Grazing-activated chemical defence in a unicellular marine alga

Research on "microbial loop" organisms, heterotrophic bacteria and phagotrophic protists, has been stimulated in large measure by Pomeroy's seminal paper published in BioScience in 1974. We now know that a significant fate of bacterioplankton production is grazing by 20-µm ciliates and heterotrophic dinoflagellates in the microzooplankton. Protists can grow as fast as, or faster than their phytoplankton prey. Phototrophic cells grazed by protists range from bacterial-sized prochlorophytes to large diatom chains (which are preyed upon by extracellularly-feeding dinoflagellates). Recent estimates of microzooplankton herbivory in various parts of the sea suggest that protists routinely consume from 25 to 100% of daily phytoplankton production, even in diatom-dominated upwelling blooms. Phagotrophic protists should be viewed as a dominant biotic control of both bacteria and of phytoplankton in the sea.