Phagotrophic Protists: Central Roles in Microbial Food Webs

Abstract

Along with the prokaryotes, bacterial and archaea, single-celled eukaryotic microbes, protists, are ubiquitous in aquatic ecosystems and have major roles in food webs and elemental cycles. The trophic modes of protists encompass photosynthesis (algae), heterotrophy (phagotrophic protists), and mixotrophy (protists capable of both photosynthesis and phagotrophy). This chapter focuses on the roles of phagotrophic protists in marine and freshwaters with a review of method development and research findings, highlighting our own work in this field. Phagotrophic protists include flagellates in the nanoplankton (2–20 μm in size) that prey on bacteria and smaller-sized phytoplankton, and ciliates and phagotrophic dinoflagellates in the microzooplankton (20–200 μm in size) that consume a wide size range of other protists, up to large diatom cells and chains. Protists in the microzooplankton are dominant herbivores in all regions of the sea. As a consequence of their high rates of grazing and high biomass-specific excretion rates, phagotrophic protists regenerate a large part of the organic nitrogen and phosphorus in bacterial and algal biomass. Phagotrophic protists are also an important food resource for copepods and other multicellular zooplankton. Productive new areas of research on aquatic protists are assessment of their molecular genetic diversity and investigation of the biochemistry of prey detection or avoidance by these abundant microbes.

Evelyn B. Sherr and Barry F. Sherr

We bonded over sewage sludge. In 1975–1977, Ev, a new research scientist at the University of Georgia Marine Institute on Sapelo Island, and Barry, a graduate student at the University of Georgia, collaborated on a project to evaluate whether salt marsh microbes could break down organic wastes and remove nitrogen via denitrification from secondarily treated sewage sludge applied to a cordgrass marsh. We asked Larry Pomeroy, who was directing a major project in the salt marshes around Sapelo, if we could borrow his research van to deliver sludge from the Athens, GA, waste treatment plant to the island. We may not have been entirely clear about what we were going to transport. After loading up the sludge in trash barrels, we drove to the coast in time for the afternoon ferry to the island. It was a warm day, and with windows open we didn’t notice the winged creatures emerging from the barrels. But when we got to the coast and opened the van, a cloud of sludge flies exploded from the interior. We tried to shoo out as many flies as we could, without great success. Pomeroy later had the van fumigated. We were married in 1979, after Barry finished his Ph.D., and went to Israel for a year and a half research “honeymoon.” We returned to work at the UGMI until 1990 when we relocated to Oregon State University with our two young sons. Projects in the Arctic and in the upwelling system off the Oregon coast occupied us until our retirement in 2012.