Maria Cellamare

Charlotte Duval

Yannick Drelin

Chakib Djediat

Nour Touibi

Hélène Agogué

Christophe Leboulanger

Magali Ader

Cécile Bernard
email: cbernard@mnhn.fr

Characterization of phototrophic microorganisms and description of new cyanobacteria isolated from the saline-alkaline crater-lake Dziani Dzaha (Mayotte, Indian Ocean)

Keywords: cyanobacteria, microalgae, crater-lake, saline-alkaline lake, polyphasic approach, Arthrospira

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Soda lakes are highly alkaline environments (pH typically between 9 and 12) characterized by large amounts of sodium carbonate (or complexes of this salt) formed by evaporative concentration [START_REF] Jones | Microbial diversity and ecology of the Soda Lakes of East Africa[END_REF][START_REF] Schagerl | Dipping into the Soda Lakes of East Africa[END_REF]. Many soda lakes also contain high concentrations of sodium chloride and other dissolved salts, making them salineor hypersaline systems [START_REF] Grant | Alkaline environments and biodiversity[END_REF]. The resulting high alkalinity and salinity make these lakes ones of the most extreme aquatic environments on Earth [START_REF] Schagerl | Dipping into the Soda Lakes of East Africa[END_REF], where only alkaliphilic, halo-and thermotolerant organisms can thrive [START_REF] Krienitz | Tiny and Tough: Microphytes of East African Soda Lakes[END_REF]. In saline-alkaline lakes from tropical and subtropical areas, the alkaliphilic filamentous cyanobacterium Arthrospira is frequently the dominant taxon and the responsible of permanent or seasonal blooms [START_REF] Ballot | Cyanobacteria and cyanobacterial toxins in the alkaline crater lakes Sonachi and Simbi, Kenya[END_REF][START_REF] Sili | Arthrospira (Spirulina)[END_REF]. However, the rest of the phototrophic microorganisms are often neglected because of their minute dimension or lower biomass. In consequence, their diversity remains unexplored and many taxa are still unknown. One of the main reasons to explain the low cyanobacterial and microalgal diversity usually reported from these systems relies on the use of traditional methods such as light-microscopy, that can underestimates true species number [START_REF] Schagerl | Algal communities of Kenyan soda lakes with a special focus on Arthrospira fusiformis[END_REF].

In the last decades, the polyphasic approach that combines phenotypic, molecular, ultrastructural, physiological and ecological characteristics has been commonly applied to unravel the complex taxonomy of cyanobacteria [START_REF] Nübel | The halotolerance and phylogeny of cyanobacteria with tightly coiled trichomes (Spirulina Turpin) and the description of Halospirulina tapeticola gen. nov., sp. nov[END_REF][START_REF] Abed | Polyphasic characterization of benthic, moderately halophilic, moderately thermophilic cyanobacteria with very thin trichomes and the proposal of Halomicronema excentricum gen. nov., sp[END_REF][START_REF] Suda | Taxonomic revision of water-bloom-forming species of oscillatorioid cyanobacteria[END_REF][START_REF] Casamatta | Molecular and ultrastructural characterization of ten polar and near-polar strains within the Oscillatoriales (Cyanobacteria)[END_REF][START_REF] Taton | Polyphasic study of Antarctic cyanobacterial strains[END_REF][START_REF] Comte | Relationships between the Arctic and the Antarctic cyanobacteria; three Phormidium-like strains evaluated by a polyphasic approach[END_REF][START_REF] Řeháková | Morphological and molecular characterization of selected desert soil cyanobacteria: three species new to science including Mojavia pulchra gen. et sp. nov[END_REF][START_REF] Dadheech | Desertifilum fontinale sp. nov. (Oscillatoriales, Cyanobacteria) from a warm spring in East Africa, based on conventional and molecular studies[END_REF][START_REF] Komárek | Taxonomic classification of cyanoprokaryotes (cyanobacterial genera) 2014, using a polyphasic approach[END_REF][START_REF] Komárek | A polyphasic approach for the taxonomy of cyanobacteria: principles and applications[END_REF]) and microalgae [START_REF] Luo | Generic concept in Chlorella-related coccoid green algae (Chlorophyta, Trebouxiophyceae)[END_REF][START_REF] Krienitz | Picocystis salinarum (Chlorophyta) in saline lakes and hot springs of East Africa[END_REF]. This approach allowed to enhance the diversity of microorganisms, mostly by revealing previously overlooked taxa [START_REF] Abed | Characterization of microbialite-forming cyanobacteria in a tropical lagoon: Tikehau atoll, Tuamotu, French Polynesia[END_REF]. For example, the cyanobacterial and microalgal diversity from saline-alkaline lakes distributed in the East African Rift valley has been well characterized using this method [START_REF] Ballot | Phylogenetic relationship of Arthrospira, Phormidium and Spirulina strains from Kenyan and Indian waterbodies[END_REF]Dadheech et al. 2012b;[START_REF] Krienitz | Picocystis salinarum (Chlorophyta) in saline lakes and hot springs of East Africa[END_REF][START_REF] Dadheech | Cyanobacterial diversity in the hot spring, pelagic and benthic habitats of a tropical soda lake[END_REF][START_REF] Krienitz | Mass developments of the cyanobacteria Anabaenopsis and Cyanospira (Nostocales) in the soda lakes of Kenya: ecological and systematic implications[END_REF]. The isolation and characterization of phototrophic microorganisms living in saline-alkaline lakes are of great importance to solve taxonomic questions and contribute to the knowledge about their local environment, geographical distribution, ecology, physiology and behavior [START_REF] Taton | Polyphasic study of Antarctic cyanobacterial strains[END_REF][START_REF] Andreote | Non-heterocytous cyanobacteria from Brazilian saline-alkaline lakes[END_REF].

Dziani Dzaha is a small saline-alkaline crater-lake located in Mayotte Island (Comoros Archipelago, Western Indian Ocean) and situated very close to the ocean (0.1 km). Because Dziani Dzaha means "lake of the volcano" in shimaore, we refer to the lake as Dziani Dzaha in order to avoid redundancy. The physical and chemical characteristics of this lake are significantly different from other tropical soda lakes, mainly by its relatively balanced Na+ / Cl-composition that suggests significant contribution of seawater inputs [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF]. Besides its high salinity (>50 psu) and alkalinity (>10 g L -1 CO 3 2-), this lake also presents a number of analogies with Precambrian environments as the permanent anoxia in depth, the presence of numerous stromatolites and a trophic chain mostly reduced to prokaryotes [START_REF] Cadeau | Cycles biogéochimiques du carbone, de l'azote et du soufre dans un analogue des océans précambriens : le lac Dziani (Mayotte[END_REF][START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF]. Similar to other saline-alkaline lakes, the water column is characterized by cyanobacteria blooms dominated by Arthrospira, but the bloom appears permanent in Dziani Dzaha [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF].

Despite the extremely high algal biomass production all year round in Dziani Dzaha, only one study to date has focused on the microbial compartment [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF]. Using the polyphasic approach, we characterize several strains of phototrophic microorganisms (cyanobacteria and microalgae) isolated from the water column and stromatolites of this unique ecosystem. Two new genera and four new species of filamentous cyanobacteria are described. Besides, ecological considerations regarding the morphological plasticity of several strains identified are highlighted.

MATERIALS AND METHODS

Study site

Dziani Dzaha is a tropical crater lake located in the Petite Terre of Mayotte Island (Comoros archipelago, Western Indian Ocean) (12°46'15"S; 45°17'17"E) (Fig. 1). The lake surface is close to 25 ha., with 600 m diameter and an altitude close to the mean sea level. It is a shallow lake with a maximum depth of 4.5 to 5.5 m depending on the rainfall. A pit of 18 m depth is located in the eastern part of the lake. The watershed is reduced to the crater walls, with no river inflow. The lake is chemically stratified in the dry season (May to October), forming two layers in the water column: an oxic layer (0-2.5 m) and an anoxic layer (>2.5 m). Over its entire area and depth, the water has high salinity (>50 psu), high alkalinity (dissolved inorganic carbon concentration in the range160-220 mM), conductivity between 77.1 and 79.7 mS cm -1 and pH about 9 to 9.5. The lake is very turbid (Secchi transparency <0.15 m) and the chlorophyll a concentrations are extremely high (up to 875 µg L -1 ). Both parameters define the hypereutrophic nature of this system. Water temperature is around 29 °C below 1 m, whereas surface temperatures show daily fluctuations, reaching up to 35 °C. Numerous stromatolites patches are distributed all around the lake shore and occasional gases bubbling occur on the surface [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF]. More details about the lake characteristics are given in [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF].

