Temporal changes in the dominance of major planktonic bacterial groups in an alpine lake: discrepancy with their contribution to bacterial production

We tested the idea that temporal changes in the numerical dominance of major bacterial groups are not necessarily coupled to those in bacterial production. The seasonal dynamics of the bacterial community in the water column of an alpine lake were followed from late spring until the autumn at either bi-weekly or monthly intervals. Changes in community structure were tracked by fluorescence in situ hybridization combined with catalyzed reporter deposition (CARD-FISH); the contribution of the dominant bacterial groups to leucine incorporation was assessed by microautoradiography (MAR). Before ice-out, the bacterial assemblage was dominated by Actinobacteria. Immediately afterwards, their relative abundance, and that of Cytophaga-like bacteria, increased in the water column, suggesting that they had taken advantage of the input of dissolved organic matter (DOM) during the melting of the ice. During summer, Betaproteobacteria and Actinobacteria together accounted for > 50% of the cell counts; however, their maxima were separated temporally and vertically. Water temperature, DOM, and grazing influenced their antagonistic behavior. Alpha-proteobacteria were consistently present, although their abundance was generally low. Regardless of their relative abundance, Betaproteobacteria dominated the assemblage of leucine-positive cells, and the relative abundance of their R-BT subgroup closely matched that of active Betaproteobacteria. Of the cells positive for leucine incorporation, the contribution of Actinobacteria was lower than expected from their relative abundance. Our results show that bacterial abundance and production are not necessarily dominated by the same phylogenetic groups.