Rapid successions affect microbial N-acetyl-glucosamine uptake patterns during a lacustrine spring phytoplankton bloom.

The vernal successions of phytoplankton, heterotrophic nanoflagellates (HNF) and viruses in temperate lakes result in alternating dominance of top-down and bottom-up factors on the bacterial community. This may lead to asynchronous blooms of bacteria with different life strategies and affect the channelling of particular components of the dissolved organic matter (DOM) through microbial food webs. We followed the dynamics of several bacterial populations and of other components of the microbial food web throughout the spring phytoplankton bloom period in a pre-alpine lake, and we assessed bacterial uptake patterns of two constituents of the labile DOM pool (N-acetyl-glucosamine [NAG] and leucine). There was a clear genotypic shift within the bacterial assemblage, from fast growing Cytophaga-Flavobacteria (CF) affiliated with Fluviicola and from Betaproteobacteria (BET) of the Limnohabitans cluster to more grazing resistant AcI Actinobacteria (ACT) and to filamentous morphotypes. This was paralleled by successive blooms of viruses and HNF. We also noted the transient rise of other CF (related to Cyclobacteriaceae and Sphingobacteriaceae) that are not detected by fluorescence in situ hybridization with the general CF probe. Both, the average uptake rates of leucine and the fractions of leucine incorporating bacteria were approximately five to sixfold higher than of NAG. However, the composition of the NAG-active community was much more prone to genotypic successions, in particular of bacteria with different life strategies: While 'opportunistically' growing BET and CF dominated NAG uptake in the initial period ruled by bottom-up factors, ACT constituted the major fraction of NAG active cells during the subsequent phase of high predation pressure. This indicates that some ACT could profit from a substrate that might in parts have originated from the grazing of protists on their bacterial competitors.