Sampling

Samples were collected for strains isolation in October 2010 (end of the dry season) and April 2014 (end of the rainy season). Water column samples were collected at the surface, with a 20-μm-mesh size net and benthic samples were collected directly from stromatolite biofilms (Fig. 1f).

Cyanobacterial and microalgal isolation and culture conditions

Water and stromatolite samples were inoculated on solid medium (5 or 10 g L -1 of agar) with several culture mediums: Z8-salt or Z8-salt minus nitrogen (Z8X) [START_REF] Rippka | Isolation and purification of cyanobacteria[END_REF] irradiance level of 16 μmol photons cm -2 s -1 , with a photoperiod of 16L: 8D. Strains and cultures were monoclonal and non-axenic.

Morphological analyses

Morphological analyses of cyanobacteria and microalgae strains were carried out using a light microscope NIKON Eclipse Ni and photographed with a coupled NIKON DS-Fi2 camera.

The parameters analyzed included cell width and length, cell and filament morphology and motility presence. Strains identification by the traditional method was carried out using updated taxonomic literature [START_REF] Lewin | Picocystis salinarum gen. et sp. nov. (Chlorophyta)a new picoplanktonic green alga[END_REF][START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF][START_REF] Komárek | Phenotype diversity of the cyanobacterial genus Leptolyngbya in the maritime Antarctic[END_REF]Dadheech et al. 2012a;Dadheech et al. 2012b;[START_REF] Dadheech | Desertifilum fontinale sp. nov. (Oscillatoriales, Cyanobacteria) from a warm spring in East Africa, based on conventional and molecular studies[END_REF][START_REF] Komárek | Filamentous cyanobacteria[END_REF].

Ultrastructural analyses

Cyanobacterial and microalgal strains were analyzed by Transmission Electron Microscopy (TEM) as described by [START_REF] Parveen | Bacterial communities associated with Microcystis colonies differ from free-living communities living in the same ecosystem[END_REF], with few modifications. Cells or filaments harvested from a growing culture by centrifugation were fixed with a mix of 2% glutaraldehyde, 2% formaldehyde, 0.18 M sucrose, 0.1% picric acid in 0.1 M, pH 7.4

Sorensen phosphate buffer (SPB) for 1h. The specimens were then washed three times with SPB and post fixed with 1% osmium tetraoxide for 1 h. Subsequently, they were washed with distilled water before being dehydrated in a graded ethanol series (30%, 50%, 70%, 90% and 100% ethanol), with agitation and centrifugation. The samples were embedded in EPON

Resin and sectioned at 0.5 µm using an ultra-microtome (Reichert-Jung Ultracut) with a diamond knife and transferred onto 150-mesh copper grids. The prepared sections were stained with 2% uranyl acetate/50% ethanol for 15 min and washed three times in 50% distilled water/50% ethanol and finally twice in distilled water. The copper grids were then dried and examined using a transmission electron microscope (TEM, Hitachi HT-7700, Japan). Images were taken using a digital camera (Hamamatsu, Japan).

Molecular and phylogenetic analyses

DNA extraction from cyanobacteria and microalgae strains was carried out with a QiaGen kit (Cat. N° 69506) following manufacturer's protocol. Mechanical lysis was carried out through an ultrasonic cell probe Vibra Sonic (Granuloshop) for 30 s at a range of 100 % to 32.5 W.

The amplification of the 16S rRNA gene, 16S-23S internal transcribed spacer region (ITS)

and cpcBA-IGS locus was done with primers and PCR programs already described in Gugger and Hoffman (2004), [START_REF] Iteman | Comparison of conserved structural and regulatory domains within divergent 16SrRNA-23S rRNA spacer sequences of cyanobacteria[END_REF] and [START_REF] Kumar | Similarity analysis of Spirulina/Arthrospira strains on the basis of phycocyanin operon locus (cpcB-IGS-cpcA) and 16S rRNA gene sequences[END_REF], respectively.

Amplification of 18S rRNA gene was performed using the universal primer 515F [START_REF] Caporaso | Global patterns of 16S rRNA diversity at a depth of millions of sequences per sample[END_REF]) and the eukaryotic primer 951R (TTG-GYR-AAT-GCT-TTC-GC). All PCR reactions were carried out using the thermocycler Techne Echne FTC-Plus / 02. The PCR products were then sequenced by GENOSCREEN (Lille, France).

Sequences were assembled and corrected using ClustalW in MEGA7 version software [START_REF] Kumar | MEGA7: Molecular Evolutionary Genetics Analysis Version 7.0 for Bigger Datasets[END_REF]. For 16S rRNA gene, a partial sequence of minimum 1 325 base pairs (bp) from the complete gene sequence (1 550 bp) was obtained. For 16S-23S ITS, cpcBA-IGS locus and 18S rRNA gene, partial sequences were obtained with a minimum of 460, 520 and 390 bp, respectively.

Phylogenetic analyses were performed according to three methods: neighbor joining (NJ), maximum likelihood (ML) and maximum parsimony (MP), using the MEGA7 version software with an equal to 1 000 iteration. For 16S rRNA gene phylogeny, an overall alignment (n = 114) was generated, including the new produced sequences and those available in GenBank belonging to Oscillatoriales, Synechococcales and Spirulinales. The selected sequences were all longer than 1000 bp. Gloeobacter violaceus PCC 7421 (order Chroococcales) was chosen as outgroup. A cut-off value of 95% of 16S rRNA gene sequence identity was used for genus definition [START_REF] Komárek | Recent changes (2008) in cyanobacteria taxonomy based on a combination of molecular background with phenotype and ecological consequences (genus and species concept)[END_REF]. The 18S rRNA gene phylogeny included the five ALCP strains and 26 sequences of GenBank. Euglena gracilis ASW 08025 was chosen as outgroup.

For Arthrospira, the 16S-23S ITS and cpcBA-IGS phylogenies were performed with four PMC Arthrospira strains (PMC 737.11,738.11,851.14,894.15) Identification numbers and habitat of all the strains are indicated in Table 1.

Morphological, ultrastructural and molecular characterization

Arthrospira fusiformis (Voronichin) Komárek et Lund Arthrospira fusiformis was found as floating filaments, in mats covering the lake surface or attached to some stromatolites distributed on the edge of the lake. The thallus was blue-green and diffluent. Trichomes were solitary, olive green, isopolar, cylindrical, variously lengthened (170-2 390 µm), always straight (Fig. 2a-c) or slightly wavy (Fig. 2 d-e). Coiled trichomes were never observed. Trichomes were often gradually attenuated at the ends (Fig. 2a), not constricted at the cross-walls. Cells were always shorter than wide, 9-12 μm wide, 3-7 µm long. Cell content was granular with abundant aerotopes (Fig. 2c andd), mainly concentrated near the cross-wall (Fig. 3a-c). Sheaths were generally absent, but they were observed in some trichomes (Fig. 2e). Motility (gliding) was always present. The apical cell was rounded (Fig. 3d), often with thickened outer cell wall (Fig. 2b andc) and sometimes calyptrate (Fig. 2a-c).

Cross-walls were visible under light microscope (Fig. 2d). Multiplication occurred by fragmentation of the trichome into hormogonia (Fig. 2f). The thylakoids showed a radial arrangement in the longitudinal and cross sections of the trichomes (Fig. 3a-f). The cells contained several cyanophycin granules (Fig. 3a-f) and carboxysomes (Fig. 3a, b ande). The septa between cells was strongly and regularly undulated (Fig. 3b andc). The 16S rRNA gene sequence analysis showed that Arthrospira strains from Dziani Dzaha shared ≥ 98% sequence identity with other Arthrospira strains (Fig. 4). The ITS region of Arthrospira strains comprised both tRNA Ile and tRNA Ala genes (data not shown). The phylogenetic tree of the 16S-23S ITS sequences of Arthrospira strains produced two main clusters (Fig. 5). The first cluster was mainly composed of strains of African or Indian origin (27 strains), four strains from Dziani Dzaha, two strains from other countries (Spain, Peru) and one strain of doubtful origin. The second cluster contained nine strains from the American continent (USA, Mexico, Peru), one strain from Thailand and one strain of doubtful origin. The D1-D1' secondary structure of Arthrospira strains from Dziani Dzaha was identical in terms of nucleotide number (62 nt) and sequence to those of African (A. fusiformis AY575929, Kenya) and Asian (A. indica PD1998/pus, India) strains (Fig. 6), while considerably different from those of American origin (GenBank no. FJ001892 and AJ292337). The phylogenetic tree of the cpcBA-IGS locus showed a similar pattern to that of the ITS tree and produced three clusters: one big cluster (I) mainly containing strains of African or Indian origin (23 strains), the Dziani Dzaha strains (4 strains) and one strain of doubtful origin (Thailand?), and two small clusters (II and III) mainly containing strains from Mexico (Supplementary figure 1). Strains of the genus Desertifilum were isolated from the stromatolites distributed on the edge of the lake. The thallus was dark green or blackish green, forming mats or empty balls in culture conditions. Filaments were solitary or entangled, more or less straight or wavy (Fig. 7a andb). Trichomes were cylindrical, pale blue-green, not or slightly constricted at cross-wall (Fig. 7a and b, 8a), with a colourless, thin and firm sheath attached to the trichome (Fig. 7b, 8a-d). Motility was present (slow gliding and oscillation). Cells were mainly longer than wide (rarely isodiametric), 2.4-3.4 µm wide, 3.4-5.9 µm long. The cell content was homogeneous, with numerous small granules and without aerotopes (Fig. 8a andb). The apical cell was conical-rounded (Fig. 7a and b, 8b), slightly hooked or bent (Fig. 7c), sometimes with extrusions (Fig. 7d). Cells contained numerous cyanophycin granules (Fig. 8a) and carboxysomes (Fig. 8c), and the thylakoids were parietally arranged in the peripheral zone (Fig. 8a andc). Type strain: a living strain deposited at Paris Museum Collection (PMC) under the no. PMC 872.14, Genbank MF579897.

