Grazing of protozoa and its effect on populations of aquatic bacteria

Predation by bactcrivorous protists in aquatic habitats can influence the m orphological structure, taxonomic composition and physiological status of b acterial communities. The protistan grazing can result in bacterial respons es at the community and the species level. At the community level, grazing- induced morphological shifts have been observed, which were directed toward s either larger or smaller bacterial sizes or in both directions. Morpholog ical changes have been accompanied by changes in taxonomic community struct ure and bacterial activity. Responses at the species level vary from specie s to species. Some taxa have shown a pronounced morphological plasticity an d demonstrated complete or partial shifts in size distribution to larger gr owth forms (Filaments, microcolonies). However, other taxa with weak plasti city have shown no ability to reduce grazing mortality through changes in s ize. The impact of protistan grazing on bacterial communities is based on t he complex interplay of several parameters. These include grazing selectivi ty (by size and other features), differences in sensitivity of bacterial sp ecies to grazing, differences in responses of single bacterial populations to grazing (size and physiology), as well as the direct and indirect influe nce of grazing on bacterial growth conditions (substrate supply) and bacter ial competition (elimination of competitors). (C) 2001 Federation of Europe an Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.