Grazing by bacterivorous protists and the availability of nutrients strongly affect the taxonomic composition of freshwater bacterioplankton. However, so far, studies have focussed mainly on changes in abundance. Therefore, we studied biomass reallocation within different phylogenetic lineages of bacteria under varying regimes of protistan grazing and nutrient supply in the oligo-mesotrophic Piburger See (Austria). Size fractionation of lake water was used to create setups with and without bacterivores. These treatments were incubated in bottles (with and without added P) and in dialysis tubes (allowing for free nutrient exchange) for 96 h. The release of bacteria from grazing resulted in a small increase in total abundance, but more pronounced changes of production and biomass. Addition of P to bottles seemed to indicate P limitation of bacterial growth. However, this was contradicted by the results of dialysis tube incubations. In these treatments, highest bacterial biomass and production were observed as well as a substantial increase in particulate P in the bacterial size fraction. While Betaproteobacteria abundance and biomass increased greatly in P-surplus bottles (4x and 12x, respectively), biomass increased even more in the dialysis tubes (28x). After 48 h incubation, virtually all imported P in predator-free dialysis tubes could be attributed to newly produced betaproteobacterial biomass, These bacteria were significantly reduced by the presence of predators in the bottle incubations only, probably due to the delayed growth of bacterivores in the dialysis tubes. By contrast, although Actinobacteria numerically dominated in the lake, as well as in the grazing-exposed bottle treatments, their contribution to total biomass was always low. Our results illustrate that the quantification of the biomass of specific lineages may allow for a better assessment of C and P fluxes within microbial food webs. Moreover, nutrient addition during bottle incubations might result in misleading conclusions about growth limitation.
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