Desertifilum dzianense

Type locality: stromatolites, saline-alkaline environment, crater-lake Dziani Dzaha, Mayotte.

Etymology: the specific epithet (dzianense) refers to the name of the lake where the strain was isolated (Dziani Dzaha).

Leptolyngbya-like filaments

Strains with thin filaments (≤3 µm wide) were divided into four morphotypes (I, II, III, IV) on the basis of cell dimension, morphology and presence of aerotopes (Table 3).

Morphotype I

Strains of the morphotype I were found as floating filaments, in mats covering the lake surface or attached to some stromatolites. The thallus was emerald green, forming thin mats.

These strains were characterized by filaments solitary, nearly straight, flexuous (Fig. 10a andb) or forming tight circular bundles in culture conditions (Fig. 10c-f). Trichomes were cylindrical, blue-green or yellow-green, slightly constricted at the cross walls (Fig. 11a andb)

and very slightly attenuated at the ends (Fig. 11c). The trichomes sometimes possessed a very fine, attached and diffluent sheath (Fig. 11c, d ande). Motility was always present, with gliding in straight trichomes and rotation in spirally coiled trichomes. Cells were longer than wide (up to 2 times), 2.4-3 μm wide, 3.2-4.9 µm long, with a refractive granule (potentially a polyphosphate granule) on either or both sides of the cross-walls (Fig. 10a, 11a-d), cyanophycin granules (Fig. 11e) and carboxysomes (Fig. 11f). The apical cell was rounded, hemispherical and often possessed a large polar granule (aerotope?) (Fig. 10b and11c). The thylakoids were parietally and concentrically arranged, parallel to the cell wall (10 to 11 layers) (Fig. 11a, b and f). Comparison of 16S rRNA gene sequences revealed that the morphotype I strains formed a separate clade with Geitlerinema from black band disease (BBD) of corals (93% of similarity) (Fig. 4).

Formal description of the genus and species: Type strain: a living strain deposited at Paris Museum Collection (PMC) under the no. PMC 869.14, GenBank MG772676.

Sodalinema komarekii M.
Type locality: water column and stromatolites, saline-alkaline environment, crater-lake Dziani Dzaha, Mayotte.

Etymology: Sodalinema= the name of the genus refers to the alkaline-saline habitat where this cyanobacterium was found (soda lake) and to the cyanobacterium morphology (nema= filament); komarekii= named in honour of Prof. RNDr. Jiří Komárek, a Czech phycologist from the Institute of Botany, Academy of Sciences of the Czech Republic.

Morphotype II

The strains corresponding to the morphotype II were isolated from the stromatolites. The thallus was bright-green, forming thin, flattened and expanded mats. Filaments were solitary or entangled, nearly straight or slightly flexuous (Fig. 12a andb). Trichomes were thin, cylindrical, pale blue-green, not attenuated at the ends and slightly constricted at cross-walls (Fig. 12a, b andc). The sheath covering the trichome was attached, colourless and diffluent (Fig. 12c, d ande). Motility was present (gliding). Cells were always longer than wide (up to 2.9 times) (Fig. 12c), 1.5-2 µm wide, 1.9-4.8 µm long. The apical cell was rounded, without calyptra or thickened outer cell wall, and often presented a refractive granule (Fig. 12b), corresponding to numerous aerotopes (Fig. 12d). Cells contained a few cyanophycin granules and the thylakoids were parietally arranged, parallel to the cell wall (5-6 layers) (Fig. 12e andf). Comparison of 16S rRNA gene sequences revealed that the morphotype II strains formed a single clade (Fig. 4).

Formal description of the genus and species: Type locality: stromatolites, saline-alkaline environment, crater-lake Dziani Dzaha, Mayotte.

Sodaleptolyngbya stromatolitii M.
Etymology: Sodaleptolyngbya = the name of the genus refers to the alkaline-saline habitat where this cyanobacterium was found (=soda lake) and to the morphological similarity to members of the genus Leptolyngbya; stromatolitii = the specific epithet refers to the sampling environment (=stromatolite).

Morphotype III

Haloleptolyngbya alcalis Dadheech, Mahmoud, Kotut et Krienitz Strains corresponding to the morphotype III were isolated from the water column. The thallus was downy, soft, forming mats, bright blue-green. Filaments were solitary or entangled, flexuous (Fig. 13a andb), sometimes wavy. Trichomes were pale blue-green, cylindrical, very constricted at cross-walls (Fig. 13a, c andd), not attenuated towards ends, consisting of very few (2) to several cells. Trichomes possessed a firm sheath with a thin transparent zone (Fig. 13c andd). Motility was not observed. Cells were isodiametric, shorter or longer than wide, barrel-shaped, 1.4-1.9 µm wide, 1.2-2.7 long, well separated from one another by cross-walls (Fig. 13c andd). The apical cell was conical-rounded, without calyptra or thickened outer cell wall (Fig. 13e). Thylakoids were parietally and concentrically arranged, parallel to the cell wall (5 layers) (Fig. 13c-f). Comparison of 16S rRNA gene revealed that the morphotype III strains were closely related to Haloleptolyngbya alcalis (GenBank no. JN712770.1) with 99% sequence similarity (Fig. 4). The ITS region of the morphotype III (with both tRNA Ile and tRNA Ala genes, data not shown) showed 98% similarity with H. alcalis (GenBank no.

JN712771) (Supplementary figure 3). The ITS secondary structures (D1-D1', Box-B and V3 regions) of the morphotype III were almost identical to those of H. alcalis (Fig. 14). Although the D1-D1' region of the morphotype III showed a few differences with H. alcalis in number (6 and 11 nt instead of 7 and 10 nt, respectively) and sequence of nucleotides in the two terminal loops, the rest of the structure was conserved (Fig. 14). The Box-B and V3 regions were identical to those of H. alcalis in terms of number and sequence of nucleotides (Fig. 14).

Morphotype IV

Strains corresponding to the morphotype IV were isolated from the water column. The thallus was bright-green, forming mats. Filaments were solitary or entangled, straight or slightly arcuated, flexuous, sometimes undulated (Fig. 15a andb). Trichomes were pale blue-green, thin, cylindrical, consisting of very few (2) to several cells, very constricted at cross-walls (Fig. 15c), not attenuated towards the ends (Fig. 15b). A firm sheath with a thin transparent zone covered the trichome (Fig. 15c-f). Cells were cylindrical, always longer than wide (up to 3 times) (Fig. 15c ande), 0.9-1.5 µm wide, 1.5-3.9 µm long. The apical cell was conicalrounded (Fig. 15e), without calyptra or thickened outer cell wall. The cell content possessed polyphosphate granules (Fig. 15c ande). Motility was not observed. Thylakoids were parietally and concentrically arranged, parallel to the cell wall (4 layers) (Fig. 15c-f).

Comparison of 16S rRNA gene sequences indicated that the strains of the morphotype IV were closely related to the Haloleptolyngbya alcalis (GenBank no. JN712770.1) cluster, but in a separated lineage, and shared 96% sequence similarity with this species (Fig. 4). The ITS sequence analysis (with both tRNA Ile and tRNA Ala genes, data not shown) revealed that the morphotype IV only shared 85% similarity with H. alcalis (GenBank no. JN712771) (Supplementary figure 3). ITS secondary structures analysis showed that the D1-D1' region of the morphotype IV (61 nt) differed from H. alcalis in the nucleotide sequence in both the terminal region and the main stem (Fig. 14). The Box-B region also showed substantial differences in terms of sequence and number of nucleotides (35 nt) with H. alcalis (49 nt).

