Simon M Dittami
email: simon.dittami@sb-roscoff.fr

Svenja Heesch

Jeanine L Olsen

Jonas Collén

TRANSITIONS BETWEEN MARINE AND FRESHWATER ENVIRONMENTS PROVIDE NEW CLUES ABOUT THE ORIGINS OF MULTICELLULAR PLANTS AND ALGAE

Keywords: APG -Angiosperm Phylogeny Group Macroalgae, aquatic plants, marine-freshwater and freshwater-marine transitions, adaptation, evolution

Ca -calcium

CaCO3-calcium carbonate Cl --chloride Gya

FRESHWATER AND MARINE HABITATS

Water bodies cover 71% of the Earth's surface, either as oceans, lakes, rivers or as ice [START_REF] Ott | Meereskunde. Ulmer[END_REF]). Approximately 97.6% of all water on Earth is saline and only 2.4% is fresh water (although estimates vary depending on the author). Most of this fresh water is found as groundwater or as ice, and only 0.009% occurs within lakes and rivers [START_REF] Wetzel | Limnology[END_REF]. Fresh water is defined as water with a salinity ≤ 0.5 g dissolved salts•L -1 (International Symposium for the Classification of Brackish Waters 1958), and the global average for lakes and rivers is 0.12 g•L -1 of salt [START_REF] Wetzel | Limnology[END_REF]. The reason for differences in salinity among water bodies is the water cycle: surface water (both salty and fresh) evaporates and precipitates as fresh water. A part of this fresh water precipitates on land and gradually flows back to the sea, dissolving and transporting salts on its way. In parallel, the evaporation of large basins in combination with the rise of land masses generates new terrestrial salt reserves. Ocean salinity has been modeled to have varied over the last 600 My between 30 to 60 g•L -1 with a current mean of 35 g•L -1 [START_REF] Hay | Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life[END_REF]. Mainly the sodium(Na)/potassium(K) balance in seawater is thought to have shifted in favor of Na over time due to the higher potential of K to bind to clay and thus to be removed from seawater [START_REF] Macintyre | Why the Sea is Salt[END_REF].

Seawater is not just a concentrated version of fresh water, as a number of processes impact ion distribution in each environment. For one, salts (including nutrients) are washed into the ocean at different rates. Thus, the availability of macro-and micronutrients differs significantly between freshwater and marine environments. Primary production in the majority of freshwater ecosystems is most often limited by the availability of phosphorus [START_REF] Schindler | Evolution of phosphorus limitation in lakes[END_REF], whereas marine environments are primarily limited by nitrogen [START_REF] Hecky | Nutrient limitation of phytoplankton in freshwater and marine environments: A review of recent evidence on the effects of enrichment[END_REF][START_REF] Zehr | Nitrogen cycling in the ocean: new perspectives on processes and paradigms[END_REF] or trace elements such as iron [START_REF] Martin | Iron deficiency limits phytoplankton growth in Antarctic waters[END_REF]). Also, once eluted from the soil, calcium ions (Ca 2+ ) readily precipitate as calcium carbonate (CaCO3), whereas Na + remains in solution and is efficiently transported to the ocean.

Freshwater environments are therefore mainly Ca 2+ -dominated whereas seawater is Na +dominated. Nevertheless, there is usually more total carbonate in marine environments, compared to fresh water (Table 1).

Fresh water composition depends more on the surrounding terrestrial environment and is highly variable compared to seawater. Factors that influence fresh water composition include chemical interactions with the soil, the geological context, weathering, precipitation (quantity and quality), temperature, stream discharge, nutrient uptake, and physical and biological transformations [START_REF] Feller | Human impacts on streams and rivers[END_REF]. Geologists distinguish between rock-dominated and precipitation-dominated fresh water [START_REF] Gibbs | Mechanisms controlling world water chemistry[END_REF]. Rock-dominated fresh water is usually found at higher altitudes and salt concentrations are highly dependent on the material of the basin; it is usually rich in silicate (Si) and K. Precipitation-dominated fresh water occurs primarily in leached areas with high rainfall. Its ion composition reflects that of the rain, with ions occurring in similar proportions as in seawater, even in water bodies that are far inland.

Finally, evaporation has a strong impact on the ion composition of fresh water, increasing salinity and removing CaCO3 due to precipitation from solution.

Life on Earth is generally thought to have originated 4 billion years ago in primordial, hypersaline oceans [START_REF] Martin | Hydrothermal vents and the origin of life[END_REF] with salinities > 80 g•L -1 [START_REF] Pinti | The origin and evolution of the oceans[END_REF]. From there, bacteria and eukaryotes gradually colonized freshwater and then terrestrial environments.

Recently, an alternative scenario has been proposed in which life first arose in non-marine settings (i.e. subaerial or freshwater) [START_REF] Wellman | The terrestrial biota prior to the origin of land plants (embryophytes): a review of the evidence[END_REF] and only later colonized marine habitats. It is possible that early eukaryotes including the ancestors of the major macro-algal lineages also originated and diversified in this non-marine setting 2-1.1 Gya, and that the colonization of marine environments may have been stimulated during times of lethal land surface conditions. In either scenario, habitat transitions were key events in the early evolution of photosynthetic, multicellular eukaryotes.

However, most extant lineages -from archaeans to metazoans -are found in both fresh water and seawater, indicating that each of them also independently made a transition between these environments at least once in their recent evolutionary history. When, how often, and under what conditions these transitions have occurred (and can occur) are questions fundamental to our understanding of habitat diversification in the broadest sense and to explain the distribution of marine, freshwater, and terrestrial biodiversity.

In this review, we first compile an inventory of extant marine-freshwater and freshwatermarine transitions in multicellular, photosynthetic eukaryotes (Figure 1) and then discuss our findings in the context of current hypotheses about their evolutionary and geological history.

We define multicellular photosynthetic protists/algae as a physically connected set of genetically identical cells with the ability to produce distinctive reproductive structures, although we also included some siphonous algae in our review. We consider an alga as marine if it naturally occurs and reproduces in seawater (see above), and as freshwater, if it naturally occurs and reproduces in fresh water (salinity ≤ 0.5 g•L -1 ). Species found in brackish water are, in almost all cases, also able to tolerate marine conditions and were thus considered marine. Terrestrial species are those found outside of waterbodies, although some species also occur in fresh water, and some might require regular exposure to fresh water (e.g. abundant rain for reproduction). Please note that, across the different lineages examined in this review, particular taxonomic ranks (families, genera, species) are not always equivalent.

CENSUS OF MARINE-FRESHWATER AND FRESHWATER-MARINE

TRANSITIONS

Unicellular and non-photosynthetic multicellular organisms

Unicellular protists and bacteria occur in extremely large populations with generally short generation times and fast evolutionary rates [START_REF] Baer | Mutation rate variation in multicellular eukaryotes: causes and consequences[END_REF]). These properties facilitate large-scale dispersal into different environments. Nevertheless, in a recent study [START_REF] Logares | Infrequent marine-freshwater transitions in the microbial world[END_REF] demonstrated that among these organisms, separate clades have adapted to either marine or freshwater environments, a separation probably due in part to the energetic costs associated with osmoregulation and ion homeostasis.

