porites displaying signs of PBPS were counted within each transect. The prevalence was estimated as the number of diseased colonies divided by the total number of Porites colonies (>2 cm) counted within each transect in 1 m × 1 m photo-quadrats. Disease fronts were monitored on a monthly basis from 30 November 2010 to 10 December 2011 to ascertain whether PBPS progressed. Nails were driven into the dead portions of five Porites lutea colonies behind disease front as reference markers for this purpose. The progression rate was recorded as the linear distance between the nails and the nearest live tissue by superimposing pictures taken in consecutive surveys.

Figure 21 :

 21 Figure 21: Histological sections of Porites lutea: A) Healthy tissue (ht). Note the integrity of the epidermis. B) Living polyp from PBPS-infected tissue (dt) showing high concentration of granular pigment cells (pc). Note the basophilic bodies (bb) surrounded by granular pigment cells. C) Degraded tissue with cell debris (cd) and residual of pigment cells (pc). Note the presence of ciliate-like organisms (ci). D) Disease (dt) band boundary characterised by a dense mat of filamentous cyanobacteria (cy) invading the fragmented tissue (ft). E) and (F) Penetration of filamentous cyanobacteria (white arrow) into the infected tissue (dt).

Figure 22 :

 22 Figure 22: Cyanobacteria retrieved from Porites black patch syndrome (PBPS): A) Clumps of brown cyanobacterial filamments growing in a petri dish with Z8 medium. B) Photomicrograph of the cyanobacterial strain CYPBD1, closely related to the cyanobacterium Pseudoscillatoria coralii (FJ210722) and Roseofilum reptotaenium (HM048872), isolated from pure cultures. C) Neighbour-joining phylogenetic tree showing the relatedness of the strains CPPBPS1, CPPBPS2, CPPBPS3, and CPPBPS4 with reference cyanobacterial strains. Numbers at each node are bootstraps values (%) obtained after 1000 iterations.

Figure 23 :

 23 Figure 23: Comparative analysis of bacterial communities associated with three tissue categories of Porites lutea: A) Multidimensional scaling ordination, B) cluster diagram and C) rarefaction curves of bacterial communities associated with VHT1-2 A and B = apparently healthy tissue, HTA and B = healthy tissue and DT 1-2 A and B = diseased tissue of Porites lutea, created using MEGAN software version 5.0.78 beta.

Figure 25 :

 25 Figure 25: Taxonomic tree in MEGAN, profiling bacterial communities of two PBPS samples (DT1 and DT2) replicated in two subsamples (A and B). Each square and bar represents one subsample of diseased tissue. The scale of each bar was normalised and represents the number of sequence reads.

Figure 26 :

 26 Figure 26: Taxonomic tree in MEGAN, profiling bacterial communities of VHT samples (DT1 and DT2) replicated in two subsamples (A and B). Each square and bar represents one subsample of diseased tissue. The scale of each bar was normalised and represents the number of sequence read

  are on other reef ecosystems(McClanahan et al. 2004b), regional and seasonal field surveys conducted at three geographically distant WIO locations(Reunion, South Africa and Mayotte) during two consecutive years have demonstrated their potential threat on coral reefs. Principal findings from this study revealed the presence of six main coral diseases including white syndromes (WS), black band disease (BBD), skeleton eroding band (SEB), pink line syndrome (PLS), growth anomaly (GA), and Porites white patch syndrome (PWPS). Their prevalence was low on Mayotte and South African reefs (< 4%) compared with other coral reefs, but were relatively high in Reunion (7.5 %; Fig.6.1). The genera Acropora and Porites, two important reef-building corals commonly found in Reunion, South Africa and Mayotte were the most vulnerable to disease. Acropora were more susceptible to WS, SEB, Nec and GA while Porites were regularly affected by signs of BBD, PLS and PWPS. Surveys conducted on different geomorphologic reef zones and between consecutive seasons provided important insights on potential factors driving coral diseases on WIO coral reefs. Indeed, spatial patterns in the prevalence of BBD and WS were found on both Reunion and South Africa reefs with more disease cases at shallow than deep sites. Additionally, BBD in Reunion and WS in Mayotte and South Africa seemed to manifest seasonality with higher prevalence during summer months. As warm temperature anomalies are expected to become more frequent or extreme during the next few decades, it is likely that coral diseases outbreaks will become more prevalent and severe. A recent study has predicted that WIO Sea Surface Temperature (SST) is increasing in most locations at about 0.01°C per year(Maina et al. 2008). Furthermore, reefs located 10-15° South will be affected by lethal SST to corals every 5 years by 2010-2025[START_REF] Sheppard | Predicted recurrences of mass coral mortality in the Indian Ocean[END_REF].

Figure 27 :

 27 Figure 27: Map of the Western Indian Ocean showing results of the coral disease composition of each sampling and survey location and mean disease prevalence (% ± SE). Red bands represent the main coral reefs.

and 4 .

 4 Alphaproteobacteria-affiliated to BBD dominated by sequences closey related to Oscillatoria sp. and Gammaproteobacteria. Potential sources of ubiquitous bacteria associated with both PWPS and PBPS When comparing all the 16S rRNA gene sequences taken from the diseased and healthy tissues (chapters 3 and 5) certain groups or species of bacteria were only present within infected one. For instance, 16S rRNA sequences closely related to Vibrio hepatarius, Shimia marina and Pseudoruegeria aquimaris were consistently found from PWPS while the bacterial genera Desulfovibrio, Clostridium, Arcobacter, Flexibacter, Cytophaga, Oceanospirillum, and Spirochaeta were found only in PBPS samples suggesting they might be introduced from external sources. It is possible that these bacteria which are potentially implicated in those two pathologies may have been transmitted by infected corallivorous organisms on healthy colonies. For instance during this study almost every PWPS-infected colony presented at the middle of the lesion a visible scar (Fig 6.2A, B) probably left by corallivorous fishes. Additionally, intense feeding or predation scars were regularly observed

  . More recently Chong-Seng et al. (2011) have shown that 25 species of fishes were observed to feed on coral infected with BBD and brown band disease (BrB) suggesting their potential role in disease transmission among adjacent healthy coral colonies. Finally, a preliminary work conducted in Reunion during this study has shown the presence of several bacterial strains closely related to known pathogens associated with three species of damselfish, Stegastes lividus, S. limbatus and S. nigricans known to inflict intensive scars to corals (Appendix 3).