Only the V3 region was identical for both taxa in terms of sequence and number of nucleotides (23 nt).

Formal description of the species: Diagnosis: Thallus bright-green, forming mats. Filaments solitary or entangled, straight or slightly arcuated, flexuous, sometimes undulated. Trichomes pale blue-green, thin, very constricted at cross-walls, not attenuated towards the ends, with a firm sheath with a thin transparent zone. Cells cylindrical, always longer than wide (up to 3 times), 0.9-1.5 µm wide.

Haloleptolyngbya
Apical cell conical-rounded. Motility was not observed.

Holotype: a cryopreserved and formaldehyde-fixed sample of the strain PMC 893.15 deposited at Paris Museum Collection (PMC), Paris, France.

Type strain: a living strain deposited at Paris Museum Collection (PMC) under the no. PMC 893.15, GenBank MF579910.

Type locality: water column, saline-alkaline environment, crater-lake Dziani Dzaha, Mayotte.

Etymology: the specific epithet (elongata) refers to the dimension of the cells which are up to 3 times longer than wide.

Spirulina subsalsa Oersted ex Gomont

Strains of Spirulina subsalsa were isolated from the stromatolites. The thallus was bright green and formed thin and flattened mats. Filaments were always entangled (Fig. 16a) Spirulina subsalsa (96% and 98% of similarity, respectively).

Trichomes

Picocystis salinarum Lewin

Strains of Picocystis salinarum were isolated from the water column. Cells were green, generally globular to ellipsoidal, some were trilobate, 2.5-4.5 µm diameter (Fig. 17a andb).

TEM observations showed a single nucleus in the cell and a lateral, usually cup-shaped and bilobed chloroplast (Fig. 17c, e and f), without pyrenoid but containing several starch grains (Fig. 17c). When cells were trilobate, the plastid was extending into two of the lobes and the nucleus into the third (Fig. 17c). A single elongated mitochondrion was located in the central cytoplasm, closely associated with the concavity of the chloroplast (Fig. 17c). Reproduction occurred by autosporulation leading to two daughter cells (Fig. 17d ande). Comparison of 18S rRNA gene sequences revealed that the strains from Dziani Dzaha shared 99% sequence identity with P. salinarum (GenBank no. KF615769.1) (Fig. 18).

DISCUSSION

This is the first study that characterizes the cyanobacteria and microalgae from Dziani Dzaha from strains isolated from the water column and stromatolites. This crater-lake is characterized by high salinity and alkalinity [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF]) as many soda lakes in Eastern Africa [START_REF] Ballot | Cyanobacteria and cyanobacterial toxins in the alkaline crater lakes Sonachi and Simbi, Kenya[END_REF][START_REF] Krienitz | Picocystis salinarum (Chlorophyta) in saline lakes and hot springs of East Africa[END_REF]. These extreme environmental conditions lead to high productivity but low diversity of phototrophic microorganisms [START_REF] Vonshak | Arthrospira (Spirulina): Systematics and ecophysiology[END_REF][START_REF] Melack | Saline and soda lakes[END_REF]), mostly dominated by Arthrospira [START_REF] Tomaselli | Spirulina platensis (Arthrospira): Physiology[END_REF][START_REF] Sili | Arthrospira (Spirulina)[END_REF][START_REF] Schagerl | Algal communities of Kenyan soda lakes with a special focus on Arthrospira fusiformis[END_REF]. In some way, the results obtained here agree with these findings since Arthrospira was the dominant taxon in Dziani Dzaha (>97% of the total biomass in all sampling occasions; [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF]).

However, using the polyphasic approach, several filamentous cyanobacteria belonging to the orders Oscillatoriales, Synechococcales and Spirulinales, and the prasinophyte Picocystis salinarum were also identified. The degree of coiling of the trichome has been considered as one of the main criteria to differentiate the two planktic Arthrospira species, the loosely coiled trichomes correspond to A. maxima and the tightly coiled trichomes to A. fusiformis [START_REF] Komárek | What is 'Spirulina platensis' in fact?[END_REF][START_REF] Li | Isolates identifiable as Arthrospira maxima and Arthrospira fusiformis (Oscillatoriales, Cyanobacteria) appear identical on the basis of a morphological study in culture and 16S rRNA gene sequences[END_REF][START_REF] Mussagy | The cyanophyte Arthrospira fusiformis from Mozambique, Africa: morphological and molecular characterization[END_REF]. However, the taxonomical separation of these two species based on this feature is questionable due to the high variability in the degree of coiling of the trichomes (from straight to helical and vice versa) observed in laboratory or culture conditions [START_REF] Hindák | Morphology of trichomes in Spirulina fusiformis Voronichin from lake Bogoria, Kenya[END_REF][START_REF] Bai | Competitive exclusion or morphological transformation? A case study with Spirulina fusiformis[END_REF][START_REF] Dadheech | Phylogenetic relationship and divergence among planktonic strains of Arthrospira (Oscillatoriales, Cyanobacteria) of African, Asian and American origin deduced by 16S-23S ITS and phycocyanin operon sequences[END_REF][START_REF] Sili | Arthrospira (Spirulina)[END_REF]. The finding of the straight morphotype of Arthrospira in Dziani Dzaha confirms that the coiled trichome criterion is unreliable to identify the planktic Arthrospira species. Indeed, this feature rendered difficult the identification of the material from Dziani Dzaha when only using the light microscope, but TEM observations provided several evidences to relate the studied strains with a planktic Arthrospira (e.g. radial thylakoids arrangement, presence of numerous aerotopes), later confirmed by the molecular analyses (≥ 98% 16S rRNA gene sequence similarity with other Arthrospira strains).

Arthrospira

Results based on 16S-23S ITS region and cpcBA-IGS locus showed that Arthrospira strains from Dziani Dzaha were phylogenetically related to strains originating from Africa and Asia, that according to [START_REF] Dadheech | Phylogenetic relationship and divergence among planktonic strains of Arthrospira (Oscillatoriales, Cyanobacteria) of African, Asian and American origin deduced by 16S-23S ITS and phycocyanin operon sequences[END_REF] correspond to A. fusiformis. This result was confirmed by the ITS secondary structure (D1-D1') analysis that showed that Arthrospira strains from Dziani Dzaha possessed the same number and sequence of nucleotides than the Arthrospira strains from African and Asian origin. This genotypic conservation suggests that Arthrospira strains from Dziani Dzaha, Africa and Asia belong to a unique taxonomic unit.

The use of diacritical (and more stable) features in the morphological and ultrastructural analyses, and the investigation of other gene loci or complete genome phylogenies will be necessary to resolve the complex taxonomy of this genus.

Soda lakes are often characterized by hidden diversity dominated by cyanobacterial taxa that possess simple morphology and lack of conspicuous traits [START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF][START_REF] Andreote | Non-heterocytous cyanobacteria from Brazilian saline-alkaline lakes[END_REF]. This makes their identification problematic and as a result many taxa are still unknown or loosely assigned. Using the polyphasic approach, new taxa of filamentous cyanobacteria were characterized from Dziani Dzaha. Within the Oscillatoriales, a new Desertifilum species was identified. Comparison of 16S rRNA gene sequences revealed that strains of this cyanobacterium shared high sequence similarity (≥98%) with Desertifilum tharense, D. fontinale and D. salkalinema, those are the only three species of the genus until now described (Dadheech et al. 2012a;[START_REF] Dadheech | Desertifilum fontinale sp. nov. (Oscillatoriales, Cyanobacteria) from a warm spring in East Africa, based on conventional and molecular studies[END_REF][START_REF] Cai | Desertifilum salkalinema sp. nov. (Oscillatoriales, Cyanobacteria) from an alkaline pool in China[END_REF]). Considerable differences in the ITS secondary structures (D1-D1', V2 and V3 regions) allowed to discriminate the Desertifilum strains from Dziani Dzaha from the three other Desertifilum species. In addition to the phylogenetic analyses, morphological observations showed that the filaments from Dziani Dzaha were narrower than in D. fontinale, whereas the difference from D. tharense and D. salkalinema mainly concerns the absence of gas vacuoles and the apical cell shape, respectively. Additionally, the ecology of the four taxa is different. While D.

tharense was isolated from desert crusts (Thar Desert, Rajasthan, India) (Dadheech et al. 2012a), D. fontinale from free floating mats in a warm spring (near Lake Bogoria, Kenya) [START_REF] Dadheech | Desertifilum fontinale sp. nov. (Oscillatoriales, Cyanobacteria) from a warm spring in East Africa, based on conventional and molecular studies[END_REF]) and D. salkalinema from an Arthrospira culture pool (Zhejiang Province, China) [START_REF] Cai | Desertifilum salkalinema sp. nov. (Oscillatoriales, Cyanobacteria) from an alkaline pool in China[END_REF], the Desertifilum strains from Dziani Dzaha were isolated from stromatolites. This study also showed that Desertifilum can thrive in ecosystems with higher salinity and alkalinity than those where the other members of the genus were found (Dadheech et al. 2012a;[START_REF] Dadheech | Desertifilum fontinale sp. nov. (Oscillatoriales, Cyanobacteria) from a warm spring in East Africa, based on conventional and molecular studies[END_REF][START_REF] Cai | Desertifilum salkalinema sp. nov. (Oscillatoriales, Cyanobacteria) from an alkaline pool in China[END_REF]. The habitat specificity of the strains studied here together with morphological, ultrastructural and molecular differences with the three Desertifilum mentioned above (Table 2) justify its classification as a new taxon.

