Manual of Environmental Microbiology

Protistan assemblages of aquatic ecosystems are the focus of extensive research in aquatic ecology. One stimulus for this work has been the long-standing recognition that phototrophic protists (the unicellular algae) constitute a major fraction of the primary productivity within aquatic ecosystems. Another incentive has been the realization that protozoa (heterotrophic protists) play a pivotal role in the flow of energy and elements in these communities (109, 118). Studies of the abundance, biomass and trophic activities of protists have now been conducted in a wide range of aquatic ecosystems. In addition, many laboratory experiments have examined the general biology and physiology of various protistan species grown under carefully controlled conditions. The synthesis of this information into models of how protistan assemblages are structured and how they function in nature has advanced considerably during the last few decades. Protozoa traditionally have been distinguished from unicellular eukaryotic algae as heterotrophic organisms that exist by absorbing dissolved organic substances (osmotrophy) or more commonly by engulfing prey (phagocytosis). It is important to recognize that the term Protozoa now has more historical significance than phylogenetic or ecological meaning. Modern phylogenetic schemes have merged many 'protozoan' taxa with 'algal' taxa and vice versa. For example, numerous heterotrophic species of chrysophytes and dinoflagellates exist within these traditionally-algal taxa. Heterotrophic species within each group are closely related to chloroplast-bearing species based on ultrastructural features and DNA sequence data but obviously have quite different modes of nutrition than their photosynthetic counterparts (4, 80, 99, 115). In addition to the existence of 'apochlorotic algae', chloroplast-bearing genera exist within the chrysophyte, dinoflagellate, prymnesiophyte, cryptophyte and euglenophyte algae that are capable of phagotrophy in addition to photosynthesis (112, 129, 130). This mixotrophic behavior obscures the distinction between traditional definitions of algae and protozoa and has lead ecologists to conceptualize protistan trophic activity as a continuum of nutritional modes (76). There are also difficulties in the classification of some ciliated protozoa as phototrophs or heterotrophs. Some ciliate species ingest and digest algal prey but are able to retain the chloroplasts of their prey in a functional state, thereby providing those ciliates with limited photosynthetic ability (112, 127). Photosynthesis in green ciliates contributes significantly to the overall nutrition of these protozoa, and may form a notable fraction of the primary productivity of some planktonic communities (33). The close phylogenetic affinities of some flagellated protozoa with algal taxa, as well as the mixed nutrition of many …