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

This paper emphasizes some general aspects of soil protozoa as indicators of environmental quality--pros and cons, methods, and diversity. Protozoa are at the base of the heterotrophic eukaryotic food web and an essential component in marine, freshwater, and soil ecosystems because they consume a significant portion (usually >50%) of the bacterial productivity, enhancing nutrient cycles and energy flows to the benefit of microorganisms, plants and animals. Accordingly, studies of their dynamics and community structures provide 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 higher animals (meso- and macrofauna) as indicator organisms because they have unique physiological properties: they consume more food and have a higher respiration rate per mass unit, have shorter generation and life times, and reproduce much faster. Direct counting methods should be given preference over dilution culture techniques, which are beset with uncontrollable deficiencies. Thus, ciliates and, especially, testate amoebae, whose abundance and 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 suggest that this is only a minor portion (20-30%) of the species actually present, most of which are still undescribed. Overcoming the methodological and 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 ecological information available.