Multicellular organisms (with exceptions) are generally characterized by comparatively smaller population sizes, longer generation times and a slower rate of evolution [START_REF] Baer | Mutation rate variation in multicellular eukaryotes: causes and consequences[END_REF]). Nevertheless, many of these lineages too, are found in both marine and freshwater environments. In animals, marine to freshwater transitions (and vice versa) have been particularly well-studied in fish [START_REF] Vega | Why are there so few fish in the sea?[END_REF]. In addition, several species of fish are also able to migrate between fresh water and seawater in the course of their lives. These diadromous fish rely on specialized membrane transporters and compatible osmolytes to adjust intracellular osmolarity and ion concentrations. Active osmoregulation, however, depends on a sufficient supply of energy. Plants and macroalgal species that live permanently submerged have to deal with reduced light quality and quantity and thus reduced energy supply. Adaptations in photosynthetic pigments [START_REF] Dawes | Marine Botany[END_REF] and/or specific structural and physiological adaptations related to osmotolerance for marine vs. freshwater conditions are therefore required (see section 4).

Archaeplastida

The supergroup Archaeplastida or Plantae (includes: Glaucophyta, the Viridiplantae, and the Rhodophyta) arose from a primary endosymbiosis event with a cyanobacterium and a heterotrophic eukaryote [START_REF] Mcfadden | Primary and secondary endosymbiosis and the origin of plastids[END_REF]. It is the most diverse group of multicellular photosynthetic eukaryotes, reaching substantial abundances in marine, freshwater, and terrestrial habitats. The Glaucophytes are rare, unicellular organisms at the root of the Archaeplastida that occur predominantly in fresh water, although recent 18S metabarcoding data from the Tara Oceans Project revealed glaucophyte sequences in oceanic environments (see Supplementary dataset W5 in [START_REF] De Vargas | Eukaryotic plankton diversity in the sunlit ocean[END_REF]. The Viridiplantae (the green lineage) split into two monophyletic groups, the Streptophyta, and the Chlorophyta, approximately 1,200-750 million years ago [START_REF] Becker | Streptophyte algae and the origin of embryophytes[END_REF] and includes green algae, charophytes, and terrestrial plants. Rhodophytes (red algae) are mainly multicellular marine algae but also comprise freshwater and a very few, partially terrestrial species [START_REF] Sheath | Freshwater algae of North America: ecology and classification[END_REF].

Streptophyta

The most basal multicellular streptophytes are the Klebsormidiophyceae (Figure 1A). They are small, filamentous freshwater and terrestrial algae; only 41 different species of this class are currently registered in AlgaeBase (Guiry and Guiry 2015). Some members of the Klebsormidiophyceae can survive and grow in saline waters up to 60 g•L -1 (Karsten and Rindi 2010), and Klebsormidium marinum (Deason) P.C.Silva, K.M.Mattox & W.H.Blackwell has been found close to the sea [START_REF] Deason | Filamentous and colonial soil algae from Dauphin Island, Alabama[END_REF]. However, we are unaware of any confirmed findings in marine environments. The Zygnematophyceae (= Conjugatophyceae) is a group of small unicellular or filamentous algae with over 3000 species (Guiry and Guiry 2015).

Among them, the Zygnematales are generally considered a freshwater order [START_REF] Lee | Phycology[END_REF], although a few findings of Spirogyra spp. have also been recorded from saline lakes [START_REF] Hammer | Saline lake ecosystems of the world[END_REF]) and estuaries [START_REF] Attrill | A Rehabilitated Estuarine Ecosystem[END_REF]. The second order within this class, the Desmidiales, comprises thousands of microscopic freshwater species [START_REF] Brook | The biology of desmids[END_REF][START_REF] Hall | Conjugating green algae including desmids[END_REF]. The multicellular representatives (Spondylosium, Desmidium, Onychonema, Hyalotheca, Cosmocladium, Groenbladia, Sphaerozosma) (McCourt et al. 2000), are almost exclusively freshwater organisms. To our knowledge, the only (possible) exception is an unconfirmed report of Sphaerozosma vertebratum Brébisson ex Ralfs from Indian coastal waters [START_REF] Thayalan | Comparative spatial assessment of phytoplankton and productivity in coastal fresh water pond, estuary and neritic water of Palk Bay, South East India[END_REF]. The Coleochaetophyceae, with 35 species, are also predominantly found in fresh water [START_REF] Cook | Chlorokybophyceae, Klebsormidiophyceae, Coleochaetophyceae. Handbook of the Protists[END_REF].

Higher morphological complexity, including tissue differentiation and more elaborate body plans, is found in the Charophyceae, comprising only a single order, the Charales [START_REF] Lee | Phycology[END_REF].

Charales, also called stoneworts due to the fact that some species in the group can become heavily calcified, are closely related to terrestrial plants. Their thalli can be up to 1 m long, and they form underwater meadows in freshwater ponds and streams. Although the vast majority of the ~700 known species of Charales (Guiry and Guiry 2015) grow in fresh water [START_REF] John | Filamentous (nonconjugating) and plantlike green algae[END_REF], there are also examples from brackish water, notably in the upper Baltic Sea [START_REF] Schubert | Charophytes of the Baltic Sea[END_REF]) and salt-contaminated lakes in Australia [START_REF] Burne | Saline lake charophytes and their geological significance[END_REF]. Lamprothamnium spp. generally inhabit estuaries and coastal pools where the salinity can vary rapidly between fresh and full strength seawater. Tolypella salina R.Corillion is found in salt marshes along the French Atlantic coast [START_REF] Lambert | Tolypella salina R. Cor. on the French Atlantic coast: biology and ecology[END_REF], while the majority of Tolypella species occur in fresh water. It may be assumed that within the Charales at least two independent transitions from freshwater to marine environments have taken place.

The largest group of streptophytes are the Embryophyta (land plants). They separated from the charophytes and the Zygnematophyceae [START_REF] Wodniok | Origin of land plants: Do conjugating green algae hold the key?[END_REF], and colonized a terrestrial habitat approximately 470 million years ago [START_REF] Sanderson | Molecular evidence on plant divergence times[END_REF]. Embryophytes are currently estimated to comprise 200,000 to 450,000 species [START_REF] Scotland | How many species of seed plants are there?[END_REF]. About 4,500 of them live in aquatic 1 environments. However, [START_REF] Les | Phylogenetic studies in Alismatidae, II: Evolution of marine angiosperms (seagrasses) and hydrophily[END_REF] estimated that only 130 species of land plants have entirely returned to fully submerged aquatic environments. This is presumed to have happened during several independent transitions, which have occurred in the Alismatidae, the Ceratophyllaceae (Ceratophyllales), and Callitrichaceae (Asterids). While in the cases of Ceratophyllaceae and Callitrichaceae the entire families are restricted to fresh water (possibly due to their lack of roots and rhizomes) [START_REF] Arber | Water plants. a study of aquatic angiosperms[END_REF], five families within the lower monocot order Alismatales made the transition to 1 Please note that our use of aquatic includes both fresh water and seawater marine environments on at least two occasions (Figure 1B). As the sister-group of each of the marine groups of seagrasses is found in fresh water, and as freshwater forms are clearly more common than marine forms, a transition from fresh water to seawater is more parsimonious than a direct transition from terrestrial environments as noted by [START_REF] Les | Phylogenetic studies in Alismatidae, II: Evolution of marine angiosperms (seagrasses) and hydrophily[END_REF]. In addition, there may have been a secondary re-colonization of freshwater environments by a marine ancestor within the Potamogetonaceae (suggested in Figure 1).