Thus, Desertifilum dzianense sp. nov. is described here as a benthic filamentous cyanobacterium associated to stromatolites from tropical saline-alkaline lakes.

The order Synechococcales was the most diversified group from Dziani Dzaha isolated strains and included four Leptolyngbya-like cyanobacteria. The taxonomic identification of members of the genus Leptolyngbya is very problematic due to its simple morphology and minute dimension that limit the number and strength of discriminating features [START_REF] Komárek | Phenotype diversity of the cyanobacterial genus Leptolyngbya in the maritime Antarctic[END_REF]. In consequence, the genetic diversity exceeds the morphological diversity in this genus [START_REF] Albertano | Is the genus Leptolyngbya (Cyanophyte) a homogeneous taxon?[END_REF][START_REF] Casamatta | Molecular and ultrastructural characterization of ten polar and near-polar strains within the Oscillatoriales (Cyanobacteria)[END_REF][START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF][START_REF] Johansen | Leptolyngbya species from selected seep walls in the Great Smoky Mountains National Park[END_REF][START_REF] Andreote | Non-heterocytous cyanobacteria from Brazilian saline-alkaline lakes[END_REF][START_REF] Komárek | Filamentous cyanobacteria[END_REF]. The genus Leptolyngbya is defined by strains with very thin and simple trichomes (0.5-3.5 µm wide), sheaths present or facultative, without gas vesicles and peripherally arranged thylakoids [START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF][START_REF] Komárek | Filamentous cyanobacteria[END_REF]. These few morphological features are not discriminative enough to ensure a proper identification and as a consequence misidentifications often occur. For this reason, the different Leptolyngbya-like strains characterized in this study were separated in four morphotypes (I-IV) based onto morphological and ultrastructural features (Table 3).

Two of these morphotypes (I and II) were classified into new genera/species since they had very distinct morphological and ultrastructural characteristics and they were placed in single clusters in the phylogenetic tree based on 16S rRNA gene sequence data (Fig. 4). These two morphotypes possessed several morphological, ultrastructural and behavioral characters common with the genus Leptolyngbya [START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF][START_REF] Komárek | Filamentous cyanobacteria[END_REF]. However, apical cells in the strains from Dziani Dzaha always presented polar aerotopes or prominent granules and cells were always distinctly longer than wide, which according to [START_REF] Komárek | Filamentous cyanobacteria[END_REF] likely correspond to other genera. 16S

rRNA gene sequence data showed that the strains of the morphotype I were unrelated to the clusters containing Leptolyngbya and formed a separate clade with Geitlerinema from black band disease (BBD) (93% sequence similarity). Since the absence of sheath is defined as a diacritical character in Geitlerinema [START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF], the strains from Dziani Dzaha could not be classified into this genus as a sheath was observed in some filaments. In addition, these strains were located phylogenetically distant from clusters containing Geitlerinema sensu stricto (that include G. splendidum, G. carotinosum and G.

pseudacutissimum). This result also suggests that the Geitlerinema from BBD (GenBank no.

78064498, DQ151461.1) probably belongs to a different genus. The presence of sheath and the parietal and concentrically arranged thylakoids confirmed the affiliation of the strains from Dziani Dzaha to the order Synechococcales, family Leptolyngbyaceae [START_REF] Komárek | Taxonomic classification of cyanoprokaryotes (cyanobacterial genera) 2014, using a polyphasic approach[END_REF]. Since the studied strains of this cyanobacterium could not be assigned to any known taxon, the new genus Sodalinema with the type species S. komarekii was established based on the phylogenetic position (16S rRNA gene sequences) and substantial differences in the phenotypic features (morphology and ultrastructure) with other genera of Leptolyngbyaceae.

Interestingly, straight and dense spirally coiled filaments were observed within the same strains of this cyanobacterium. Although the occurrence of both morphologies in one genotype is not frequent (Komárek com. pers.), this phenomenon was also observed in Leptolyngbya borchgrevinkii from Antarctic marine waters [START_REF] Komárek | Phenotype diversity of the cyanobacterial genus Leptolyngbya in the maritime Antarctic[END_REF] and

Leptolyngbya tentaculiformis from an Australian thermal spring [START_REF] Mcgregor | Cyanobacterial composition of microbial mats from an Australian thermal spring: a polyphasic evaluation[END_REF]. The explanation for this morphological variability remains unknown.

The lineage of the morphotype II strains showed their distinctiveness from other cyanobacteria genera (boostrap value <50%). Although morphological and ultrastructural characteristics of the morphotype II correspond to the genus Leptolyngbya, the very distant position from Leptolyngbya sensu stricto clades (Fig. 4) and the presence of numerous aerotopes in the terminal cell suggest that it belongs to a new genus into the order Synechococcales. The strains from Dziani Dzaha also showed >6% 16S rRNA gene sequence divergence to its closest related taxa that also possess aerotopes in the terminal cell (i.e.

Limnothrix).

Based on these results, the morphotype II fulfill the two criteria required to characterize a new genus that include less than 95% genetic similarity combined with at least one autapomorphic character [START_REF] Komárek | Recent changes (2008) in cyanobacteria taxonomy based on a combination of molecular background with phenotype and ecological consequences (genus and species concept)[END_REF]. Thus, the strains of the morphotype II were classified into the new genus Sodaleptolyngbya with the type species S. stromatolitii, that was found only associated to the stromatolites.

Regarding the morphotype III, the results based on the morphological and ultrastructural features (cell dimension and shape, thylakoids arrangement) together with the gene sequence analyses (16S rRNA gene, 16S-23S ITS region and ITS secondary structures) demonstrated that it can be classified as Haloleptolyngbya alcalis. This finding confirmed the affinity of this species to the high salinity and alkalinity typically found in soda lakes (Dadheech et al. 2012b). The morphotype IV formed a closely (but separated) related lineage with H. alcalis cluster (Fig. 4). The cut-off value of 95% of 16S rRNA gene sequence identity suggested for genus definition [START_REF] Komárek | Recent changes (2008) in cyanobacteria taxonomy based on a combination of molecular background with phenotype and ecological consequences (genus and species concept)[END_REF]) allowed to classify the morphotype IV into the genus Haloleptolyngbya (96% sequence similarity with H. alcalis). However, morphological and ultrastructural features indicated that this morphotype did not belong to the species H. alcalis.

While the cells in H. alcalis are wider and barrel-shaped (Dadheech et al. 2012b, this study), the morphotype IV is characterized by cylindrical, longer and thinner cells. The 16S-23S ITS region and secondary structures analyses confirmed the distinction between both taxa. These results allowed to describe a new Haloleptolyngbya species, identified as Haloleptolyngbya elongata sp. nov., a planktic filamentous cyanobacterium from tropical saline-alkaline lakes.

These findings highlighted the importance of using the polyphasic approach to solve the complex taxonomy of Leptolyngbya-like filaments and suggest the verification of the different Leptolyngbya strains that occur in different clades in the phylogenetic tree of 16S rRNA gene [START_REF] Albertano | Is the genus Leptolyngbya (Cyanophyte) a homogeneous taxon?[END_REF]Dadheech et al. 2012b).

Among the order Spirulinales, Spirulina subsalsa was identified in the strains isolated from Dziani Dzaha, whose environment corresponds well to the typical habitat of the species (marine biotopes, inland salty waters) [START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF]. Although the phototrophic community was dominated by cyanobacteria, bi-and trilobate cells of the prasinophyte Picocystis salinarum were also identified. Picocystis was previously reported in ponds and lakes with high salinity in North America, Asia and Africa [START_REF] Lewin | Picocystis salinarum gen. et sp. nov. (Chlorophyta)a new picoplanktonic green alga[END_REF][START_REF] Hollibaugh | Stratification of microbial assemblages in Mono Lake, California, and response to a mixing event[END_REF][START_REF] Fanjing | Characterization of a eukaryotic picoplankton alga, strain DGN-Z1, isolated from a soda lake in Inner Mongolia, China[END_REF][START_REF] Krienitz | Picocystis salinarum (Chlorophyta) in saline lakes and hot springs of East Africa[END_REF][START_REF] Krienitz | Tiny and Tough: Microphytes of East African Soda Lakes[END_REF].

