SEASONAL COMMUNITY AND POPULATION-DYNAMICS OF PELAGIC BACTERIA AND ARCHAEA IN A HIGH-MOUNTAIN LAKE

The seasonal variations in community structure and cell morphology of pelagic procaryotes from a high mountain lake (Gossenkollesee, Austria ) were studied by in situ hybridization with rRNA-targeted fluorescent ly labeled oligonucleotide probes (FISH) and image-analyzed microscopy . Compositional changes and biomass fluctuations within the assemblage were observed both in summer and beneath the winter ice cover and are discussed in the context of physicochemical and biotic parameters. Pr oteobacteria of the beta subclass (beta-proteobacteria) formed a domin ant fraction of the bacterioplankton (annual mean, 24% of the total co unts), whereas alpha-proteobacteria were of similar relative importanc e only during spring (mean, 11%). Bacteria of the Cytophaga-Flavobacte rium cluster, although less abundant, constituted the largest fraction of the filamentous morphotypes during most of the year, thus contribu ting significantly to the total microbial biomass. Successive peaks of threadlike and rod-shaped archaea were observed during autumn thermal mixing and the period of ice cover formation, respectively. A set of oligonucleotide probes targeted to single phylotypes was constructed f rom 16S rRNA-encoding gene clone sequences. Three distinct populations of uncultivated microbes, affiliated with the alpha- and beta-proteob acteria, were subsequently monitored by FISH. About one-quarter of all of the beta-proteobacteria (range, 6 to 53%) could be assigned to onl y two phylotypes. The bacterial populations studied were annually recu rrent, seasonally variable, and vertically stratified, except during t he periods of lake overturn. Their variability clearly exceeded the fl uctuations of the total microbial assemblage, suggesting that the appa rent stability of total bacterioplankton abundances may mask highly dy namic community fluctuations.