Freshwater protozoa: biodiversity and ecological function

The purpose of this article is to pull together various elements from curre nt knowledge regarding the natural history of free-living protozoa in fresh waters. We define their functional role, set the likely limits of 'biodive rsity', and explore how the two may be related. Protozoa are unicellular, p hagotrophic organisms, and 16 phyla of protists contain free-living freshwa ter protozoan species. They are the most important grazers of microbes in a quatic environments and the only grazers of any importance in anoxic habita ts. In sediments, ciliates are usually the dominant protozoans. Benthic cil iate biomass accounts for slightly less than 10% of total benthic invertebr ate biomass: but ciliate production may equal or even exceed invertebrate p roduction. Freshwater protozoan species are probably ubiquitous, although m any may persist locally for long periods in a cryptic stale - 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 sma ller, more numerous microbes are also ubiquitous. The number of protozoan s pecies recorded in local surveys (232) is about 10% of the estimated global species richness (2390). The 'seedbank' of protozoan (and microbial) speci es ensures that local microbial diversity is never so impoverished that it cannot play its full part in ecosystem functions such as carbon fixation an d nutrient cycling.