Vertical and longitudinal distribution patterns of different bacterioplankton populations in a canyon-shaped, deep prealpine lake

The pelagic zone of large, deep freshwater lakes features pronounced horizontal and vertical gradients of physicochemical parameters, which in turn might allow for a nonuniform occurrence of specifically adapted bacterial taxa. We, therefore, studied the spatial distribution patterns of different heterotrophic bacteria, picocyanobacteria, and the dominant primary producer, the filamentous cyanobacterium Planktothrix rubescens, in a large, canyon-shaped, prealpine lake (Lake Zurich, Switzerland), in six vertical profiles along a 21.7-km longitudinal transect. Highest total densities and proportions of cells with high nucleic acid content were in the warm epilimnion and the hypoxic zone of the hypolimnion. P. rubescens formed a dense layer in the metalimnion throughout the lake, whereas picocyanobacteria populated the water layers above. The epilimnion was mainly inhabited by ultramicrobacteria related to the LD12-lineage of Alphaproteobacteria and to Actinobacteria; the latter group preferred the shallow regions. Cytophaga–Flavobacteria, in particular a population related to Fluviicola sp. were more frequent in and below the layer of maximal P. rubescens abundances. Betaproteobacteria, on the other hand, were highly abundant in the epi- and hypolimnion, but not in the P. rubescens layer. Four betaproteobacterial subpopulations with contrasting longitudinal and/or vertical habitat preferences were distinguished: putatively methylotrophic bacteria of the LD28 lineage (beta IV) preferentially inhabited the hypolimnion, Polynucleobacter acidiphobus was found throughout the epilimnion, Limnohabitans (R-BT065) more in the shallow regions of the lake, and Polynucleobacter necessarius ssp. asymbioticus only in hypoxic waters. Our results stress the potential importance of spatial niche differentiation in freshwater bacterioplankton.