According to [START_REF] Lewin | Picocystis salinarum gen. et sp. nov. (Chlorophyta)a new picoplanktonic green alga[END_REF], trilobate forms of P. salinarum occur in old, nutrientdepleted cultures. However, trilobate cells were found here in both culture and environmental conditions where no nutrient limitation occurred.

Based on the isolated strains, the phytoplankton diversity in Dziani Dzaha was similar to that found by several authors in other saline-alkaline lakes worldwide, mainly characterized by Arthrospira fusiformis, Leptolyngbya-like filaments, Haloleptolyngbya alcalis and Picocystis salinarum [START_REF] Tomaselli | Spirulina platensis (Arthrospira): Physiology[END_REF][START_REF] Lewin | Picocystis salinarum gen. et sp. nov. (Chlorophyta)a new picoplanktonic green alga[END_REF]Dadheech et al. 2012b;[START_REF] Krienitz | Picocystis salinarum (Chlorophyta) in saline lakes and hot springs of East Africa[END_REF][START_REF] Sili | Arthrospira (Spirulina)[END_REF][START_REF] Dadheech | Cyanobacterial diversity in the hot spring, pelagic and benthic habitats of a tropical soda lake[END_REF][START_REF] Andreote | Non-heterocytous cyanobacteria from Brazilian saline-alkaline lakes[END_REF][START_REF] Schagerl | Algal communities of Kenyan soda lakes with a special focus on Arthrospira fusiformis[END_REF]. Since microalgal and cyanobacteria studies on stromatolites are scarce [START_REF] Stal | Cyanobacterial mats and stromatolites[END_REF][START_REF] Abed | Characterization of microbialite-forming cyanobacteria in a tropical lagoon: Tikehau atoll, Tuamotu, French Polynesia[END_REF][START_REF] Foster | Molecular and morphological characterization of cyanobacterial diversity in the stromatolites of Highborne Cay, Bahamas[END_REF]), many taxa still remain unknown. This was the case in this study, where apart from Spirulina subsalsa, which was already known to be associated to stromatolites [START_REF] Abed | Characterization of microbialite-forming cyanobacteria in a tropical lagoon: Tikehau atoll, Tuamotu, French Polynesia[END_REF], the other cyanobacteria isolated from this structures are reported here for the first time (Arthrospira fusiformis, Desertifilum dzianense sp. nov., Sodalinema komarekii gen. nov., sp. nov. and Sodaleptolyngbya stromatolitii gen. nov., sp.

nov.). Stromatolites have a long fossil record and are considered among the oldest evidence of the first forms of life on earth [START_REF] Knoll | A new molecular window on early life[END_REF]. In modern environments, benthic cyanobacteria are thought to be responsible for the formation of these organosedimentary structures in terms of organic input and structural integrity [START_REF] Abed | Characterization of microbialite-forming cyanobacteria in a tropical lagoon: Tikehau atoll, Tuamotu, French Polynesia[END_REF], thus their polyphasic characterization from modern stromatolites in Dziani Dzaha could offer interesting clues about the primitive microbial assemblage on early life.

In this lake, only the top meter is photosynthetically active because of the high turbidity and low light penetration [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF]. These conditions favor the dominance of cyanobacteria, that are the most low light-adapted group among photosynthetic organisms [START_REF] Schwaderer | Eco-evolutionary differences in light utilization traits and distributions of freshwater phytoplankton[END_REF]. All the cyanobacteria taxa isolated from Dziani Dzaha possess straight or elongate morphology (= high surface:volume ratio) which allows to maximize the surface exposed to light (Naselli-Flores and Barone 2011) in this extremely turbid ecosystem.

Straight filaments are also more susceptible to grazing than the coiled ones [START_REF] Padisák | Sinking properties of some phytoplankton shapes and the relation of form resistance to morphological diversity of planktonan experimental study[END_REF][START_REF] Kaggwa | Ecomorphological variability of Arthrospira fusiformis (Cyanoprokaryota) in African soda lakes[END_REF] but the risk of being grazed in Dziani Dzaha seems to be low as no primary consumers as zooplankton, fishes or Lesser Flamingos have been identified so far [START_REF] Leboulanger | Microbial diversity and cyanobacterial production in Dziani Dzaha crater lake, a unique tropical thalassohaline environment[END_REF]. The real factors leading to the morphological specificity of the strains characterized in this study should be explored in greater detail in the future. Furthermore, studies focusing on the environmental microbial diversity in relation with the geochemical processes will be helpful to understand the functioning of this lake.

  or 0.2 μm filtered Dziani Dzaha raw water. Isolations were carried out by repeated transfers of single cells or filaments on solid or liquid media (at least 3 times) under an inverted microscope (Nikon ECLIPSE TS100). Growing clones were later cultured in 25 cm 2 culture flasks (Nunc, Roskilde, Denmark) containing 10 ml of Z8X-salt medium. Cyanobacteria strains were maintained in the Paris Museum Collection (PMC) and microalgae strains in the Algothèque Laboratoire Cryptogamie Paris (ALCP) at 25°C, using daylight fluorescent tubes providing an

  M. Cellamare, C. Duval, N. Touibi, C. Djediat et C. Bernard sp. nov.

  figure 2). Substantial differences were observed between the ITS secondary structures (D1-D1', V2 and V3 regions) of Desertifilum dzianense and the three Desertifilum species mentioned above (Fig.9). The number of nucleotides of the D1-D1' region of D. dzianense was identical to that from D. fontinale (nt=57) but differed from D. tharense and D. salkalinema (nt=56). In terms of nucleotide sequence, the D1-D1' region of D. dzianense differed from the three other Desertifilum species in the terminal loop, with D. fontinale in the basal and side loops, and with D. salkalinema in the basal loop. Significant differences between the V2 region of D. dzianense and the other Desertifilum species were observed in the different portions in terms of sequence and number of nucleotides. Comparing the V3 secondary structure of D. dzianense with the other Desertifilum species, differences in the nucleotide sequence were observed in the terminal portion. Differences in the number of nucleotides with D. fontinale were also observed in the terminal portion of this region.

  elongata M. Cellamare, C. Duval, N. Touibi, C. Djediat et C. Bernard sp. nov.

  were blue-green, 1.6-2.2 µm wide, 50-287 µm long, regularly densely screw-like coiled (Fig. 16 b), straight or variously curved. Trichomes were rapidly motile, with screwlike rotation and simultaneous gliding. Cell content was homogeneous and the apical cell was rounded. Sequence analyses of the 16S rRNA gene and 16S-23S ITS region (with only the tRNA Ile gene, data not shown) confirmed that the strains from Dziani Dzaha belong to

  strains from Dziani Dzaha shared several morphological characteristics with planktic Arthrospira species, including presence of aerotopes, cell dimension (width and length), apical cell rounded with thickened outer cell wall, sometimes calyptrate, trichome gradually attenuated at the ends, not constricted at the evident cross-walls and motility by gliding[START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF], However, the strains from Dziani Dzaha always presented straight trichomes. To the best of our knowledge, this is the first time that Arthrospira trichomes with constant straight morphology are observed in natural populations, which are typically characterized by spirally coiled trichomes[START_REF] Vonshak | Arthrospira (Spirulina): Systematics and ecophysiology[END_REF][START_REF] Komárek | Cyanoprokaryota 2. Teil: Oscillatoriales[END_REF] and only become straight under laboratory conditions or intensive cultures[START_REF] Hindák | Morphology of trichomes in Spirulina fusiformis Voronichin from lake Bogoria, Kenya[END_REF][START_REF] Bai | Competitive exclusion or morphological transformation? A case study with Spirulina fusiformis[END_REF][START_REF] Tomaselli | Spirulina platensis (Arthrospira): Physiology[END_REF][START_REF] Li | Isolates identifiable as Arthrospira maxima and Arthrospira fusiformis (Oscillatoriales, Cyanobacteria) appear identical on the basis of a morphological study in culture and 16S rRNA gene sequences[END_REF] Wang and Zhao 2005;[START_REF] Mühling | Phenotypic analysis of Arthrospira (Spirulina) strains (cyanobacteria)[END_REF][START_REF] Noor | Spirulina culture in Bangladesh XII. Effects of different culture media, different culture vessels and different cultural conditions on coiled and straight filament characteristics of Spirulina[END_REF][START_REF] Sili | Arthrospira (Spirulina)[END_REF]. The straight morphology of the trichomes isolated from Dziani Dzaha was conserved also under culture conditions.

