Closely related dinoflagellate species in vastly different habitats – an example of a marine–freshwater transition

Phytoplankton are present in a large variety of aquatic environments, ranging from small freshwater ponds to the oceans. Typically, freshwater and marine species are not closely related, indicating an ancient divergence and that salinity poses a strong dispersal barrier. Here we reveal a common recent origin of two dinoflagellates that are well adapted to different habitats. <i>Gymnodinium baicalense</i> inhabits the geologically old freshwater Lake Baikal, which is located in the middle of the Eurasian continent. <i>Gymnodinium corollarium</i>, on the other hand, is a brackish water species from the Baltic Sea. Both species form blooms under ice during the spring. We generated 10 DNA sequences from these species. The partial 28S rRNA gene from <i>G. baicalense</i> and the partial 18S rRNA gene and internal transcribed spacer-2 (ITS2) from <i>G. corollarium</i> were sequenced for the first time. A more detailed description of <i>G. baicalense</i>, which was previously known only from light microscopy observations, is also provided. In the laboratory we demonstrated that <i>G. baicalense</i> is strictly adapted to fresh water, while <i>G. corollarium</i> had a wide salinity tolerance. However, the two species have almost identical morphology, identical SSU rRNA gene sequences, and only small differences in the LSU rRNA gene and ITS2. We infer a common ancestor, which was a species from the Arctic region with a tolerance to a range of salinities. Our data support the scenario that the under-ice phytoplankton community in Lake Baikal has been formed recently, even though the lake is the oldest in the world.