Chlorophyta

Approximately 80% of the >6,000 green algal species (Guiry and Guiry 2015) are freshwater and about 20% are marine or brackish species [START_REF] John | Filamentous (nonconjugating) and plantlike green algae[END_REF]. Microscopic species are usually found in fresh water and macrophytic taxa in marine waters [START_REF] John | Filamentous (nonconjugating) and plantlike green algae[END_REF].

Multicellular chlorophytes have developed in four lineages, the Palmophyllales (Mamiellophyceae), the Trebouxiophyceae, the Chlorophyceae, and the Ulvophyceae [START_REF] Leliaert | Phylogeny and molecular evolution of the green algae[END_REF]. Here, we attempt to give an overview of marine-freshwater or freshwater-marine transitions in these classes, keeping in mind that for many of the species, molecular phylogenies are lacking and highly convergent morphology severely limits the utility of morphological classification.

The Palmophyllales constitute a small group of early-branching chlorophytes with only three genera (Palmophyllum, Verdigellas, and Palmoclathrus) restricted to deep-water or low light marine environments [START_REF] Zechman | An unrecognized ancient lineage of green plants persists in deep marine waters[END_REF][START_REF] Leliaert | Into the deep: new discoveries at the base of the green plant phylogeny[END_REF].

The Trebouxiophyceae comprise 700 species (Guiry and Guiry 2015), of which some members enter symbiotic relationships with fungi to form lichens, while others are common in freshwater plankton. Although this group consists mainly of unicellular or simple colonyforming representatives, the Microthamniales, Phyllosiphonales, and Prasiolales also form filaments and simple thalli. Freshwater forms comprise the majority of the Trebouxiophyceae, and multicellular representatives include only freshwater (Microthamniales) and terrestrial forms (Phyllosiphonales). To our knowledge, there are no known multicellular marine forms within the Trebouxiophyceae [START_REF] Leliaert | Phylogeny and molecular evolution of the green algae[END_REF]). At least one clade of the Prasiolales, however, frequently occurs in the supralittoral of cold-temperate and polar oceans, and a few species can be cultivated in vitro in full-strength seawater medium [START_REF] Rindi | Molecular phylogeny of the green algal order Prasiolales (Trebouxiophyceae, Chlorophyta)[END_REF][START_REF] Heesch | Prasiolales (Trebouxiophyceae, Chlorophyta) of the Svalbard Archipelago: diversity, biogeography and description of the new genera Prasionella and Prasionema[END_REF].

The Chlorophyceae comprise ~3,500 species (Guiry and Guiry 2015), many of which are unicellular or colonial (notably the Chlamydomonadales). These groups are not included in this review. Among the filamentous forms, most occur primarily in fresh water [START_REF] John | Filamentous (nonconjugating) and plantlike green algae[END_REF], although several freshwater to marine transitions have occurred. One freshwater to seawater transition probably arose in the Chaetopeltidales, a freshwater/terrestrial order that also comprises marine species in the genus Pseudulvella [START_REF] Sanchez-Puerta | Pseudulvella americana belongs to the order Chaetopeltidales (class Chlorophyceae), evidence from ultrastructure and SSU rDNA sequence data[END_REF]). While mostly from fresh water, the Chaetophorales [START_REF] Caisová | Polyphyly of Chaetophora and Stigeoclonium within the Chaetophorales (Chlorophyceae), revealed by sequence comparisons of nuclear-encoded SSU rRNA genes[END_REF] and references therein), comprise eleven marine or partially marine genera (Arthrochaete, Didymosporangium, Elaterodiscus, Endophyton, Entodictyon, Gongrosira, Kymatotrichon, Protoderma, Sporocladopsis, Uronema, Thamniochloris; see e.g. [START_REF] Lee | A catalogue of the marine algae of the Canadian Arctic[END_REF][START_REF] Nielsen | Marine algae within calcareous shells from New Zealand[END_REF][START_REF] Stuercke | Variation in algal turf species composition and abundance on two Hawaiian shallow subtidal reefs[END_REF].

The Ulvophyceae are a very diverse group of multicellular green algae with 1,700 species (Guiry and Guiry 2015), including unicellular, multicellular, siphonous/coenocytic (single cell with multiple nuclei), and siphonocladous (several cells each with multiple nuclei) forms.

Different groups are thought to have independently evolved multicellularity (Cocquyt et al. 2010a). The Ulvophyceae are mainly marine, but some species are also found in freshwater (Ichihara et al. 2009a) or terrestrial habitats. A few orders of Ulvophyceae such as the Scotinosphaerales and the Oltmannsiellopsidales are not included here as they comprise only simple or unicellular algae (e.g. Škaloud et al. 2013 and references therein).

Within the multicellular Ulvophyceae, only the Trentepohliaceae (100 species; Guiry and Guiry 2015) are thought to contain exclusively terrestrial/freshwater species [START_REF] Lopez-Bautista | Molecular systematics of the subaerial green algal order Trentepohliales: an assessment based on morphological and molecular data[END_REF]. Its sister clade, comprising the Cladophorales, Bryopsidales, and Dasycladales (Cocquyt et al. 2010b), together accounting for >1,000 species (Guiry and Guiry 2015), is predominantly marine. The Bryopsidales include siphonous and siphonocladous algae that can form dense seabed meadows. All known members are marine. This order also comprises the invasive species Caulerpa taxifolia (M.Vahl) C.Agardh. The Cladophorales are generally considered a marine order, but freshwater taxa are known from both of its principal clades [START_REF] Hanyuda | Phylogenetic relationships within Cladophorales (Ulvophyceae, Chlorophyta) inferred from 18s rRNA gene sequences, with special reference to Aegagropila linnaei[END_REF], thus corresponding to at least two independent adaptations. One clade comprises the genera Aegagropila (freshwater, including the so-called lake balls or Marimo balls), Pithophora (freshwater), Arnodiella (freshwater), Wittrockiella (marine), and some Cladophora (marine and freshwater). Of the species examined by [START_REF] Hanyuda | Phylogenetic relationships within Cladophorales (Ulvophyceae, Chlorophyta) inferred from 18s rRNA gene sequences, with special reference to Aegagropila linnaei[END_REF], the second clade comprises only two species (Rhizoclonium hieroglyphicum (C.Agardh) Kützing and Cladophora glomerata (L.) Kützing) that occur in seawater and fresh water, and the remaining 29 are marine. The Dasycladales are mainly unicellular or siphonous and are dominantly marine [START_REF] Lee | Phycology[END_REF], although e.g. Batophora oerstedii J.Agardh was also found in freshwater environments [START_REF] Valet | Approche paléoécologique du monde des Dasycladales à partir de l'écologie des formes actuelles[END_REF].