Figure 28 :

 28 Figure 28: Massive colonies of Porites lutea exhibiting signs (A and B) of PWPS and (C and D) PBPS. Ct = compromised tissue; dt= diseased tissue; ht= healthy tissue; bi= bite; Nud= nudibranch

Chapter 2 Identification and prevalence of coral diseases observed on three Western Indian Ocean (WIO) coral reefs

	1. Abstract
	Coral diseases have caused a substantial decline in the biodiversity and abundance of reef-
	building corals. To date, more than 30 distinct diseases of scleractinian corals have been
	reported and are known to generate progressive tissue loss and affect coral growth rate,
	reproductive capacity, recruitment, species diversity and the abundance of reef-associated
	organisms. While coral disease investigations have increased over the last 40 years, very little
	is known about coral diseases in the Western Indian Ocean (WIO). Surveys conducted at
	multiple sites in Reunion, South Africa and Mayotte between August 2010 and June 2012
	revealed the presence of six coral diseases: black band disease (BBD), white syndromes
	(WS), pink line syndrome (PLS), growth anomalies (GA), skeleton eroding band (SEB) and
	Porites white patch syndrome (PWPS). The overall disease prevalence was higher in Reunion
	(7.5 ± 2.2%; mean ± SE) compared to South Africa (3.9 ± 0.8%; mean ± SE) and Mayotte
	(2.7 ± 0.3%; mean ± SE). Compiling results from the three distant locations, Acropora and
	variation on three target coral reefs in Reunion, South African and Mayotte. Chapter 3 Porites were the genera most vulnerable to disease. Spatial variability was detected in both
	provides the first characterisation of bacterial communities associated with the recently Reunion and South Africa with BBD and WS more prevalent on shallow than deep reefs.
	reported disease, Porites white patch syndrome (PWPS, Séré et al. 2012) using both
	histopathology and culture-independent molecular techniques. The study in chapter 4
	identifies the causal agent(s) of PWPS by combining traditional culturing methods and
	molecular techniques coupled with inoculation trials to test for Koch's postulates. Strains
	inducing PWPS in aquaria were further characterised using standard bacteriological tests. This chapter has been submitted for publication in the journal Coral Reefs on 12 December
	Effects of elevated seawater temperature, identified as an important abiotic factor enhancing 2013 as: Séré, M.G., Chabanet, P., Turquet, J., Quod, J-P., Schleyer M.H. Identification and
	prevalence of coral diseases observed on three Western Indian Ocean (WIO) coral reefs.
	the virulence several coral diseases were also tested in aquarium experiments. Chapter 5 Coral Reefs (under review)
	provides a comprehensive characterization of an atypical black band disease by using a
	multidisciplinary approach involving field surveys, gross lesion monitoring, histopathology
	and 16S barcoding by 454-pyrosequencing to investigate bacterial community compositions.

Table 1 :

 1 Geographic coordinates and details of the survey sites.

	Scleractinian corals displaying evidence of disease were identified to the genus level and
	counted within each transect. Additionally, all coral genera exhibiting comparable gross
	lesions were considered to have the same disease (e.g. BBD, WS, etc.). Bleaching and
	necrosis were considered an impairment of normal function and were also recorded. The

Table 2 :

 2 Description and characterisation of coral diseases observed on Reunion, Mayotte and South African reefs.

Table 3 :

 3 Overall

	Coral diseases (code)	Description/Characteristics	Infected hosts
	White syndrome
	(WS)

occurrence (number of diseased coral colonies) and prevalence (the number of diseased coral colonies divided by the total number of colonies identified to the genus level within each transect; ± SE) of the main coral diseases in coral genera on Reunion (n= 23562 coral colonies), South African (n= 17140 coral colonies) and Mayotte (n= 19426 coral colonies) reefs between 2010 and 2012.

Table 4 :

 4 Mean coral disease prevalence (± SE) in three Western Indian Ocean regions during successive winters and summers in 2010-2012.

	Location	Period/Dates	Season	Prevalence
	Reunion	September 2010	Winter 1	6.8 (2.2)
		December 2011	Summer 1	7.2 (2.4)
		2010-2011		7.0 (2.4)
		October 2011	Winter 2	8.3 (2.5)
		January 2012	Summer 2	7.8 (2.2)
		2011-2012		8.1 (0.2)
		Total		7.5 (2.2)
	South Africa	February 2011	Summer 1	3.9 (1.8)
		July 2011	Winter 1	1.9 (0.9)
		2011		2.9 (0.8)
		February 2012	Summer 2	4.1 (0.3)
		June 2012	Winter 2	5.7 (0.7)
		2012		4.9 (0.9)
		Total		3.9 (0.8)
	Mayotte	August 2011	Winter	2.3 (2.2)
		March 2012	Summer	3.1 (1.4)
		2011-2012		2.7 (0.3)
		Total		2.7 (0.3)

Table 5 :

 5 Summary of factorial ANOVA testing of seasonal and spatial variations in coral diseases on Reunion (REU), South African (RSA) and Mayotte (MAY) reefs across reef zones (Shallow=S; Deep=D) over two consecutive summers (S1-2) and winters (W1-2). Analyses were performed on the mean prevalence of coral disease between regions, the most prevalent diseases (BBD, PWPS and WS) and the most susceptible coral Porites white patch syndrome (PWPS). Except for PWPS, diseases recorded during this study have been previously reported within other regions across the Indian Ocean including the Chagos Archipelago[START_REF] Sheppard | Reefs and islands of the Chagos Archipelago, Indian Ocean: why it is the world's largest no-take marine protected area[END_REF], Republic of Maldives

