Predation and protists

Abstract Protists are involved in many ecological roles in natural environments, including primary production, herbivory and carnivory, and parasitism. Microbial ecologists have been interested in these single-cell eukaryotes since Antonie van Leeuwenhoek saw them in his stool and scum from his teeth. This chapter focuses on the role of protozoa (purely heterotrophic protists) and other protists in grazing on other microbes. Heterotrophic nanoflagellates, 3–5 microns long, are the most important grazers of bacteria and small phytoplankton in aquatic environments. In soils, flagellates are also important, followed by naked amoebae, testate amoebae, and ciliates. Many of these protists feed on their prey by phagocytosis, in which the prey particle is engulfed into a food vacuole into which digestive enzymes are released. This mechanism of grazing explains many factors affecting grazing rates, such as prey numbers, size, and composition. Ingestion rates increase with prey numbers before reaching a maximum, similar to the Michaelis–Menten equation describing uptake as a function of substrate concentration. Protists generally eat prey that are about ten-fold smaller than they are. In addition to flagellates, ciliates and dinoflagellates are often important predators in the microbial world and are critical links between microbial food chains and larger organisms Many protists are capable of photosynthesis. In some cases, the predator benefits from photosynthesis carried out by engulfed, but undigested photosynthetic prey or its chloroplasts. Although much can be learnt from the morphology of large protists, small protists (<10 μm) often cannot be distinguished by morphology, and as seen several times in this book, many of the most abundant and presumably important protists are difficult to cultivate, necessitating the use of cultivation-independent methods analogous to those developed for prokaryotes. Instead of the 16S rRNA gene used for bacteria and archaea, the 18S rRNA gene is key for protists. Studies of this gene have uncovered high diversity in natural protist communities and, along with sequences of other genes, have upended models of eukaryote evolution. These studies indicate that the eukaryotic Tree of Life consists almost entirely of protists, with higher plants, fungi, and animals as mere branches.