The Ulotrichales are a heterogeneous group of primarily uniseriate, filamentous green algae comprising 170 species (Guiry and Guiry 2015). They are found in marine, brackish, freshwater, and terrestrial environments (e.g. [START_REF] Friedl | Phylogenetic relationships of green algae assigned to the genus Planophila (Chlorophyta): evidence from 18S rDNA sequence data and ultrastructure[END_REF]Lee 2008 and references therein). As the morphology of these species is highly variable and few molecular phylogenies are available, the number of transitions between freshwater and marine environments cannot be estimated. In some cases species thought to belong to the same genus Ulothrix zonata (F.Weber & Mohr) Kützing (freshwater; [START_REF] Graham | Light and temperature as factors regulating seasonal growth and distribution of Ulothrix zonata (Ulvophyceae)[END_REF]) vs. Ulothrix implexa (Kützing) Kützing (marine; [START_REF] Bartsch | The marine macroalgae of Helgoland (North Sea): an annotated list of records between 1845 and 1999[END_REF]).

The Ulvales, represented by >300 species (Guiry and Guiry 2015), are characterized by a cylindrical or sheet-like morphology [START_REF] Lee | Phycology[END_REF]. The Bolbocoleaceae and the Phaeophilaceae are considered marine (e.g. Hauck 1876; Wynne 2011), while the Cloniophoraceae are found only in freshwater or brackish environments. Members of the genus Dilabifilum are found mainly in freshwater or terrestrial habitats [START_REF] John | Filamentous (nonconjugating) and plantlike green algae[END_REF] but also hypersaline environments [START_REF] Vinogradova | Terrestrial algae of hypersaline environments of the Central Syvash islands (Kherson Region, Ukraine)[END_REF]. The largest families within the Ulvales, i.e.

the Kornmanniaceae, Ulvaceae, and Ulvellaceae, are represented by both freshwater and marine taxa, though they are mainly marine (Wehr and Sheath 2003). In each of these families, it is likely that multiple independent colonizations of fresh water have occurred. For example, most members of the genus Blidingia are marine, but some populations of the otherwise marine Blidingia marginata (J.Agardh) P.J.L.Dangeard ex Bliding have been encountered in fresh water (as Blidingia minima var. ramifera Bliding, nom. inval.) [START_REF] Iima | Morphology, ontogeny and phylogeny of freshwater green alga, Blidingia minima var. ramifera (Ulvales, Ulvophyceae) from Nikko, Tochigi prefecture, Japan[END_REF]). Four species in the genus Pseudendoclonium (P. akinetum Tupa, P. basiliense Vischer, P. laxum D.M.John & L.R.Johnson, and P. prostratum Tupa) have been described from fresh water [START_REF] Tupa | An investigation of certain chaetophoralean algae[END_REF][START_REF] Whitton | Coded List of Freshwater Algae[END_REF]. This may well constitute another transition, although the monophyly of the genus remains questionable [START_REF] Mullins | A molecular phylogenetic assessment of Pseudendoclonium[END_REF]. Most species within the Ulvaceae are marine or brackish water species, but several independent transitions to fresh water have occurred in the genus Ulva. U. flexuosa Wulfen (comprising U.

Rhodophyta

Red algae occur in both freshwater and marine environments [START_REF] Hirano | Freshwater algae in the Antartic regions[END_REF][START_REF] Vis | Distribution and systematics of Batrachospermum (Batrachospermales, Rhodophyta) in North America. 9. Section Batrachospermum: description of five new species[END_REF]. Approximately 97% of the 7,000 described species (Guiry and Guiry 2015) are marine, occurring in both intertidal and subtidal zones. Compared with green algae, there are only very few unicellular representatives.

Freshwater species exist in almost all red algal orders [START_REF] Sheath | Freshwater algae of North America: ecology and classification[END_REF]: only one higher taxon of red algae, the subclass Ahnfeltiophycidae with its 11 described species, lacks freshwater representatives (Figure 1A). Most freshwater red algae live in rivers and streams rather than in lakes (Wehr and Sheath 2003). They are usually filamentous (Figure 2) and smaller than marine species, with a narrower size range from 1-10 cm vs. <1-30 cm for marine species [START_REF] Sheath | Freshwater ecology[END_REF].

There are also red algal species that occur in both fresh water and seawater, e.g. Bangia atropurpurea (Mertens ex Roth) C.Agardh and Bostrychia moritziana (Sonder ex Kützing) J.Agardh [START_REF] Youngs | The cell wall chemistry of Bangia atropurpurea (Bangiales, Rhodophyta) and Bostrychia moritziana (Ceramiales, Rhodophyta) from marine and freshwater environments[END_REF]). The related species, Bostrychia scorpioides (Hudson) Montagne, is common in saltmarshes worldwide. Members of the genus Hildenbrandia (but different species) are likewise present in both environments. Based on a molecular phylogeny of North American taxa there are several independent marine and freshwater clades (Sherwood and Sheath 1999). In European Hildenbrandia spp., the freshwater species form a monophyletic group, suggesting that only one transition to fresh water took place in Europe [START_REF] Sherwood | European freshwater Hildenbrandia (Hildenbrandiales, Rhodophyta) has not been derived from multiple invasions from marine habitats[END_REF] and that the ancestral state was marine.

The Porphyridiophyceae, the Stylonematophyceae, the Compsopogonophyceae, and the Rhodellophyceae (together comprising 130 species; Guiry and Guiry 2015) contain species whose unicellular or simple filamentous morphologies occur in both marine and freshwater environments; as do members of the subclass Nemaliophycidae ( 900 species; Guiry and Guiry 2015). Of the eight orders three are exclusively freshwater, one is mixed and four are marine [START_REF] Lam | Multigene phylogeny of the red algal subclass Nemaliophycidae[END_REF]. Only very few microscopic species of red algae have been reported as terrestrial, notably Porphyridium purpureum (Bory) K.M.Drew & R.Ross [START_REF] Hoffmann | Algae of terrestrial habitats[END_REF], P. sordium Geitler, and Rufusia pilicola Wujek & Timpano [START_REF] Sheath | Freshwater algae of North America: ecology and classification[END_REF].