		Factorial ANOVA				FISHER LSD		Interpretation
	Location	Disease	Factor tested	F	p	REU RSA MAY	Significant
	All locations	All diseases Season	8.58	>0.05	ns	ns	ns	S = W
			Zone	5.71	*	**	ns	ns	REU: DE ≠ SH
			Season×Zone	0.97	>0.05	ns	ns	ns	S×DE.SH = W×DE.SH
		BBD	Season	2.04	0.1	ns	ns	ns	S = W
			Zone	16.62	**	**	ns	ns	REU: DE ≠ SH
			Season×Zone	5.35	*	**	ns	ns	REU: SH×S ≠ SH×W
		PWPS	Season	0.93	0.33	ns	ns	ns	Summer = winter
			Zone	0.70	0.49	ns	ns	ns	DE = SH
			Season×Zone	0.85	0.49	ns	ns	ns	S×DE.SH = W×DE.SH
		WS	Season	1.5	0.22	ns	ns	ns	S = W
			Zone	5.45	*	**	ns	ns	REU: DE ≠ SH
			Season×Zone	3.13	*	ns	ns	*	MAY: SH×S ≠ SH×W
	Reunion	PorBBD	Season	5.07	*	**	-	-	S1 ≠ W1
			Zone	38.67	**	**	-	-	DE ≠ SH
			Season×Zone	2.73	*	*	-	-	S×SH≠W×SH
	South Africa	AcrWS	Season	4.42	*	-	**	-	S1 ≠ W1
			Zone	1.12	0.30	-	ns	-	DE = SH
			Season×Zone	0.71	0.55	-	ns	-	S×SH=W×SH
		PocWS	Season	14.15	**	-	**	-	S1≠W1≠S2=W2
			Zone	17.87	**	-	**	-	DE ≠ SH
			Season×Zone	4.26	*	-	*	-	Year 1: S*DE.SH≠W×DE.SH Year 2: S×*DE≠W×DE
	Mayotte	AcrWS	Season	4.85	*	-	-	*	S1 ≠ W1
			Zone	0.49	0.48	-	-	ns	DE = SH
			Season×Zone	0.10	0.74	-	-	ns	S×SH=W×SH

genera including Acropora (Acr), Pocillopora (Poc) and Porites (Por). *: Significant difference (p<0.05); **: Highly significant difference (p< 0.01); ns: no significant difference (p>0.05).

[START_REF] Onton | Distribution and drivers of coral disease at Ningaloo reef, Indian Ocean[END_REF]), South India (Thinesh et al. 2009;[START_REF] Thinesh | Coral disease prevalence in the Palk Bay, Southeastern India-With special emphasis to black band[END_REF]

) and the Indopacific region

[START_REF] Willis | Coral disease on the Great Barrier Reef[END_REF][START_REF] Raymundo | Coral diseases on Philippine reefs: genus Porites is a dominant host[END_REF] Aeby et al. 2006)

.

The overall disease prevalence in Reunion (Table

6

; 7.0-8.1%) was higher than that found in either South Africa (2.9-4.9%) or Mayotte (2.7%). Disease levels on Reunion reefs were also higher than those reported on other Indian Ocean reefs such as Ningaloo Reef in Australia

[START_REF] Onton | Distribution and drivers of coral disease at Ningaloo reef, Indian Ocean[END_REF]

, the Chagos Archipelago

[START_REF] Sheppard | Reefs and islands of the Chagos Archipelago, Indian Ocean: why it is the world's largest no-take marine protected area[END_REF]

or the Maldives

[START_REF] Montano | First report of coral diseases in the Republic of Maldives[END_REF]

. However, values obtained for Reunion were similar to those on Mandapam reefs in South India but were substantially lower than those recorded at Palk Bay, also in South India. This relatively high disease level may be attributable to the fact that fringing reefs in Reunion are young and adjacent to areas of high coastal development, and are thus subjected to stressors such as poor water quality caused by anthropogenic activities (urbanisation and agriculture in watersheds, waste-water discharge, sedimentation, over-exploitation and overfrequentation of reefs). This assumption was also proposed by

[START_REF] Thinesh | Coral disease prevalence in the Palk Bay, Southeastern India-With special emphasis to black band[END_REF]

to explain diseases patterns in Palk Bay and on Mandapam reefs. Nevertheless, more investigations are needed to identify factors that may facilitate disease outbreaks or exacerbate their effects on coral reefs.

Table 6 :

 6 Overall coral disease prevalence (± SE) and diseases recorded on Indian Ocean coral reefs. WS: white greater energy to growth and reproduction (e.g. Acroporidae and Pocilloporidae) are more susceptible to disease than massive corals which seem to develop greater resistance through the allocation of more energy to colony maintenance. However, our results have shown that massive colonies of Porites lutea and P. lobata, generally considered robust and slow-growing genera[START_REF] Raymundo | Coral diseases on Philippine reefs: genus Porites is a dominant host[END_REF], were affected by multiple infections, exhibiting signs of BBD, PLS and PWPS. The susceptibility of this genus to disease has also been reported worldwide (Table7), notably on south-eastern Indian[START_REF] Thinesh | Coral disease prevalence in Mandapam group of islands, Gulf of Mannar, Southeastern India[END_REF][START_REF] Onton | Distribution and drivers of coral disease at Ningaloo reef, Indian Ocean[END_REF][START_REF] Montano | First report of coral diseases in the Republic of Maldives[END_REF], Philippine[START_REF] Santavy | Quantitative assessment of coral diseases in the Florida Keys: strategy and methodology[END_REF][START_REF] Raymundo | Coral diseases on Philippine reefs: genus Porites is a dominant host[END_REF]

