Soil protozoa as bioindicators: pros and cons, methods, diversity, representative examples

This paper emphasizes some general aspects of soil protozoa as indicators o f environmental quality-pros and cons, methods, and diversity. Protozoa are at the base of the heterotrophic eukaryotic food web and an essential comp onent in marine, freshwater, and soil ecosystems because they consume a sig nificant portion (usually >50%) of the bacterial productivity. enhancing nu trient cycles and energy flows to the benefit of microorganisms, plants and animals. Accordingly, studies of their dynamics and community structures p rovide a powerful means for assessing and monitoring changes in the biotic and abiotic environment. This is exemplified by some representative studies focusing on soil oxygen regime, differentiation of humus types, pesticides , global warming, forest decline, movement of protozoan pathogens in soil, and soil protozoan bioassays. Usually, protozoa are not replaceable by high er animals (meso- and macrofauna) as indicator organisms because they have unique physiological properties: they consume more food and have a higher r espiration rate per mass unit, have shorter generation and life times, and reproduce much faster. Direct counting methods should be given preference o ver dilution culture techniques, which are beset with uncontrollable defici encies. Thus, ciliates and, especially, testate amoebae, whose abundance an d diversity can be reliably estimated in simple soil suspensions, should be preferred in environmental studies. About 1600 protozoan species are known to live in terrestrial habitats. However, data from studies of ciliates su ggest that this is only a minor portion (20-30%) of the species actually pr esent, most of which are still undescribed. Overcoming the methodological a nd taxonomic problems are urgent needs which, at present, limit the use of protozoa as bioindicators in terrestrial environments. Furthermore, species monographs are required to compile the taxonomic, faunistic, and ecologica l information available. (C)1999 Elsevier Science B.V. All rights reserved.