Exclusively marine groups have mainly been confined to the Corallinophycidae (>700 species; Guiry and Guiry 2015) since members of this group have calcified cell walls [START_REF] Lee | Phycology[END_REF]) (but see Charales). It was only in 2013 that the first freshwater species of Corallinophycidae, Pneophyllum cetinaensis Kaleb, Zuljevic & Peña, was discovered in a Croatian river characterized by extremely high pH and concentrations of calcium carbonate (Žuljević et al. 2016). Among most freshwater habitats, there is a general over-representation of red algae in softer water, however, it has been suggested that recent invaders of fresh water are restricted to ion-rich environments [START_REF] Sheath | Freshwater ecology[END_REF].

Stramenopila

Phaeophyceae

The largest class among multicellular stramenopiles is the Phaeophyceae, which comprises forms from tiny filaments to giant kelps. Brown algae are very rare in fresh water. Of 2,000 currently accepted brown algal species (Guiry and Guiry 2015), only seven (0.35%) have been found in fresh water, and all are small, filamentous or crust-forming (Figure 2; see below). They have been classified within three families (although one genus, Porterinema, is currently incertae sedis): the Ectocarpaceae, the Lithodermataceae, and the Sphacelariaceae, the latter two families both members of the order Sphacelariales [START_REF] Silberfeld | An updated classification of brown algae (Ochrophyta, Phaeophyceae)[END_REF].

The Ectocarpaceae are small and filamentous. Two species have independently colonized fresh water. One transition was made by Pleurocladia lacustris A. Braun, which has been found in both marine and in freshwater environments, but more frequently in freshwater [START_REF] Wehr | Discovery of the rare freshwater brown alga Pleurocladia lacustris (Ectocarpales, Phaeophyceae) in California Streams[END_REF]). Its sister species, P. lucifuga (Kuckuck) Wilce, is exclusively marine. The second transition may have occurred in Ectocarpus. A strain of E. subulatus Kützing [START_REF] Peters | Barcoding of cryptic stages of marine brown algae isolated from incubated substratum reveals high diversity in Acinetosporaceae (Ectocarpales, Phaeophyceae)[END_REF]) isolated from a freshwater environment in Australia [START_REF] West | Ectocarpus siliculosus (Dillwyn) Lyngb. from Hopkins River Falls, Victoria -the first record of a freshwater brown alga in Australia[END_REF] is still able to grow in full marine medium [START_REF] Dittami | Towards deciphering dynamic changes and evolutionary mechanisms involved in the adaptation to low salinities in Ectocarpus (brown algae)[END_REF]. It is currently the only freshwater report of Ectocarpus, with the exception of one report from a highly salt-contaminated river in Germany [START_REF] Geissler | Die salzbelastete Flusstrecke der Werra -ein Binnenlandstandort für Ectocarpus confervoides (Roth) Kjellmann[END_REF]. Given the phylogenetic position of the freshwater strains of Ectocarpaceae, the most parsimonious explanation for these results would be two independent colonizations of fresh water within the family.

Two additional and probably independent transitions to fresh water have occurred within the Sphacelariales. At least one occurred in the Lithodermaceae, which comprises four genera of small crust-or tuft-forming algae; two of which (Lithoderma and Pseudolithoderma) are exclusively marine, and two others (Bodanella and Heribaudiella), with one species each, are restricted to fresh water. Heribaudiella fluviatilis (Areschoug) Svedelius can, at times, be one of the dominant species of benthic algae in smaller rivers (Wehr and Sheath 2003). Within the Sphacelariaceae, Sphacelaria has two freshwater species: Sphacelaria lacustris Schloesser & Blum reported from Lake Michigan, USA [START_REF] Schloesser | Sphacelaria lacustris sp. nov., a freshwater brown alga from Lake Michigan[END_REF] [START_REF] Belcher | An unknown filamentous brown alga (Phaeophyceae) from a domestic freshwater aquarium in Northampton UK[END_REF]).

Among the larger brown algae, especially among species belonging to the Fucales, temporary tolerance for low salinity is commonly observed. For example, Fucus ceranoides var. limnicola S.M.Baker & M.H.Bohling is frequently found in upper estuaries with high freshwater influence [START_REF] Khfaji | The effects of salinity on the distribution of Fucus ceranoides[END_REF] and F. radicans L.Bergström & L.Kautsky is permanently submerged in waters of low salinity (3-5 g•L -1 ) in the northern Baltic [START_REF] Bergstrom | Genetic and morphological identification of Fucus radicans sp. nov. (Fucales, Phaeophyceae) in the brackish Baltic Sea[END_REF]. Also, populations of F. vesiculosus L. have been observed at low salinities in the Baltic, but have lost sexual reproduction [START_REF] Tatarenkov | Intriguing asexual life in marginal populations of the brown seaweed Fucus vesiculosus[END_REF]. Several ecophenes of Fucus are also known from salt marshes and have previously been referred to as F. cottonii M.J. Wynne & Magne (Neiva et al. 2012). There is no documented occurrence of wracks/rockweeds in freshwater or terrestrial habitats. Likewise, the largest brown algae, belonging to the Laminariales (or kelps), are exclusively marine.

In summary, we know of maximally six marine-freshwater transitions in brown algae. None have led to diversification or wide-spread dominance in freshwater systems. Given the overwhelming dominance of marine representatives it is likely that the common ancestor of brown algae was indeed marine; however, this does not extend to all stramenopiles.

MARINE AND FRESHWATER ORIGINS OF PHOTOSYNTHETIC

EUKARYOTES

In order to understand the directionality of transitions between fresh water and seawater in photosynthetic eukaryotes, we have to understand the evolution of these organisms in their deep time, ancestral environments. It is generally assumed that photosynthesis was first acquired by a common eukaryotic ancestor of the red, green and glaucophyte lineages; and then subsequently transmitted to the haptophyte, cryptophyte, and stramenopile lineages via secondary or tertiary endosymbiosis events [START_REF] Archibald | The puzzle of plastid evolution[END_REF]. The timing of the primary endosymbiosis event is still a matter of debate but estimated at between 2.1 Gya and 900 mya, depending on the methods used and the interpretation of fossils [START_REF] Han | Megascopic eukaryotic algae from the 2.1-billion-year-old negaunee iron-formation[END_REF][START_REF] Cavalier-Smith | Megaphylogeny, cell body plans, adaptive zones: causes and timing of eukaryote basal radiations[END_REF][START_REF] Parfrey | Estimating the timing of early eukaryotic diversification with multigene molecular clocks[END_REF][START_REF] Mcfadden | Origin and evolution of plastids and photosynthesis in eukaryotes[END_REF]. The physical environment of the primary endosymbiosis event remains uncertain, but some indications can be derived from the cyanobacterium that became the plastid and from the host.

On the plastid side, phylogenetic analyses based on 30 different cyanobacterial taxa have show that the cyanobacterial group closest to plastids contains both freshwater/terrestrial (e.g. Synechocystis) and marine genera (e.g. Trichodesmium) (Ochoa de [START_REF] Ochoa De Alda | The plastid ancestor originated among one of the major cyanobacterial lineages[END_REF].

