Ecological roles of protists in the microbial loop of planktonic food webs in marine and freshwater systems

Abstract

The aquatic food linkage between heterotrophic bacteria and protists is so-called “microbial loop”, functioning as important matter cycling in pelagic food webs of marine and freshwater systems. Organic matter transfer from heterotrophic bacteria to protists has been intensively studied by numerous researchers all over the world. The ecological roles of planktonic protists, such as heterotrophic nanoflagellates and ciliates, in microbial loop are to consume bacteria that are too small to serve directly as major prey items for most zooplankters, and to be consumed by the zooplankton. There is the consensus that food linkages between bacteria and protists are substantial in many lakes and oceans. The present review provides the overview on the trend and future stage of microbial loop research. A review on unique microbial loop developed in the hypolimnion of Lake Biwa is also provided.