Freshwater protozoa: biodiversity and ecological function

The purpose of this article is to pull together various elements from current knowledge regarding the natural history of free-living protozoa in fresh waters. We define their functional role, set the likely limits of biodiversity, and explore how the two may be related. Protozoa are unicellular, phagotrophic organisms, and 16 phyla of protists contain free-living freshwater protozoan species. They are the most important grazers of microbes in aquatic environments and the only grazers of any importance in anoxic habitats. In sediments, ciliates are usually the dominant protozoans. Benthic ciliate biomass accounts for slightly less than 10% of total benthic invertebrate biomass, but ciliate production may equal or even exceed invertebrate production. Freshwater protozoan species are probably ubiquitous, although many may persist locally for long periods in a cryptic state – as potential rather than active biodiversity. As protozoa are among the largest and most complex of micro-organisms, it follows that bacteria and all other smaller, more numerous microbes are also ubiquitous. The number of protozoan species recorded in local surveys (232) is about 10% of the estimated global species richness (2390). The 'seedbank of protozoan (and microbial) species ensures that local microbial diversity is never so impoverished that it cannot play its full part in ecosystem functions such as carbon fixation and nutrient cycling.