	Geographic distribution	Prevalence (%)	Coral diseases observed	Sources
	Western Indian Ocean
	Reunion (2010-2011)	7.0 (2.4)	WS. BBD. PWPS. PLS. SEB. Nec. GA	Present study
	Reunion (2011-2012)	8.1 (0.2)	WS. BBD. PWPS. PLS. SEB. Nec. GA	Present study
	South Africa (2010-2011)	2.9 (0.8)	WS. BBD. PWPS. PLS. Nec. GA	Present study
	South Africa (2011-2012)	4.9 (0.9)	WS. BBD. PWPS. PLS. Nec. GA	Present study
	Mayotte (2011-2012)	2.7 (0.3)	WS. BBD. PWPS. PLS. Nec. GA	Present study
	Eastern Indian Ocean
	Ningaloo Reef	2.3 (0.39)	WS. BBD. SEB. BrB	(Onton et al. 2011)
	Central Indian Ocean
	Chagos Archipelago	5.2 (0.2)	WS. GA	(Sheppard et al. 2012)
	Republic of Maldives	< 2	WS. BBD. SEB. PDDr. BrB. UWS	(Montano et al. 2012)
	Northern Indian Ocean
	Mandapam (South India)	8.9 ± ND	WS. BBD.WBD. WP. YB. GA. PS	(Thinesh et al. 2009)
	Palk Bay (South India)	21.0 ± ND	WS. BBD. WBD. PS. WP. YB	(Thinesh et al. 2011)

syndrome; BBD: black band disease; PWPS: Porites white patch syndrome; PLS: pink line syndrome; SED: skeleton eroding band; Nec: necrosis; GA: growth anomaly; BrB: brown band disease. WBD: white band disease; WP: white plague disease; YB: yellow band disease; PS: pink spot; UWS: ulcerative white spot; PDDr:

Porites dark discoloration response.

Among the coral communities at the three localities, the genera most vulnerable to disease were generally Acropora and Porites. These genera, commonly found on back, lagoon and fringing reefs, are important reef-building corals in Reunion, South Africa and Mayotte

[START_REF] Turner | Coral reefs of the Mascarenes, western Indian Ocean[END_REF]

. Acropora was mainly represented by the species Acropora muricata in Reunion and Mayotte and exhibited signs of WS, SEB, GA and Nec. This high susceptibility is consistent with other research which has shown Acropora species to be particularly vulnerable to disease on Indian Ocean

(McClanahan et al. 2004b;[START_REF] Thinesh | Coral disease prevalence in Mandapam group of islands, Gulf of Mannar, Southeastern India[END_REF][START_REF] Thinesh | Coral disease prevalence in the Palk Bay, Southeastern India-With special emphasis to black band[END_REF]

) and indo-Pacific reefs

[START_REF] Willis | Coral disease on the Great Barrier Reef[END_REF] Aeby et al. 2006;[START_REF] Haapkylä | Spatiotemporal patterns of coral disease prevalence on Heron Island, Great Barrier Reef, Australia[END_REF][START_REF] Aeby | Growth anomalies on the coral genera Acropora and Porites are strongly associated with host density and human population size across the Indo-Pacific[END_REF]

.

[START_REF] Haapkylä | Spatiotemporal patterns of coral disease prevalence on Heron Island, Great Barrier Reef, Australia[END_REF]

suggested that corals which allocate Indo-Pacific reefs

[START_REF] Sutherland | Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals[END_REF][START_REF] Haapkylä | Spatiotemporal coral disease dynamics in the Wakatobi Marine National Park, South-East Sulawesi, Indonesia[END_REF]

). Therefore, these results seem to contradict

Haapkylä and colleague's (2009)

assumption that disease vulnerability is related to life history traits, with the investment of energy into growth by fast-growing species being to the detriment of their pathogen resistance. Alternatively, the high susceptibility of massive Porites colonies to disease may be attributed to predation that compromises their health.

During this study, fish bites were observed on almost every colony of Porites lutea and P. lobata. Corallivorous fishes are considered potential vectors of coral disease

[START_REF] Aeby | Factors affecting susceptibility of the coral Montastraea faveolata to black-band disease[END_REF][START_REF] Raymundo | Functionally diverse reef-fish communities ameliorate coral disease[END_REF][START_REF] Chong-Seng | Selective feeding by coral reef fishes on coral lesions associated with brown band and black band disease[END_REF][START_REF] Chong-Seng | Selective feeding by coral reef fishes on coral lesions associated with brown band and black band disease[END_REF]

found that fishes belonging to the families Blennidae, Chaetodontidae, and Pomacentridae feed preferentially on infected coral colonies and may spread coral diseases. Finally, Hydnophora spp. and Goniopora spp. exhibited particularly high susceptibility/sensitivity to BBD on both Reunion and South African reefs. For instance, all infected colonies of Hydnophora sp. recorded on the reef flat in Reunion died and, in subsequent surveys, were recorded as being colonised by opportunistic algae; no further Hydnophora colonies were encountered. This may suggest that BBD can re-structure reefs at the local scale, highlighting the importance of frequent monitoring to assess disease-related shifts in coral community structure.

Table 7 :

 7 Disease conditions recorded worldwide on the genus Porites. WS: white syndrome; BBD: black band Spatial variability was detected in BBD and WS on both Reunion and South Africa reefs. For instance, BBD observed on Porites spp. in Reunion seemed to be depth-related, with more diseased cases observed in shallow than deep habitats. Similar trends were found in South Africa in colonies of Pocillopora sp. infected by WS. These results are consistent with patterns found in the Caribbean[START_REF] Weil | Spatial variability in distribution and prevalence of Caribbean scleractinian coral and octocoral diseases-I Community-level analysis[END_REF], Republic of Maldives[START_REF] Montano | First report of coral diseases in the Republic of Maldives[END_REF]) and South India[START_REF] Thinesh | Coral disease prevalence in Mandapam group of islands, Gulf of Mannar, Southeastern India[END_REF][START_REF] Thinesh | Coral disease prevalence in the Palk Bay, Southeastern India-With special emphasis to black band[END_REF] where both WS and BBD are more abundant at shallow than deep sites. In Mayotte, no spatial variations were detected, despite fringing reefs being exposed to increasing and greater anthropogenic pressures than the barrier reef. As suggested previously byWeil et al. (2002), corals exposed frequently and continuously to the same stressors may develop resistance to them.

