The importance of phagotrophic protists in Lake Pavin

Abstract

Phagotrophic protists include unicellular taxa from different lineages of eukaryotes. Most of these freshwater microorganisms are free-living, heterotrophic forms, consuming bacteria in the water column of lakes. In Lake Pavin, we focused on the seasonal and spatial dynamic and the diversity of phagotrophic protists and estimated in situ specific-grazing rates of both ciliates and flagellates. We shown that a large part of bacterivory was the result of the phagotrophy of attached (i.e. epiphytic) ciliates and flagellates. Pigmented flagellates have also an important grazing impact on bacteria, with clearance rates equivalent to those estimated for heterotrophic taxa of similar size. Total impact of phagotrophic protists on bacteria indicates that bacterial productivity might be totally consumed by unicellular eukaryotes. In a short-term study, we estimated that flagellate bacterivory dominated the bacterial mortality in Lake Pavin while viral lysis accounted for less than 10 % of the bacterial mortality. Finally, the use of microcosms suggested that large-size ciliates ingest heterotrophic nanoflagellates and may in turn be a key resource for metazoa during spring. We conclude that phagotrophic protists are involved in the conversion of bacterial biomass and then constitute a relevant source of carbon for metazoans in the planktonic food web of Lake Pavin.