Free-Living Protozoa

Abstract PROTOZOA are microscopic, single-celled eukaryotic organisms. Each protozoon typically exists as an independent cell, and all free-living protozoa are defined as phagotropic microorganisms. In some species, the cells unite to form colonies. Many protozoa can cause disease (e.g., malaria), others are commensals in the digestive tracts of ruminants and wood-eating insects. In this chapter we will focus on the free-living protozoa. The definition of this group is not taxonomic but one that is based on its key function in the natural environment. Protozoa are capable of phagotrophy, i.e. the ability to capture and ingest food particles. Many protozoa are “mixotrophs” and capable of both phagotrophy and phototrophy (photosynthesis), the latter via functional chloroplasts or endosymbiotic algae. Like most microorganisms, protozoa have very large population sizes, and they are the most abundant group of phagotrophs in the biosphere. Biodiversity at the level of protozoa has characteristics that are not shared by macroscopic animals and plants. Most protozoan species are probably globally ubiquitous. As a consequence, a significant proportion of local protozoan species richness at any moment in time, is rare or cryptic and awaiting the arrival of conditions suitable for growth and reproduction.