	Species affected	Geographic distribution	Disease condition	Sources
	Porites lutea	Reunion	WS, PWPS, PLS, BBD, SEB	This study
	Porites lutea	South Africa	WS, PWPS, PLS, BBD, GA	This study
	Porites lutea	Mayotte	WS, PWPS, PLS, BBD, GA	This study
	Porites sp.	Indonesia	WS, PUWS, GA	(Haapkylä et al. 2009)
	Porites sp.	Heron Island, Australia	PUWS	(Haapkylä et al. 2010)
	Porites spp.	Florida keys	WS	(Porter et al. 2001)
	Porites spp.	Philippines	GA, NS, PR, PUWS	(Raymundo et al. 2005)
	Porites spp.	Caribbean	WS, BBD, GA, WP	(Sutherland et al. 2004)
	Porites lutea	Indo-Pacific	BBD, WPL, GA, YBD, PLS	(Sutherland et al. 2004)
	Porites lobata		GA
	Porites spp.		GA, YBD
	Porites lobata	Indo-Pacific	PUWS, PR	(Weil et al. 2012)
	Porites lutea		PUWS, PR
	Porites spp.	Mexico	BBD, WP, YBD	(Ward et al. 2007)
	Porites massive	Central Philippines	GA, PUWS	(Kaczmarsky 2006)
	Porites asteroides	Eastern Brazil	BBD	(Francini-Filho et al. 2008)
	Porites spp.	Hawaiian Archipelago	Trem, PorTLS, GA, PorBND, Por bl w/TL, PorDTTS	(Aeby et al. 2011)
	Porites spp.	New-Caledonia	GA, MFBS	(Tribollet et al. 2011)
	Porites spp.	South-western Caribbean	WP, YBD	(Garzón-Ferreira et al. 2001)
	Porites sp.	Eilat, Gulf of Aqaba	WS, Nec	(Ainsworth et al. 2007)
	Porites astreoides	Virgin Islands United states	WS, YBD	(Calnan et al. 2007)
	Porites spp.	US Pacific Remote Island	WS, GA	(Vargas-Angel 2009)
	Porites spp.	Red Sea	SEB	(Winkler et al. 2004)
	Porites astreoides	Dominica, West Indie	WS	(Borger and Steiner 2005)
	Porites lutea	Arabian Gulf Dubai	YBD	(Riegl 2002)
	Porites lutea	Kavarati, Gulf of Kutch	BBD	(Ravindran et al. 1999)
	Porites lutea	Papua New Guinea	BBD	(Frias-Lopez et al. 2004)
	Porites lutea	Negros Island, Philippines BBD	(Myers et al. 2007)
	Porites spp.	Guam, Micronesia	BBD, SEB, WS, GA, PUWS	(Myers and Raymundo 2009)
	Porites spp.	South-eastern India	PS, WS, WP, YBD, PR, PDDr	(Thinesh et al. 2011)
	Poritidae	Ningaloo Reef (Australia) Atramentous necrosis, SEB	(Harvell et al. 2007)

disease; PWPS: Porites white patch syndrome; PLS: pink line syndrome; SED: skeleton eroding band; Nec: necrosis; GA: growth anomaly; WBD: white band disease; WP: white plague disease; YBD: yellow band disease; PS: pink spot; UWS: ulcerative white spot; PDDr: Porites dark discoloration response; PR: pigmentation response; Trem: trematodiasis; TLS: tissue loss syndrome; BND: brown necrotizing disease; Por bl w/TL: Porites bleaching with tissue loss; PorDTTS: Porites discolored tissue thinning syndrome.

Table 8 :

 8 Number of samples collected for histopathology, sections analysed and diagnosis partitioned by tissue category and region. HT = healthy tissue. DT = disease tissue.

Table 9 :

 9 Number of samples collected, clones retrieved by tissue category and region and diversity index (Shannon-Weaver). HT = healthy tissue. DT = disease tissue.

		Reunion	South Africa	Mayotte
		HT	DT	HT	DT	HT	DT
	Samples collected	3	3	3	3	3	3
	Pooled samples	1	1	1	1	1	1
	Random clones from pooled samples	92	100	101	150	94	100
	Consensus sequences	74	60	100	145	91	91
	Sequence lengths (bp)	905-1498	904-1490	842-150	850-1498	1240-1483	855-1477
	Sequence quality (%)	77.8-100	72.2-100	74.2-100	66.2-100	78.7-100	67.1-100
	Class/subdivision	4	5	6	11	8	9
	Species	12	26	21	53	36	39
	Diversity (Shannon-Weaver index)	1.20	2.83	1.70	3.37	2.86	3.29

  , have been described worldwide e.g.

	white plague (WP), white band disease (WBD), Australian subtropical white syndrome
	(AWS) (Godwin et al. 2012), progressive white syndrome (PWS) (Sweet and Bythell 2012),
	Porites ulcerative white spot disease (PUWS) (Arboleda and Reichardt 2010), white pox

disease (WPD)

[START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF]

, Montipora white syndrome (MWS)

[START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF]

, Indo-Pacific white syndrome

[START_REF] Sussman | Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific[END_REF]

, acroporid white syndrome (AWS)

[START_REF] Aeby | Growth anomalies on the coral genera Acropora and Porites are strongly associated with host density and human population size across the Indo-Pacific[END_REF][START_REF] Roff | The ecology of 'acroporid white syndrome', a coral disease from the Southern Great Barrier Reef[END_REF]

) and white plague-like disease

[START_REF] Barash | Bacterial Strain BA-3 and a filterable factor cause a white plague-like disease in corals from the Eilat coral reef[END_REF]

.

Several studies have proposed bacteria as causative agents of these WS

[START_REF] Patterson | The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata[END_REF][START_REF] Sussman | Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific[END_REF][START_REF] Arboleda | Vibrio sp. causing Porites ulcerative white spot disease[END_REF][START_REF] Luna | Vibrio harveyi as a causative agent of the White Syndrome in tropical stony corals[END_REF][START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF][START_REF] Godwin | The role of coral-associated bacterial communities in Australian subtropical white syndrome of Turbinaria mesenterina[END_REF][START_REF] Sweet | Ciliate and bacterial communities associated with White Syndrome and Brown Band Disease in reef-building corals[END_REF][START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF]

. For example,

Table 10 :

 10 List of strains retrieved from samples of PWPS collected on Mayotte and Reunion reefs and used during phase 1 inoculation trials.

