Taxon-specific and seasonal variations in flagellates grazing on heterotrophic bacteria in the oligotrophic Lake Annecy - importance of mixotrophy.

We investigated the taxonomic composition of flagellate assemblages and taxon-specific bacterial grazing rates of heterotrophic and mixotrophic flagellates in the oligotrophic Lake Annecy (France). The comparison of bacterial grazing rates to bacterial production demonstrated a high transfer efficiency from the bacterial compartment up to flagellates. Per capita grazing rates ranged from 1.2x10(3) to 5.1x10(6) bacteria l(-1) h(-1) for heterotrophic flagellates, and from 4.8x10(6) to 6.8x10(7) bacteria l(-1) h(-1) for mixotrophic flagellates. The main bacterial grazers were Katablepharis within heterotrophic flagellates and Dinobryon within mixotrophic flagellates. Our results show that bacterial ingestion by a given flagellate taxon changed seasonally and could vary up to 30-fold. We also provide evidence that mixotrophic flagellates represent an important link in the flux of materials through planktonic food webs in Lake Annecy, suggesting that the introduction of mixotrophs within functional groups could improve our understanding of carbon flux pathways.