Biodiversity Researches on Microbial Loop in Aquatic Systems

Abstract

The food linkage between heterotrophic bacteria and protists is so-called "microbial loop," functioning as important matter cycling in pelagic food webs. Biomass of heterotrophic bacteria sometimes predominates total heterotrophic biomass in lakes, and organic matter transfer started from heterotrophic bacteria has been intensively studied by numerous researchers all over the world. The 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 themselves utilized by the zooplankton. There is a consensus that food linkages between bacteria and protists are substantial in many lakes. In addition, bacterial loss due to viral lysis has also been paid great attention by many researchers since the late 1990. The present review provides the overview on the trend and future stage of microbial loop researches in freshwater systems, with special reference to culture-independent molecular techniques such as PCR cloning and sequencing, fluorescently in situ hybridization or denaturing gradient gel electrophoresis for phylogenetic analyses on microbial communities. A review on ecology and biodiversity researches on microbial loop in Lake Biwa is also provided.