	Class/Subdivision	Closest species in GenBank database	Best match (%)	Similar 16S rRNA sequences gene MAY REU	Strain Code	Sequence lengths (bp)	GenBank Acc. No
	Actinobacteria	Micrococcus luteus	99	*	*	P230R	812-947	NR_037113.1
	Alphaproteobacteria	Paracoccus yeei	97-98	*	*	P144M	749-800	NR_029038
	Firmicutes	Staphylococcus aureus	99	*	*	P207R	748-928	CP003033
		Shimia marina	99	*	*	P57M	769-775	NR_043300
	Gammaproteobacteria	Alteromonas marina	99	*	*	P74M	847-853	NR_025260.1
		Photobacterium rosenbergii	99	*	*	P3R	857-891	NR_042343.1
		Vibrio coralliilyticus	98	*	*	P74M	852-739	NR_028014.1
		Vibrio fortis	98-100	*	*	P6M	857-980	NR_025575.1
		Vibrio harveyi	97-99	*	*	P197R	832-920	NR_043165.1
		Vibrio hepatarius	99	*	*	P180R	819-890	NR_025491.1
		Vibrio parahaemolyticus	99-100	*	*	P160R	635-893	NR_041838.1
		Vibrio rotiferianus	99	*	*	P275R	824-955	NR_042081
		Vibrio rumoiensis	99	*	*	P17R	885-898	NR_024680.1
		Vibrio sinaloensis	97-99	*	*	P86R	888-963	NR_043858.1

Table 11 :

 11 Sequencing

	Gene	Primer	Sequence (5' -> 3')	PCR amplification conditions
	16S rRNA pyrH rpoA recA	27F 1492R pyrH-04-F pyrH-02-R rpoA-01-F rpoA-03-R recA-01-F recA-02-R	AGAGTTTGATCMTGGCTCAG GGTTACCTTGTTACGACTT ATGASNACBAAYCCWAAACC GTRAABGCNGMYARRTCCA ATGCAGGGTTCTGTDACAG GHGGCCARTTTTCHARRCGC TGARAARCARTTYGGTAAAGG TCRCCNTTRTAGCTRTACC	Initial denaturing step of 4 min at 95.5˚C followed by 30 cycles at 94˚C for 30 sec, 55˚C for 1 min, and 72˚C for 1 min 30 sec, with a final extension step of 15 min at 72˚C Initial denaturing step of 4 min at 95˚C followed by 30 cycles at 95˚C for 1 min, 52˚C for 1 min, and 72˚C for 1 min, with a final extension step of 10 min at 72˚C Initial denaturing step of 5 min at 95˚C followed by 30 cycles at 95˚C for 35 sec, 55˚C for 1 min 15 sec, and 72˚C for 1 min 15 sec, with a final extension step of 7 min at 72˚C Initial denaturing step of 5 min at 95˚C followed by 30 cycles at 95˚C for 35 sec, 46˚C for 1 min 15 sec, and 72˚C for 1 min 15 sec, with a final extension step of 7 min at 72˚C

primers for 16S rRNA

[START_REF] Frias-Lopez | Partitioning of bacterial communities between seawater and healthy, Black Band Diseased, and dead coral surfaces[END_REF]), pyrH, rpoA, recA, (Thompson et al. 2005)

genes and PCR amplification conditions used for multilocus sequence typing (MLST) analysis.

Table 13 :

 13 Location and depth of the reef sites and stations selected for this study.

	Sites	Stations	Habitat	Reef depth (m)	Latitude/ longitude
	L'Ermitage 3-Chameaux	Reef flat	0.5-1.0	-21.080351° ; 55.219576°
	La Saline	Trou d'Eau		0.5-1.0	-21.103312° ; 55.242294°
	Saint-Leu	La Corne		0.5-1.0	-21.165960° ; 55.285080°
	Saint-Leu	Ravine des Poux		0.5-1.0	-21.176397° ; 55.285985°
	L'Ermitage 3-Chameaux	Reef slope	10.0-12.0	-21.081281° ; 55.217590°
	La Saline	Trou d'Eau		10.0-12.0	-21.106160° ; 55.239540°
	Saint-Leu	La Corne		10.0-12.0	-21.165940° ; 55.281930°
	Saint-Leu	Ravine des Poux		10.0-12.0	-21.175490° ; 55.283460°
	3.2. Sample collection
	Samples of Porites lutea exhibiting signs of PBPS were collected using SCUBA and
	snorkelling. Core samples of visually healthy (VHT) and diseased tissues (DT) from two
	diseased colonies and fixed in 4% formalin for histological examination of their tissue

Table 14 :

 14 Total sequences read before and after sequence trimming, number of bacterial classes and genera and diversity indices for each sample and subsample of PBPS (DT1A-DT1B and DT2A-DT2B), apparently healthy tissue (HT1A-HT1B and HT2A-HT2B) and healthy tissue (HTA and HTB).

		DT1	DT1	DT2	DT2	VHT1	VHT1	VHT2	VHT2	HT	HT
		A	B	A	B	A	B	A	B	A	B
	Σ raw sequences	11 320 12 703 14 848 13 386	15 660	14 646 19 008 15 221	12 563	8746
	Σ trimmed bacterial sequences	5785	5636	6505	5837	10 496	9419	10 687	8176	8259	5547
	Σ bacterial Phylum	8	8	8	8	2	4	4	4	4	2
	Σ genera	22	22	16	17	5	8	8	5	7	8
	Shannon index	4.310	4.357	4.158	4.349	2.882	3.092	2.638	2.638	2.743	3.152
	Simpson reciprocal index	11.947 12.162 10.756 13.063	6.730	7.745	6.059	6.061	6.064	8.131

Remerciements Acknowledgments

This work was co-funded by the European Union (EU, FEDER), the Regional Council of Reunion, the French Ministry of Higher Education and Research (DRRT), the French Acknowledgments ..................................................................................................................... Résumé ......................................................................................................................................

dear friend and field buddy Stephanie Bollard. This study would not have been the same without her generous assistance, advices and valuable support. We miss you sorely! Department of Ecology, Sustainable Development, Transportation and Housing (DEAL), the South African Association for Marine Biological research (SAAMBR), the French Ministry of Overseas (MOM) and the Western Indian Ocean Marine Science Association (WIOMSA). First of all I would like to thanks my academic supervisors Prof. Michael Schleyer, Prof. Pascale Chabanet (Plume) and Dr. Jean-Pascal Quod. Mike, thank you so much for the extraordinary opportunity you afforded me to join your "Coral Team" at the Oceanographic encouragement and support, professionally and personally you gave me all along the study. Your support has meant a great deal to me. Jean Pascal, thank you for trusting in me from the beginning to the end and afford me this great opportunity to conduct and complete this PhD. Special thanks go to my darling Lola. I could not have completed this project without her constant love and encouragements. You were always there when I doubted myself. I love you! I would like to express my sincere gratitude to Dr Jean Turquet (ARVAM), for his assistance in fieldtrip, laboratory logistic, funding proposals and administrative issues. Thanks are also due to Dr Pablo Tortosa (CRVOI), Dr Davis Wilkinson (CRVOI), Dr.

