When ecological transitions are not so infrequent: independent colonizations of athalassohaline water bodies by Arcellidae (Arcellinida; Amoebozoa), with descriptions of four new species

The salinity and humidity barriers divide biodiversity and strongly influence the distribution of organisms. Crossing them opens the possibility for organisms to colonize new niches and diversify, but requires profound physiological adaptations and is supposed to happen rarely in evolutionary history. We tested the relative importance of each ecological barrier by building the phylogeny, based on mitochondrial cytochrome oxidase gene (COI) sequences, of a group of microorganisms common in freshwater and soils, the Arcellidae (Arcellinida; Amoebozoa). We explored the biodiversity of this family in the sediments of athalassohaline water bodies (i.e. of fluctuating salinity that have non-marine origins). We found three new aquatic species, which represent, to the best of our knowledge, the first reports of Arcellinida in these salt-impacted ecosystems, plus a fourth terrestrial one in bryophytes. Culturing experiments performed on Arcella euryhalina sp. nov. showed similar growth curves in pure freshwater and under 20 g/L salinity, and long-term survival at 50 g/L, displaying a halotolerant biology. Phylogenetic analyses showed that all three new athalassohaline species represent independent transition events through the salinity barrier by freshwater ancestor, in contrast to the terrestrial species, which are monophyletic and represent a unique ecological transition from freshwater to soil environments.