  Wang ZP, Zhao Y. Morphological reversion of Spirulina (Arthrospira) platensis (Cyanophyta): from linear to helical. J Phycol 2005;41:622-8. Zuker M. Mfold web server for nucleic acid folding and hybridization prediction. Nucleic Acids Res 2003;31:3406-15.

Figure 1 .

 1 Figure 1. Location of Dziani Dzaha (Mayotte Island), situated in the northern Mozambique Channel in the Indian Ocean (a). Mayotte consists in a main island, Grande-Terre (a) and a smaller island, Petite-Terre (a, b). Dziani Dzaha is a round crater-lake in the Petite-Terre island (b, c) with two different biotopes: water column (d, e) and stromatolites (f).

Figure 2 .Figure 3 .Figure 4 .

 234 Figure 2. Light microscopy micrographs of Arthrospira fusiformis strains (PMC 851.14) from Dziani Dzaha. a: general view of a filament showing a gradual attenuation towards the ends; b-c: apical cell rounded with thickened outer cell wall and calyptrate; d: filament showing a very visible cross-wall; e: filament with sheath; f: filament with hormogonium. Scale bars: 10 µm.

Figure 5 .Figure 6 .Figure 7 .Figure 8 .

 5678 Figure 5. Consensus phylogenetic tree based on 16S-23S ITS sequences of Arthrospira strains. Arthrospira strains from Lake Dziani are shown in bold. The other strains used are from Dadheech et al. (2010) and GenBank. Bootstrap values are given in the following order: neighbor-joining/maximum likelihood/maximum parsimony. Only support values > 50% are shown.

Figure 9 .Figure 10 .

 910 Figure 9. 16S-23S ITS secondary structures (D1-D1', V2 and V3) of Desertifilum dzianense from Dziani Dzaha and other Desertifilum species (Dadheech et al. 2012a; Dadheech et al. 2014; Cai et al. 2017).

Figure 11 .Figure 12 .

 1112 Figure 11. TEM micrographs of Sodalinema komarekii (strain PMC 869.14) from Dziani Dzaha. Longitudinal (a-d) and cross sections (e-f) of the trichome showing the ultrastructural details. Figure b also shows a potentially polyphosphate granule and a cell undergoing binary fission (arrows indicate cell constriction). Abbreviations: ac: apical cell, ca: carboxysome, cc: cross-wall constriction, cy: cyanophycin granule, p: polyphosphate granule, s: sheath, t: thylakoids. Scale bars: a, b, e, f = 500 nm; c, d = 1µm.

Figure 13 .

 13 Figure 13. Light microscopy (a-b) and TEM (c-f) micrographs of Haloleptolyngbya alcalis (strain PMC 892.15) from Dziani Dzaha. Solitary filament (a), entangled filaments and trichome with sheath (b), longitudinal (c-e) and cross sections (f) of the trichome showing the ultrastructural details. Abbreviations: ac: apical cell, cc: cross-wall constriction, p: polyphosphate granule, s: sheath, t: thylakoids, tz: transparent zone. Scale bars: a, b = 10 µm, c, d, e, f = 500 nm.

Figure 14 .Figure 15 .Figure 16 .

 141516 Figure 14. 16S-23S ITS secondary structures (D1-D1', Box-B and V3) of Haloleptolyngbya alcalis and H. elongata from Dziani Dzaha and H. alcalis from Dadheech et al. (2012b). M: morphotype.

Figure 17 .

 17 Figure 17. Light microscopy (a-b) and TEM micrographs (c-f) of Picocystis salinarum (a, b, d, e, f: strain ALCP 144.1; c: environmental sample) from Dziani Dzaha. a: trilobate cells; b: trilobate and slightly bilobed cells; c: trilobate cell with starch grains and chloroplasts in the lateral lobes. The nucleus and the mitochondrion are located in the third lobe; d: autospores and daughter cells within the mother cell wall; e: daughter cells within the mother cell wall; f: detail of the figure e showing the cell-wall and the cup-shaped chloroplast. Abbreviations: cp: chloroplast; cw: cell-wall; m: mitochondrion; n: nucleus; s: starch grain. Scale bars: a-b = 10 µm; c, e = 1 µm; d = 2 µm; f = 200 nm.

Figure 18 .

 18 Figure 18. Neighbor joining phylogenetic tree based on 18S rRNA gene sequences of the prasinophyte Picocystis salinarum. Numbers above branches indicate bootstrap support (>50 %) from 1000 replicates. Picocystis strains from Dziani Dzaha strains are in bold.

  Cellamare, C. Duval, N. Touibi, C. Djediat et C. Bernard gen. Thallus emerald green, diffluent, forming thin mats. Filaments solitary, nearly straight, flexuous, blue-green or yellow-green. Trichome cylindrical, slightly constricted at the cross walls, very slightly attenuated at the ends, sometimes with a very fine, attached and diffluent sheath. Cells longer than wide, 2.4-3 μm wide. Cell content with a large refractive granule on either or both sides of the cross-walls. Apical cell rounded, hemispherical, often with a large granule (aerotope?). Motility present.

	nov., sp. nov.

Diagnosis:

Holotype: a cryopreserved and formaldehyde-fixed sample of the strain PMC 869.14 deposited at Paris Museum Collection (PMC), Paris, France.

  Cellamare, C. Duval, N. Touibi, C. Djediat et C. Bernard gen. nov., sp. nov.

	Diagnosis: Thallus bright-green, forming thin, flattened and expanded mats. Filaments
	solitary or entangled, nearly straight or slightly flexuous. Trichomes thin, cylindrical, pale
	blue-green, not attenuated at the ends, slightly constricted at cross-walls, sheaths thin,
	attached, diffluent, colourless. Cells longer than wide, 1.5-2 µm wide. Apical cell rounded,
	with a refractive granule (numerous aerotopes with TEM). Motility present.
	Holotype: a cryopreserved and formaldehyde-fixed sample of the strain PMC 867.14
	deposited at Paris Museum Collection (PMC), Paris, France.

Type strain: a living strain deposited at Paris Museum Collection (PMC) under the no. PMC 867.14, GenBank MF579918.

Table 1 .

 1 List of cyanobacteria and microalgae strains isolated from Dziani Dzaha and their corresponding strain numbers, habitat and sampling dates. PMC: Paris Museum Collection. ALCP: Algothèque Laboratoire Cryptogamie Paris. DZ10-10: October 2010. DZ14-04: April 2014.

	Taxa	Strain number	Habitat	Sampling
				date
	Arthrospira fusiformis	PMC 737.11; 738.11	Water column	DZ10-10
	Arthrospira fusiformis	PMC 851.14 to 853.14	Stromatolites	DZ14-04
	Arthrospira fusiformis	PMC 894.15; 912.15 ; 917.15	Water column	DZ14-04
	Desertifilum dzianense sp. nov. PMC 871.14 to 876.14; 890.15;	Stromatolites	DZ14-04
		901.15; 902.15; 905.15 ; 906.15;
		909.15
	Sodalinema komarekii gen.	PMC 741.11 to 746.11	Water column	DZ10-10
	nov., sp. nov. (Morphotype I)
	Sodalinema komarekii gen.	PMC 861.14 to 865.14 ; 869.14	Stromatolites	DZ14-04
	nov., sp. nov. (Morphotype I)
	Sodalinema komarekii gen.	PMC 886.15 to 889.15; 898.15;	Water column	DZ14-04
	nov., sp. nov. (Morphotype I)	899.15; 907.15
	Sodaleptolyngbya stromatolitii	PMC 866.14; 867.14	Stromatolites	DZ14-04
	gen. nov., sp. nov.
	(Morphotype II)
	Haloleptolyngbya alcalis	PMC 891.15, 892.15	Water column	DZ14-04
	(Morphotype III)
	Haloleptolyngbya elongata sp.	PMC 893.15; 895.15; 900.15;	Water column	DZ14-04
	nov. (Morphotype IV)	903.15; 904.15
	Spirulina subsalsa	PMC 857.14, 858.14	Stromatolites	DZ14-04
	Picocystis salinarum	ALCP 144.1 to 146.1; 148.1; 149.1	Water column	DZ14-04

Table 2 .

