Not Alone Anymore: an Endemic Dinoflagellate from Lake Baikal Found Its Freshwater Relative in the European Lake Ilmen

Microalgae from ice and under-ice water communities can serve as valuable models for studying how unique physiological adaptations influence the phylogeography of single-celled eukaryotes. Gymnodinium baicalense is a freshwater planktonic dinoflagellate, historically recognized as one of the few freshwater dinoflagellates with a restricted distribution, primarily within the Lake Baikal region. However, we found a similar morphospecies far from this area - in Lake Ilmen - which differs from Lake Baikal in numerous characteristics including age, depth, trophic status, oxygen saturation, transparency, and chemical stability. Genetic analyses, combined with morphological and ecological data revealed the close evolutionary relationships between these freshwater lineages and Gymnodinium corollarium from the Baltic Sea. We propose that the Lake Baikal and Lake Ilmen lineages belong to the same species, though the Baikal lineage due to its remote location has accumulated some unique mutations in its rDNA. A single episode of marine-freshwater transition, with further G. baicalense colonization of various lakes is suggested as the most likely scenario of the species dispersal. This discovery challenges the notion of G. baicalense as an endemic species and prompts to check of under-ice dinoflagellate blooms in other northern freshwater lakes.