However, a more recent and comprehensive study by [START_REF] Ponce-Toledo | An early-branching freshwater cyanobacterium at the origin of plastids[END_REF] has identified the freshwater cyanobacterium Gloeomargarita lithophora as the closest known relative of plastids. Moreover, based on ancestral state reconstructions of early plastids and cyanobacteria, a freshwater origin seems to be more consistent (Blank 2013b). A recent comparison of cyanobacterial genomes [START_REF] Dagan | Genomes of Stigonematalean cyanobacteria (subsection V) and the evolution of oxygenic photosynthesis from prokaryotes to plastids[END_REF]) concluded that a freshwater origin was probably necessary for water-splitting photosynthesis. It has also been suggested that early cyanobacteria (prior to the primary endosymbiosis event) likely lacked important genes involved in the synthesis of the compatible solutes trehalose, glucosylglycerol, glucosylglycerate and glycine betaine (Blank 2013a), implying that they may not have been able to colonize seawater at that time. This view is supported by estimates of the chemical composition of the early oceans at the time of primary endosymbiosis in which ancient seawater was probably two to three times more saline than today [START_REF] Huston | The geology and metallogeny of volcanic-hosted massive sulfide deposits: variations through geologic time and with tectonic setting[END_REF]) and much richer in Ca 2+ and iodine [START_REF] Pinti | The origin and evolution of the oceans[END_REF]. Moreover, in the period from 2500 -580 mya, the oceans were likely anoxic or only moderately oxic [START_REF] Anbar | Proterozoic ocean chemistry and evolution: a bioinorganic bridge?[END_REF][START_REF] Johnston | Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age[END_REF]).

On the host side, the earliest branching within the Archaeplastida is still not resolved with certainty (Jackson and Reyes-Prieto 2014) but is thought to have been between the ancestor of the green and red lineages, and the glaucophytes [START_REF] Rodríguez-Ezpeleta | Monophyly of primary photosynthetic eukaryotes: green plants, red algae, and glaucophytes[END_REF]. The fact that extant members of the glaucophytes occur almost exclusively in fresh water [START_REF] Kies | Typification of the Glaucocystophyta[END_REF] suggests that ancestral glaucophytes may also have been freshwater organisms.

However, even if this is true, it is still unclear whether this also applies to the ancestor of all Archaeplastida. In the case of red algae, the earliest branching group of the red algae, the Cyanidales, occurs in hot springs with varying salinity and chemical composition [START_REF] Reeb | Red Algae in the Genomic Age[END_REF]. Therefore, and based on their reduced genomes [START_REF] Collén | Genome structure and metabolic features in the red seaweed Chondrus crispus shed light on evolution of the Archaeplastida[END_REF][START_REF] Qiu | Evidence of ancient genome reduction in red algae (Rhodophyta)[END_REF], it has been suggested that the ancestor of red algae may have also emerged in a hot spring, i.e. non-marine environment [START_REF] Qiu | Evidence of ancient genome reduction in red algae (Rhodophyta)[END_REF]. Though it will always remain speculative, modern marine representatives of ancient Archaeplastida may well be descendants of non-marine organisms.

A traditional argument against an early freshwater origin of Archaeplastida is based on the supposition that lakes and rivers were ephemeral along with the probable absence of biological vectors able to transport algae or spores between freshwater systems at the time.

The oldest known lakes and rivers, such as Lake Baikal (Russia, 25 My) or the Finke River (Australia, 350 My) are still "young" [START_REF] Haines | Palaeozoic synorogenic sedimentation in central and northern Australia: A review of distribution and timing with implications for the evolution of intracontinental orogens[END_REF][START_REF] Colman | Quaternary sedimentation and subsidence history of Lake Baikal, Siberia, based on seismic stratigraphy and coring[END_REF]. Using geochemical proxies [START_REF] Wellman | The terrestrial biota prior to the origin of land plants (embryophytes): a review of the evidence[END_REF] suggest that primary production in terrestrial settings was probably established between 3.0 and 2.7 Ga. These aeroterresterial forms may have created a link between freshwater habitats. Thus, non-marine aquatic and aeroterresterial forms may have played the pivotal role in primordial times, with only later a link to the marine environment. Such early aeroterrestrial organisms would have needed protection against UV radiation [START_REF] Mulkidjanian | The cyanobacterial genome core and the origin of photosynthesis[END_REF], which today comes from the ozone layer, i.e., the interaction of photosynthetic oxygen with water. Even though oxygen

was not yet plentiful, protection against UV could have been augmented by other UV absorbing gases, such as methane [START_REF] Hessen | Solar radiation and the evolution of life[END_REF]. Accordingly, the aeroterrestrial 'flora' would have been able to take advantage of an increasingly protective habitat and greater access to newly forming freshwater environments.

The main challenge to resolving the original ancestral habitats is that the earliest divergences are not known and the deep evolutionary radiations were rapid. The best we can do with class and ordinal level phylogenies of the three lineages is to examine the transitions that have occurred in extant taxa (which we do) and estimate directionality based on available taxon sampling and sister-group observations. Though coarse, it provides some additional indications for the aeroterrestrial/freshwater -marine sequence.

But what about the stramenopiles? Here the timing of the secondary or tertiary endosymbiosis events (at the origin of the stramenopiles) is important. We speculate that, if this event (or events) occurred in seawater, the genes required for fresh water tolerance were not transferred to the nucleus of the host (or hosts) and therefore lost. If so, this would provide one possible explanation for why so few brown algae have colonized fresh water. On the other hand, unicellular stramenopiles such as diatoms are found in both environments with many transitions, possibly in both directions. Most xanthophytes also occur in fresh water, so that no clear conclusions about the ancestral state of stramenopiles can be drawn. Across all lineages, morphological complexity is clearly a factor that impacts the ease of environmental transitions. Microscopic (unicellular) representatives of all photosynthetic lineages have undergone numerous recent back-and-forth transitions between seawater and freshwater environments. Green algae have many unicellular representatives, and most of their multicellular morphologies remain simple; they have also frequently transitioned to marine environments and back (see section 2). In contrast, the red algae have relatively few unicellular representatives, and the multicellular forms are morphologically complex and dominant in marine environments; there have been few transitions to fresh water. The brown algae comprise the most morphologically complex algae and are almost exclusively marine.

Finally, the streptophytes comprise the morphologically most complex plant species dominating terrestrial and freshwater habitats. Within this group, only the seagrasses (60 species) have made a permanent transition to the marine environment [START_REF] Les | Phylogenetic studies in Alismatidae, II: Evolution of marine angiosperms (seagrasses) and hydrophily[END_REF][START_REF] Olsen | The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea[END_REF].