 2 Comparison of the morphological features of Desertifilum dzianense sp. nov. with other Desertifilum species.

	Characteristics	Desertifilum	Desertifilum	Desertifilum	Desertifilum
		dzianense	tharense	fontinale	salkalinema
	Aerotopes	Absent	Present	Absent	Absent
	Filament towards	Attenuated at the	Attenuated at the	Attenuated at the	Attenuated or
	the end	ends	ends	ends	not
					attenuated at
					the ends,
	Filament	Unconstricted or	Unconstricted or	Unconstricted or	Unconstricted
	constriction	slightly constricted	slightly constricted	slightly constricted	or slightly
		at cross walls	at cross walls	at cross walls	constricted at
					cross walls
	Filament type	Straight or slightly	Straight or slightly	Straight or slightly	Straight or
		wavy/solitary or	wavy/solitary or	wavy/solitary or	slightly
		entangled	entangled	entangled	wavy/
					entangled
	Sheath	Thin, colourless,	Thin, colourless,	Thin and	Thin,
		attached to	attached to	colourless	colourless,
		trichome	trichome		attached to
			(agglutinated)		trichome
	Filament color	Blue-green	-	Blue-green	-
	Motility	Present	Present	Present	Present
	Necridic cells	Absent	-	Present	-
	Thylakoids	Parietal	Parietal	Parietal	Parietal
	arrangement
	Apical cell	Conical shaped	Conical or variable Conical shaped	Slightly
		with rounded		with rounded	elongated
		apices, slightly		apices, sometimes
		hooked or bent,		with extrusions
		sometimes with
		extrusions
	Cell shape	Cylindrical, mainly	Cylindrical,	Cylindrical,	Cylindrical,
		longer than wide	isodiametric,	isodiametric,	longer than
		(rarely	longer than wide	shorter than wide	wide
		isodiametric)
	Cell width (µm)	2.4 -3.4	2.0-3.7	4-7	2.08 (±0.8)
	Cell length (µm)	3.4 -5.9	-	-	5.14 (±0.5)
	Habitat	Stromatolites	Desert crust or arid Free floating mats	Alkaline
		(Dziani Dzaha		in a warm spring	cultivated
		lake, Mayotte)		near Lake Bogoria,	pool
				Kenya
	Reference	This study	Dadheech et al.	Dadheech et al.	Cai et al.
			(2012a)	(2014)	(2017)

Table 3 .

 3 Comparison of the Leptolyngbya-like strains (morphotypes I-IV) isolated from Dziani Dzaha (Mayotte).

	Characteristics Morphotype I Morphotype II	Morphotype III Morphotype IV
		Sodalinema	Sodaleptolyngbya	Haloleptolyngbya	Haloleptolyngbya
		komarekii gen.	stromatolitii	alcalis	elongata sp. nov.
		nov., sp. nov.	gen. nov., sp.
			nov.
	Filament shape Nearly	Nearly straight,	Flexuous,	Straight, slightly
		straight,	slightly flexuous	sometimes wavy	arcuated,
		flexuous,			flexuous,
		forming			sometimes
		circular			undulated
		bundles
	Filament	Very slightly	Not attenuated	Not attenuated	Not attenuated
	towards the end	attenuated
	Constriction at	Slightly	Slightly	Very constricted Very constricted
	cross-wall	constricted	constricted
	Sheath	Present	Present	Present	Present
	Transparent	Absent	Absent	Present	Present
	zone
	Trichome	Present	Present	Absent	Absent
	motility
	Thylakoids	Parietal	Parietal	Parietal	Parietal
	arrangement
	Cell shape	Cylindrical,	Cylindrical,	Barrel-shaped,	Cylindrical,
		longer than	longer than wide	isodiametric,	longer than wide
		wide		shorter or longer	(up to 3 times)
				than wide
	Cell width (µm) 2.4-3	1.5-2	1.4-1.9	0.9-1.5
	Apical cell type Rounded,	Rounded	Conical-rounded Conical-rounded
		hemispherical
	Aerotopes	Refractive	Present	Absent	Absent
		granule in the	(numerous
		apical cell (?)	aerotopes in the
			apical cell)
	Particular	A large	A large refractive
	features	refractive	granule in the	-	-
		granule on	apical cell
		either or both	(numerous
		sides of the	aerotopes)
		cross-walls and
		in the apical
		cell
	Habitat	Water column	Stromatolites	Water column	Water column
		and
		stromatolites

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ACKNOWLEDGEMENTS

The authors highly thanks Professor J. Komárek for the valuable comments for the description of the new cyanobacteria taxa. They also thanks P. Got, D. Jezequel, G. Sarazin, P. Cadeau and E. Fouilland for their support during prospecting campaigns on Dziani Dzaha, N. Villeriot for 18S rRNA gene phylogeny and R. Peiffer for preliminary MET studies. This work was supported by Agence Nationale de la Recherche (France), http://www.agence-nationalerecherche.fr (grant n° ANR-13-BS06-0001), and Total Corporate Foundation, project DZAHA (grant n° C001493). Thanks to the Deep Carbon Observatory community for informative discussions on the importance of characterizing the ecosystems related to volcanic CO 2 emissions. The authors also acknowledge the anonymous referees for providing valuable suggestions that significantly improved the quality of the manuscript.

Arthrospira sp. AB2006/01 (FJ001876.1) Kenya Arthrospira sp. AB2006/02 (FJ001877.1) Kenya Arthrospira platensis Sp-2 (DQ279768.1) Chad Arthrospira indica PD1997/ram (AY575931.1) India Arthrospira fusiformis AB2002/11 (AY575929.1) Kenya Arthrospira fusiformis AB2002/10 (AY575928.1) Kenya Arthrospira fusiformis AB2002/05 (AY575927.1) Kenya Arthrospira fusiformis AB2002/04 (AY575926.1) Kenya Arthrospira fusiformis AB2002/03 (AY575925.1) Kenya Arthrospira fusiformis AB2002/02 (AY575924.1) Kenya Arthrospira fusiformis AB2002/01 (AY575923.1) Kenya Arthrospira sp. PCC 9223 (AJ292335.1) Spain Arthrospira sp. strain Titicaca (AJ292331.1) Peru Arthrospira sp. SP-10 (AJ292325.1) Madagascar Arthrospira fusiformis PMC 894.15 Dziani Dzaha, Mayotte Arthrospira fusiformis PMC 851.14 Dziani Dzaha, Mayotte Arthrospira fusiformis PMC 737.11 Dziani Dzaha, Mayotte Arthrospira fusiformis PMC 738.11 Dziani Dzaha, Mayotte Arthrospira sp. AB2006/03 (FJ001878.1) Kenya Arthrospira sp. AB2006/04 (FJ001879.1) Kenya Arthrospira sp. AB2006/05 (FJ001880.1) Kenya Arthrospira sp. AB2006/06 (FJ001881.1) Kenya Arthrospira sp. KR2005/113 (FJ001898.1) Kenya Arthrospira sp. KR2005/117 (FJ001900.1) Kenya Arthrospira sp. KR2005/118 (FJ001901.1) Uganda Arthrospira sp. PD2002/08 (FJ001906.1) India Arthrospira maxima str. Lefevre 1963/M-132-1 (FJ798612.1) Chad Arthrospira indica strain MCRC isolate straight (AJ292336.1) India Arthrospira indica PD1998/pus (AY575930.1) India Arthrospira indica PD2002/ana (AY575932.1) India Arthrospira sp. PD2002/02 (FJ001902.1) India Arthrospira sp. PD2002/04 (FJ001903.1) India Arthrospira sp. PD2002/06 (FJ001904.1) India Arthrospira sp. PCC8005 (X70769.2) India, Kenya, Mexico or Peru (?) Arthrospira platensis UTEX 1928 (AJ292322.1) USA Arthrospira platensis SP-1 (AJ292324.1) Mexico Arthrospira sp. PCC 9108 (AJ292329.1) ? Arthrospira platensis SAG 257.80 (AJ292337.1) Peru Arthrospira sp. AB2006/11 (FJ001885.1) Mexico Arthrospira sp. AB2006/12 (FJ001886.1) Mexico Arthrospira sp. AB2006/15 (FJ001889.1) Mexico Arthrospira sp. KR2004/06 (FJ001894.1) Mexico Arthrospira sp. KR2004/09 (FJ001895.1) Mexico Arthrospira sp. KR2004/12 (FJ001897.1) Mexico

CONCLUSIONS

The polyphasic approach allowed to identify a higher number of taxa than those determined using only the morphological evaluation. The identification of new taxa in such extreme environment was possible because of the strains isolation effort. They could be further used as taxonomic references for metagenomic studies on samples from Dziani Dzaha and other soda lakes. The results obtained also revealed a number of open questions concerning the morphological variability of phototrophic microorganisms and the low pressure from predators under extreme environmental conditions.

AUTHOR CONTRIBUTIONS

CB, HA, MA, CL, CD and CB have worked on the design of the experiments. CD, YD, CD, NT and CB have performed the experiments. MC, CD, NT and CB have performed the data analysis. MC, CD and CB have written the paper. CB co-led the project with CL and MA.