increase disease progression [START_REF] Willis | Coral disease on the Great Barrier Reef[END_REF]Bruno and Selig 2007) and tissue mortality [START_REF] Boyett | Elevated temperature and light enhance progression and spread of black band disease on staghorn corals of the Great Barrier Reef[END_REF][START_REF] Haapkylä | Spatiotemporal coral disease dynamics in the Wakatobi Marine National Park, South-East Sulawesi, Indonesia[END_REF][START_REF] Haapkylä | Spatiotemporal patterns of coral disease prevalence on Heron Island, Great Barrier Reef, Australia[END_REF]) by stimulating the growth of putative pathogens [START_REF] Patterson | The etiology of white pox, a lethal disease of the Caribbean elkhorn coral, Acropora palmata[END_REF]Ben-Haim et al. 2003a;[START_REF] Cervino | Relationship of Vibrio species infection and elevated temperatures to Yellow Blotch/Band Disease in Caribbean corals[END_REF][START_REF] Rosenberg | The Vibrio shiloi/Oculina patagonica model system of coral bleaching[END_REF][START_REF] Boyett | Elevated temperature and light enhance progression and spread of black band disease on staghorn corals of the Great Barrier Reef[END_REF]) and altering the structure of coralassociated bacteria [START_REF] Reshef | The coral probiotic hypothesis[END_REF][START_REF] Mouchka | Coral-associated bacterial assemblages: current knowledge and the potential for climate-driven impacts[END_REF]) which may have an important role in disease-resistance [START_REF] Ritchie | Regulation of microbial populations by coral surface mucus and mucus-associated bacteria[END_REF].

Methods

The sampling of Porites lutea colonies for this study was authorised by the French Department of Ecology, Sustainable Development, Transportation and Housing (DEAL), the Isimangaliso Wetland Park (South Africa) and CITES (Permit no. FR1197400391-FR1197400394-1).

Sample collection

Individual samples of Porites lutea exhibiting signs of PWPS were collected in South Africa, Reunion and Mayotte (Fig. 9) using SCUBA. At each location, samples were collected from three healthy corals and colonies with signs of PWPS-infection (n total =18 samples).

Results

Microscopic morphology and spatial structure

Histological cross-sections of PWPS revealed extensive tissue breakdown and necrosis within the lesion area between the exposed skeleton and living tissue (Fig. 10A). Ovoid basophilic bodies resembling bacterial aggregates were visible within the mesogloea of the body wall, mainly in DT (Table 8), especially in the area of tissue fragmentation (Figs 10B,D). These aggregates were seen in nine of the 15 samples collected from corals showing signs of PWPS at all three sampling locations. Among the 15 samples of HT, only one was found with such aggregates (Table 8). Other organisms, including Cyanobacteria (Fig. 2C, F), Nematoda (Fig. 10E), Ciliata (Fig. 10A), and algae (Fig. 10F) were also observed but only within dead tissue.

Bacterial communities associated with healthy tissues

A total of 91, 74 and 100 16S rRNA sequences (818-1627 bp), subdivided into seven, six and four classes (Table 2 and Fig. 3) were obtained from healthy tissues collected in Mayotte (HT-M), South Africa (HT-RSA) and Reunion (HT-R) respectively (Table 9). Sequences retrieved from HT-M (Table 9 and Fig. 11) were mainly identified as members of the γproteobacteria (42.0%), α-proteobacteria (19.0%), Cyanobacteria (11.0%), Firmicutes (6.0%), Cytophagia (6.0%) and ɛ-proteobacteria (3.0%).

Chapter 4

Identification of a primary pathogen involved in white patch syndrome, a newlyreported disease affecting the massive coral Porites lutea in the Western Indian Ocean This chapter has been submitted for publication in the journal Plos one as: Séré, M.G., Tortosa, P., Quod, J-P., Chabanet, P Schleyer M.H. Identification of a primary pathogen involved in white patch syndrome, a newly-reported disease affecting the massive coral Porites lutea in the Western Indian Ocean. Plos One (under review).

Methods

The sampling of Porites lutea colonies for this study was authorised by the French Department of Ecology, Sustainable Development, Transportation and Housing (DEAL), the Isimangaliso Wetland Park (KZN-Wildlife, South Africa) and CITES (Permit no.

FR1197400391-I to FR1197400394-1).

Sampling, growth conditions and isolation of bacteria

Three core samples from three individual P. lutea colonies manifesting PWPS (Fig. 14

Characterisation and identification of the primary pathogen P180R

P180R is a gram-negative bacterium forming beige/ cream colonies on MA, yellow on TCBS.

Its culture yielded high growth at 37°C and emitted a strong sweet smell similar to sugar cane molasses. Tests on its phenotypic traits and susceptibility to antibiotics are presented in Table 12. While P180R appeared to be closely related to the Gamaprotebacteria Vibrio hepatarius (AJ345063, KF179862, and KF179731) using a region of 16S rRNA gene, additional analysis performed on pyrH, recA and rpoA genes revealed a greater affinity to V. tubiashii with 100% identity. Furthermore, phylogenetic analysis of the concatamerised genes (MLST) confirmed evidence that P180R is mostly phylogenetically related to the species V. tubiashii (Fig. 19). Archive (SRA).

