| Abstract: | Free-living heterotrophic flagellates have been widely studied as key intermediates in aquatic food webs. However, very few attempts have been made (all in marine systems) to study the smallest cells belonging to these communities. In this study, we report on the quantitative importance and the vertical distribution of heterotrophic picoflagellates during spring in the oligomesotrophic Lake Pavin, using the FISH approach. The rRNA-based probes used, initially designed for two novel lineages of heterotrophic picoflagellates (i.e., the Stramenopiles NS3 and NS4) in marine systems, revealed high abundances (in the order of 104 cells ml−1 and up to about 6 × 104 cells ml−1) of the targeted picoflagellates in Lake Pavin, i.e. one order of magnitude higher than in seawater samples. 8NS39 cells were more often detected than 8NS49 cells, the average contributions of the two populations to the total abundance of heterotrophic flagellates being at 25 and 9%, respectively. However, the probes used were apparently non-specific to the targeted cells, implying that the occurrence of the two novel lineages NS3 and NS4 of Stramenopiles in freshwaters needs to be confirmed. Although, this study clearly confirms the occurrence, in high abundance, of small heterotrophic eukaryotes in Lake Pavin. In a near future, we believe that the availability of microbial clone libraries will allow us to improve our knowledge of the composition and the functional role of small heterotrophic eukaryotes in freshwaters. |
