Regional patterns of bacterial community composition and biogeochemical properties in the southern North Sea

The German Bight of the North Sea is characterized by intertidal flats, with high loads of suspended particulate matter (SPM) and estuarine inputs of organic matter, as well as pelagic off-shore regions. Due to tidal and wind-induced currents, the hydrography of this area is highly dynamic. In order to examine how these properties affect the regional distribution and composition of the bacterioplankton we conducted 2 surveys, in June 2002 and June 2003; in these surveys we assessed the composition of the free-living (FL) and particle-associated (PA) bacterial communities together with hydrographic properties (salinity, temperature) and biogeochemical properties (SPM, particulate organic carbon, chlorophyll, phytoplankton composition). The bacterial community composition was determined by denaturing gradient gel electrophoretic (DGGE) analysis of 16S rRNA gene fragments, PCR-amplified by Bacteria-, Alphaproteobacteria- and Bacteroidetes-specific primer sets, and sequencing of excised bands. The results, and a multidimensional scaling (MDS) analysis of the DGGE banding patterns, showed distinct differences in the bacterial community composition of the FL and PA fractions and also between the inshore and offshore stations. A canonical correspondence analysis (CCA), including the DGGE banding patterns, temperature, salinity and biogeochemical parameters, further showed that the community patterns and the inshore and offshore gradients can be explained to a great extent by the hydrographic and biogeochemical conditions. Several prominent bands, identified as phylotypes affiliated to the Roseobacter clade, persisted throughout all 10 stations visited, whereas other phylotypes occurred only at distinct stations, inshore and offshore.