Molecular characterisation of the small-eukaryote community in a tropical Great Lake (Lake Tanganyika, East Africa)

In aquatic environments, small eukaryotes (mainly algae and protozoa of 1 to 5 μm in size) are a key link in the carbon transfer to higher trophic levels, e.g. through primary production and grazing of picoplankton. However, the diversity of these microorganisms remains poorly investigated in freshwater habitats, and is still unknown in tropical aquatic systems. In this study, we investigated the small-eukaryote diversity in the oligotrophic Lake Tanganyika, one of the African Great Lakes, at different depths in the water column using denaturing gradient gel electrophoresis (DGGE) and gene clone libraries based on 18S rRNA genes. Each sample produced complex DGGE fingerprints clearly discriminating the epilimnion from the metalimnion. Analysis, using genetic libraries, confirmed the high level of small-eukaryote diversity in Lake Tanganyika. Organisms from 5 taxonomic groups (Stramenopiles, Alveolata, Cryptophyta, Kinetoplastea and Choanoflagellida) were dominant among the species detected. Some sequences were nearly identical to those recovered in temperate freshwaters in North America and Europe, suggesting a high dispersal ability in some small-eukaryote lineages. However, 49% of sequences were