Transitions between marine and freshwater environments provide new clues about the origins of multicellular plants and algae

Abstract

Marine-freshwater and freshwater-marine transitions have been key events in the evolution of life, and most major groups of organisms have independently undergone such events at least once in their history. Here, we first compile an inventory of bidirectional freshwater and marine transitions in multicellular photosynthetic eukaryotes. While green and red algae have mastered multiple transitions in both directions, brown algae have colonized freshwater on a maximum of six known occasions, and angiosperms have made the transition to marine environments only two or three times. Next, we review the early evolutionary events leading to the colonization of current habitats. It is commonly assumed that the conquest of land proceeded in a sequence from marine to freshwater habitats. However, recent evidence suggests that early photosynthetic eukaryotes may have arisen in subaerial or freshwater environments and only later colonized marine environments as hypersaline oceans were diluted to the contemporary level. Although this hypothesis remains speculative, it is important to keep these alternative scenarios in mind when interpreting the current habitat distribution of plants and algae. Finally, we discuss the roles of structural and functional adaptations of the cell wall, reactive oxygen species scavengers, osmoregulation, and reproduction. These are central for acclimatization to freshwater or to marine environments. We observe that successful transitions appear to have occurred more frequently in morphologically simple forms and conclude that, in addition to physiological studies of euryhaline species, comparative studies of closely related species fully adapted to one or the other environment are necessary to better understand the adaptive processes.

Repeated evolutionary transitions of flavobacteria from marine to non‐marine habitats

“…However, several ocean to freshwater transitions and the following re-colonization from freshwater to ocean are found among eukaryotic microbes, including the microalga diatoms (Alverson et al, 2007) and cryptomonads (Shalchian-Tabrizi et al, 2008), as well as the heliozoan protist group centrohelids (Cavalier-Smith and von der Heyden, 2007). In animals, bidirectional transitions are well-studied in fish, with several species able to migrate between freshwater and seawater within their lifetime (Dittami et al, 2017). One of the strategies fish use to maintain osmotic homeostasis and avoid sodium toxicity is based on dynamic osmoregulation through active salt absorption and secretion and through water excretion and retention (Kültz, 2015).…”

Revisiting australianectocarpus subulatus(phaeophyceae) from the hopkins river: distribution, abiotic environment, and associated microbiota

“…Brown algae (Phaeophyceae) form the dominant vegetation in the tidal and sub-tidal zone of rocky shores in temperate marine environments, but they are rarely found in fresh water (Dittami et al 2017): while there are ca. 2,000 known species of marine brown algae, covering a large range of morphologies from small filamentous algae to large and morphologically complex kelp species, there is only a handful of known freshwater brown algae, all of them small and with simple morphology (crust-forming or filamentous).…”

Exploring the Cultivable Ectocarpus Microbiome

“…Here, we study the microbiome of a freshwater strain of Ectocarpus subulatus ( West and Kraft, 1996 ). The transition to fresh water is a rare event in brown algae that occurred in only a few species ( Dittami et al, 2017 ). The examined strain is currently the only publicly available freshwater isolate within the Ectocarpales, and it is still able to grow in both seawater and freshwater ( Dittami et al, 2012 ).…”