PHYSIOLOGICAL AND GENOMIC ADAPTATIONS TO SEAWATER AND FRESH WATER

It is clear that marine-freshwater or freshwater-marine transitions have been common in photosynthetic eukaryotes (Figure 1). Still, we have only rudimentary understanding of the genomic changes underlying the physiological adaptations. Canonical understanding is usually derived from comparative acclimation experiments with euryhaline species that are capable of growing in both environments. For example, a recent study in the freshwater green alga Ulva limnetica used suppression subtractive hybridization in combination with quantitative real-time-PCR to compare gene expression profiles of freshwater vs. seawatergrown cultures of the same strain [START_REF] Ichihara | Isolation and temporal expression analysis of freshwater-induced genes in Ulva limnetica (Ulvales, Chlorophyta)[END_REF]). An up-regulation was observed in enzymes involved in the degradation of sorbitol, probably as a means of reducing intracellular osmotic pressure, as well as the activation of reactive oxygen species scavengers. Also, an earlier study of the same strain under the same conditions detected the accumulation of lectinlike proteins in fresh water-grown cultures (Ichihara et al. 2009b), but the molecular role of these proteins for fresh water tolerance remains unknown.

Salt tolerant Chara longifolia C.B. Robinson and Lamprothamnium sp. were shown to respond to salinity changes by increasing vacuolar concentrations of K + , Cl -and sometimes sucrose, but not Na + [START_REF] Beilby | Salt tolerance at single cell level in giant-celled Characeae[END_REF]. In both taxa, membrane potential is maintained by an increased activity of proton pumps. In an older study, Lamprothamnium papulosum (K.Wallroth) J.Groves was also shown to modify levels of cell wall sulfated polysaccharides with varying salinity [START_REF] Davis | Lamprothamnium prosperity in permanently hypersaline water[END_REF][START_REF] Shepherd | The effect of an extracellular mucilage on the response to osmotic shock in the charophyte alga Lamprothamnium papulosum[END_REF].

In red algae, the discovery of the coralline freshwater red alga, Pneophyllum cetinaensis, constitutes an interesting case study of a recent (probably within the last 120,000 years) but irreversible transition to fresh water (Žuljević et al. 2016), but so far the necessary genomic and physiological adaptations have not been studied. Another example is Bangia atropurpurea, which is able to grow in fresh water [START_REF] Reed | On the conspecificity of marine and freshwater Bangia in Britain[END_REF]. In acclimation experiments to full marine salinity, this species was shown to rapidly synthesize large quantities of floridoside, thought to serve as an osmoprotectant for the cells [START_REF] Reed | Osmoacclimation in Bangia atropurpurea (Rhodophyta, Bangiales): the osmotic role of floridoside[END_REF]. B. atropurpurea from the Great Lakes (USA) has been shown to exhibit reduced growth in seawater at a salinity of 26 g•L -1 , but this effect was reversed (i.e. reduced growth in fresh water compared to seawater) after three generations of acclimation to seawater [START_REF] Sheath | Distribution and salinity adaptations of Bangia atropurpurea (Rhodophyta), a putative migrant into the Laurentian Great Lakes[END_REF].

By analogy to the Baltic Fucus vesiculosus described in section 2 [START_REF] Tatarenkov | Intriguing asexual life in marginal populations of the brown seaweed Fucus vesiculosus[END_REF], the Bangia populations also lack sexual reproduction in fresh water, despite the fact that they have maintained their presence in the Great Lakes since the 1960s and probably originated from fresh water-adapted European populations [START_REF] Shea | Distribution, seasonality and putative origin of the non-native red alga Bangia atropurpurea (Bangiales, Rhodophyta) in the Laurentian Great Lakes[END_REF]. Indeed, sexual reproduction may merit particular attention when studying the adaptation to fresh water, as has previously been argued by [START_REF] Raven | Constraints on marine algal invasion of low-salinity environments: sex in the Baltic[END_REF]. Specifically, the fusion of gametes is highly sensitive to the surrounding osmotic conditions, and mechanisms to block polyspermy in marine algae frequently rely on the influx of Na + , while freshwater algae generally rely on the efflux of Cl -.

Within the brown algae, the aforementioned freshwater strain of Ectocarpus subulatus provides a model for the transition between both environments. Here a combination of metabolite and transcriptome profiling revealed that over half of all examined genes and most metabolites were differentially expressed between the two conditions [START_REF] Dittami | Towards deciphering dynamic changes and evolutionary mechanisms involved in the adaptation to low salinities in Ectocarpus (brown algae)[END_REF]. In this case, changes included the upregulation of genes involved in cell wall sulphation in seawater, which was confirmed using stained antibodies [START_REF] Torode | Monoclonal antibodies directed to fucoidan preparations from brown algae[END_REF]. This freshwater strain of E. subulatus has never been observed to reproduce sexually in the laboratory (A.

Peters, personal communication as well as eight years of observation in our laboratory).

Differential gene expression analyses and physiological characterization of euryhaline organisms can shed light on the physiological acclimations needed to cope with osmoregulation and ion homeostasis in fresh water vs. seawater. However, they provide little information on the adaptations to either environment and their evolution after the initial transition. It is these adaptations that, once completed, may prevent a move back to the former environment. Comparisons of sister taxa that are strictly freshwater or strictly marine would provide valuable information to answer these questions, but are still rare.

An important example for such adaptations to life in marine vs. freshwater environments is cell wall sulphation. This process is present in all marine algae (red, green, and brown) and seagrasses [START_REF] Kloareg | Structure of the cell-walls of marine-algae and ecophysiological functions of the matrix polysaccharides[END_REF]), but has not been found in freshwater plants or freshwater algae. Both aforementioned euryhaline red and brown algal species, L. papulosum and E. subulatus, have been shown to regulate cell wall sulphation depending on their environment [START_REF] Shepherd | The effect of an extracellular mucilage on the response to osmotic shock in the charophyte alga Lamprothamnium papulosum[END_REF][START_REF] Torode | Monoclonal antibodies directed to fucoidan preparations from brown algae[END_REF]. Interestingly, in a recent genome analysis of the marine angiosperm, the seagrass Zostera marina, [START_REF] Olsen | The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea[END_REF] showed that the enzymes responsible for cell wall sulphation in Z. marina re-evolved from carbohydrate sulfatases, which may be active on a wider range of substrates. Such comparisons illustrate the value of comparative genomics in deciphering the adaptations related to freshwater-marine habitat preferences and may shed light on the evolutionary events that have led to or prevented marine-freshwater transitions more recently.

CONCLUSIONS

Bidirectional marine-freshwater transitions have occurred in multicellular photosynthetic eukaryotes all across the eukaryotic tree, but branches that are specialized to one or the other habitat have clearly emerged. At one extreme, streptophytes dominate freshwater and terrestrial habitats, with only a few species (the seagrasses) having colonized the sea from freshwater ancestors. At the other extreme, brown algae, with the exception of a few morphologically simple species, are found only in seawater. Green and red algae hold an intermediate position and have successfully made the transition between seawater and fresh water many times. As a general trend, successful transitions appear to occur more frequently in morphologically simple organisms, as illustrated in the case of brown algae, but also to a certain extent in green algae and plants. Unicellular organisms have not been treated in this review but follow in parallel with this observation, frequently found in both types of habitats.

