Stefan Roesel, Hans-Peter Grossart

• Phytoplankton development affects the community structure and dynamics of freshwater bacteria by changing the availability of nutrients, algal exudates and biological surfaces. To elucidate these effects of phytoplankton development in spring in oligotrophic Lake Stechlin (Germany), we measured limnological and biological parameters, including the bacterial community composition (BCC), at the depth of the highest chlorophyll a concentration. To increase the resolution of BCC measurements, we separated particle-associated (PA) and free-living (FL) bacteria using serial filtration through 5.0 and 0.2 mu m pore size filters, respectively. The BCC of ultramicrobacteria was also determined by collecting the 0.2 mu m filtrate on 0.1 mu m filters. Changes in the community composition of Bacteria and particularly of Actinobacteria, one of the most important bacterial groups in temperate freshwater habitats, were studied via DGGE analysis of PCR-amplified 16S rRNA gene fragments. Patterns in BCC dynamics of FL Bacteria and ActinobacteriaPhytoplankton development affects the community structure and dynamics of freshwater bacteria by changing the availability of nutrients, algal exudates and biological surfaces. To elucidate these effects of phytoplankton development in spring in oligotrophic Lake Stechlin (Germany), we measured limnological and biological parameters, including the bacterial community composition (BCC), at the depth of the highest chlorophyll a concentration. To increase the resolution of BCC measurements, we separated particle-associated (PA) and free-living (FL) bacteria using serial filtration through 5.0 and 0.2 mu m pore size filters, respectively. The BCC of ultramicrobacteria was also determined by collecting the 0.2 mu m filtrate on 0.1 mu m filters. Changes in the community composition of Bacteria and particularly of Actinobacteria, one of the most important bacterial groups in temperate freshwater habitats, were studied via DGGE analysis of PCR-amplified 16S rRNA gene fragments. Patterns in BCC dynamics of FL Bacteria and Actinobacteria remained fairly constant throughout the study period, while patterns of PA Bacteria were more variable over time. At the breakdown of the diatom spring bloom, bacterial production and abundance sharply increased, indicating a close coupling between heterotrophic bacteria and algal detritus. The succession in BCC revealed life-style dependent patterns related to specific environmental variables. Our results indicate independent dynamics of PA and FL Bacteria as well as Actinobacteria during succession of phytoplankton spring blooms. These differences in bacterial lifestyle can only be resolved when the PA and FL fractions of microorganisms are separated.…