Statistical analysis

Disease prevalence was calculated per transect and for each site. Data were log-transformed [log10 (X)] to satisfy assumptions of homoscedasticity and normality of variance. Variations in the prevalence of PBPS over the two survey years in the consecutive summers and winters and across reef zones (shallow vs. deep) were tested using Factorial ANOVA (STATISTICA 8). Fisher tests were performed for post hoc multiple comparison. Rarefaction curves were performed on bacterial populations associated with each tissue category. Principal coordinate analysis (PCoA) and cluster analysis were performed using the Bray-Curtis similarity Considering the similarities between bacterial profiles and that no significant difference was found between the DT samples (t-test <0.05), the replicates were then pooled for detailed analysis. The most dominant genera observed in DTs were Vibrio (24.9%) followed by Desulfovibrio (20.1%), Clostridium (12.9%), Arcobacter (9.9%), Flexibacter (6.2%) and Spirochaeta (2.7%).

Bacterial community composition in VHT and HT

Bacterial communities of VHT1, VHT2 and HT samples were almost exclusively dominated by the Class Gammaproteobacteria comprising 99.9%, 99.7% and 99.5% of the total sequences reads respectively (Fig. 26 and Table 14). A few additional sequences attributed to Alphaproteobacteria (0.06% in VHT1, 0.11 % in VHT2 and 0.38 % in HT) and Deltaproteobacteria (0.03% in VHT1, 0.03 % in VHT2 and 0.05 % in HT) were also found in common. In VHT1, additional sequence reads attributed to Chloroflexi (0.06%) were found while Bacteroidia (0.03%) and Cytophagia (0.05%) were obtained in VHT2.

Finally, the sequences obtained from HT, considered a negative control, were composed of a few extra reads affiliated to Actinobacteria (0.03%), Photobacterium (0.3%), Acetobacter (0.09%), Wolbachia (0.09%), Rhizobium (0.08%), and Pseudomonas (0.06%). In total, only three genera were common between VHT and HT samples and included Spongiobacter (80.1% in VHT1, 74.6% in VHT2 and 67.6% in HT), Vibrio (19.4% in VHT1, 7.0% in VHT2 and 67.6% in HT) and Pseudomonas (0.06% in VHT1, 2.6% in VHT2 and 0.06% in HT). A small proportion of sequence reads affiliated to Photobacterium (0.17%), Chloroflexaceae (0.1%), Caulobacter (0.06%), and Bacteriovorax (0.04%) were also found in VHT1, while better represented genera including Porphyromonadaceae (3.5%), Arcobacter (3.5%), sulfuroxidizing symbionts (3.5%) and Flexibacter (2.6%) were obtained from VHT2.

Chapter 6

General discussion

Histopathology of PWPS-affected tissues revealed extensive tissue fragmentations inhabited by ovoid basophilic bodies resembling bacterial aggregates. Other organisms e.g. Ciliata, Cyanobacteria and Nematoda were observed within the dead epidermis and cell debris.

Principal findings from culture-independent analysis revealed substantial variability of bacterial communities not only between healthy and PWPS-infected tissues but also between the three geographically distant sampling localities. However, few bacterial ribotypes e.g. the γ-proteobacteria Vibrio hepatarius and the α-proteobacteria Shimia marina were consistently found from the lesions suggesting their potential role as primary pathogens of PWPS. In the same study, a deductively approach based on presence/absence of specific bacterial ribotypes (Chapter 3) associated with healthy and PWPS-infected tissues, traditional culturing techniques and laboratory infection trials was successfully used to identify potential primary causative agents (Chapter 4). Among the selected bacterial strains used in experimental infection trials, only the strain P180R closely related (100% sequence identity) to Vibrio tubiashii elicited similar evidence of disease to that observed in the field and satisfied the four Henle-Koch's postulates. Although no statistical difference between summer and winter was found in the field (chapter 2), seasonality was evident for PWPS. This "thermo-dependent" pattern of PWPS was demonstrated experimentally with its severity clearly increased with elevated seawater temperature (Chapter 4).

The second unreported coral disease was an atypical form of BBD termed here "Porites black patch syndrome" (PBPS) characterized by a diffuse, centrally to peripherally situated, medium to large black patch, leaving behind it at the rate of 4.4 ± 0.12 mm day -1 (mean ± SE), the dead skeleton generally covered by a whitish filamentous film (Chapter 5). Among the three distant studied locations, PBPS was only found at Reunion on two of the main framework building corals, Porites lobata and P. lutea. It manifested spatial and seasonal variability with more infected colonies observed on the reef flat and during the summer parameters (temperature, pH, salinity, nutrient load, and metal concentrations), organic matter and microbial communities in the water column should be recorded regularly at the same selected site simultaneously with coral disease monitoring. Therefore combined results may guide managers to better identified, mitigate or even control the main factors leading to disease incidence.

Among the 30 coral diseases identified to date only six bacterial species have been identified as aetiological agents [START_REF] Israely | Growth, differentiation and death of Vibrio shiloi in coral tissue as a function of seawater temperature[END_REF]Ben-Haim et al. 2003b;[START_REF] Sussman | Coral pathogens identified for White Syndrome (WS) epizootics in the Indo-Pacific[END_REF][START_REF] Arboleda | Vibrio sp. causing Porites ulcerative white spot disease[END_REF][START_REF] Sutherland | Human pathogen shown to cause disease in the threatened eklhorn coral Acropora palmata[END_REF][START_REF] Ushijima | Vibrio owensii induces the tissue loss disease Montipora white syndrome in the Hawaiian reef coral Montipora capitata[END_REF]. Therefore, it is important to develop standardized methodologies from the coral disease sampling to the characterisation and identification of primary pathogens. Thus, a multidisciplinary approach based to that used in this study could be proposed for future researches. It includes the following steps:

i. Characterisation of lesions using histopathology to describe lesions at the cellular level and detect presence (or absence) of potential pathogens.

ii. Discrimination of potential pathogens using molecular techniques (Cloning, 454 pyrosequencing) based on presence/absence of bacterial ribotypes in healthy/diseased colonies.

iii. Culturing and isolation of potential pathogens using general and specific growth media and candidates selection based on step (ii) for infections trials.

iv. Inoculation trials to test the infectivity of the selected bacteria and determine disease causality by fulfilling Koch's Postulates.

v. Metabolic characterisation and identification using MLST and phylogenetic analysis.

Nevertheless, not all bacteria are culturable with exciting commercial media. Thus, more efforts need to be allocated to the development of new specific and generic marine media since bacterial culturing is the crucial step in coral disease identification.