It is commonly assumed that the colonization of fresh water was the intermediate step that allowed a gradual colonization of land by a marine, green algal lineage [START_REF] Becker | Streptophyte algae and the origin of embryophytes[END_REF]. While there is little doubt that plants have colonized terrestrial environments from fresh water, we cannot rule out the possibility of an original freshwater/terrestrial origin of photosynthetic eukaryotes followed by secondary colonization of marine habitats. To learn more about these early and later evolutionary transitions and the underlying mechanisms responsible for habitat constraint, the availability of omics resources, in combination with comparative and experimental approaches, will allow us to unravel the determinants of these fundamentally different evolutionary trajectories.

  have different habitat preferences (e.g. Codiolum kuckuckii Skottsberg & Levring (freshwater; Silva and Chacana 2005) vs. Codiolum brevipes Foslie (marine; Lokhorst and Trask 1981);

  intestinalis L.;[START_REF] Mareš | True identity of the European freshwater Ulva (Chlorophyta, Ulvophyceae) revealed by a combined molecular and morphological approach[END_REF]) and its sister species U. meridionalis R.Horimoto & S.Shimada[START_REF] Horimoto | Enteromorpha-like Ulva (Ulvophyeae, Chlorophyta) growing in the Todoroki River, Ishigaki Island, Japan, with species reference to Ulva meridionalis Horimoto et Shimada sp. nov[END_REF] constitute the most common freshwater clade, frequently forming blooms in eutrophicated or salt-contaminated waters. A second clade consists of U. limnetica K. Ichihara & S. Shimada(Ichihara et al. 2009a). It is only known from freshwater bodies in Japan. Finally, U. maeotica (Proshkina-Lavrenko) P.Tsarenko and U. simplex (K.L.Vinogradova) H.S.Hayden, Blomster, Maggs, P.C.Silva, M.J.Stanhope & J.R.Waaland have been described in marine and freshwater habitats in the Ukraine (Burova et al. 2011), but their phylogenetic relationship has not been investigated. Within the Ulvellaceae, only two genera have freshwater representatives: Entocladia and Ulvella. Entocladia gracilis Hansgirg is the only freshwater species in the genus (John and Rindi 2015), whereas at least three species of Ulvella have colonized fresh water, i.e., U. bullata (C.-C.Jao) H.Zhu & G.Liu, U. tongshanensis H.Zhu & G.Liu, and U. prasina (C.-C.Jao) H.Zhu & G.Liu (Zhu et al. 2015).

  ; and S. fluviatilis C.-C. Jao recorded from China and the United States (McCauley and Wehr 2007; Necchi 2016). A fifth marine-freshwater transition in brown algae probably occurred within the species Porterinema fluviatile (H.C.Porter) Waern, an alga which branches very early in the brown algal tree (McCauley and Wehr 2007) and is currently not attributed to a specific order. This species has a global distribution with populations occurring both in freshwater and in marine environments in North America and in Europe. A possible sixth example is a strain of Ectocarpus-like brown algae tentatively named Ectocarpoides piscinalis nom. nud. isolated from a domestic freshwater aquarium in Northampton, UK. Preliminary molecular analyses indicate that this isolate constitutes a yet undescribed brown algal species, possibly in a new order

AmmoniumFIGURESFigure 1 :

 1 FIGURES

ACKNOWLEDGEMENTS

We thank the creators of AlgaeBase (http://www.algaebase.org/; Guiry and Guiry 2015) and its funders for this valuable resource. We are also grateful to Dr. Akira Peters for bringing important literature to our attention, Dr. Catherine Boyen and Dr. Thierry Tonon for critical reading of the manuscript and helpful comments; Dr. Gabriel Markov and Dr. Catherine Leblanc for helpful discussions; and the reviewers for the in-depth critique and suggestions. This work benefited from the support of the French Government via the National Research Agency investment expenditure program IDEALG (ANR-10-BTBR-04).

Schizocladiophyceae, Phaeothamniophyceae, and Tribophyceae

The Schizocladiophyceae comprise only one member, Schizocladia ischiensis E.C.Henry, K.Okuda & H.Kawai, which is marine and was collected at the Island of Ischia, Gulf of Naples, in the Mediterranean [START_REF] Kawai | Schizocladia ischiensis: a new filamentous marine chromophyte belonging to a new class, Schizocladiophyceae[END_REF].

The Phaeothamniales have previously been considered part of the Chrysophyceae, but based on molecular phylogenetic data have been erected as a new class, the Phaeothamniophyceae [START_REF] Bailey | Phaeothamniophyceae Classis Nova: A new Lineage of Chromophytes based upon photosynthetic pigments, rbcL sequence analysis and ultrastructure[END_REF]. The three principal genera are Phaeoschizochlamys, Phaeothamnion, and Stichogloea, all of which have been described from fresh water. Other genera tentatively associated with this class are not available in culture, and their placement in this group remains uncertain [START_REF] Bailey | Phaeothamniophyceae. The Tree of Life Web Project[END_REF]. The only known marine species of Phaeothamniales is Chrysophaeum lewisii W.R.Taylor, but its position within this order would need to be confirmed by molecular data.

Among the Tribophyceae, which comprise mostly freshwater algae, three orders are known to form filaments or to be siphonous, the Botrydiales (multinucleate cells), the Tribonematales, and the Vaucheriales. All three of these orders mainly occur in fresh water, and several have also colonized terrestrial habitats. Within the Vaucheriales 20 species have been found in marine or brackish environments [START_REF] South | An Introduction to Phycology[END_REF]. Based on a phylogenetic tree of 32 Vaucheria strains (21 species; Andersen and Bailey 2002), the marine representatives form two groups: one comprising only Pseudodichotomosiphon sp., a genus closely related to Vaucheria (exact phylogenetic position uncertain; [START_REF] Fukushi-Fujikura | Cell wall polysaccharides of Pseudodichotomosiphon constrictus with special reference to its systematics[END_REF], and the other comprising all other marine species in this group. The most parsimonious explanation for this distribution would be two separate transitions from freshwater to marine habitats, one in Pseudodichotomosiphon and one in Vaucheria.

TABLE LEGENDS

Table 1: Overview of the typical composition of seawater and fresh water in selected streams.

An asterisk (*) indicates global averages of major ions in river waters according to [START_REF] Wetzel | Limnology[END_REF], followed by ranges found in the literature for different water bodies [START_REF] Goldberg | Minor elements in sea water[END_REF][START_REF] Ott | Meereskunde. Ulmer[END_REF][START_REF] Wetzel | Limnology[END_REF]Wehr and Sheath 2003;[START_REF] Salminen | Geochemical Atlas of Europe, Part 1, Background Information, Methodology and Maps[END_REF]. The term brackish water refers to a mixture of seawater and fresh water with salinities ranging from 0.5 to 30 g•L -1 .

Seawater

Fresh water

Overall salinity 30-40 g•L -1 <0.5 g