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

Abstract

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 investi- gated in freshwater habitats, and is still unknown in tropical aquatic systems. In this study, we inves- tigated 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 finger- prints 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 taxo- nomic 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 <95% similar to any sequence in Gen- Bank. This may result from undersampling of freshwater systems, but also raises the possibility that perennially warm tropical waters harbour particular assemblages of planktonic small eukaryotes.