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Abstract

The paradigm of the 'microbial loop' has became increasingly important for understanding the structure and function of aquatic ecosystems. Most of the microbial loop studies have focused on energy flow and nutrient cycling. Much less is known, however, about the importance of grazing as a force shaping the structure and community composition of planktonic bacteria. Theoretical considerations of predator-prey interactions suggest that predator evasion mechanisms should have evolved for bacteria in the same way as in other predator-prey systems (e.g. zooplankton-phytoplankton). Consistent with this hypothesis, field data show that bacteria are often the most stable component of planktonic communities. Refuges from grazing are one of the possible mechanisms buffering bacterioplankton against strong seasonal fluctuations in abundance. Substantial direct and indirect evidence exists for the occurrence of grazing-resistant bacteria (GRB) in both marine and freshwater habitats. We summarize the potential mechanisms for grazing resistance, including morphological, chemical and behavioral defenses as well as growth in spatial refuges. Cell size appears to be an important factor influencing susceptibility to grazing, with a refuge at the lower and upper ends of the bacterial size range. Thus, a relative grazing resistance can be assumed for the large number of ultramicrobacteria as well as for morphologically complex growth forms such as filaments and aggregates. Besides morphological features, resistance may be achieved by other mechanisms for which, however, much less information is available. We describe how GRB can be included in conceptual models of the interactions among metazooplankton, bacterivorous protozoans and bacteria. It is suggested that the relative importance of GRB increases with increasing grazing pressure exerted by protozoans, whereas it decreases with increasing top-down control of protozoans by metazooplankton. GRB may reduce the productivity of planktonic systems through decreased trophic transfer efficiencies and reduced regeneration of bacterially bound nutrients.