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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 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.
Seasonal changes in environmental conditions have a strong impact on microbial community structure and dynamics in aquatic habitats. To better elucidate the response of bacterial communities to environmental changes, we have measured a large variety of limnetic variables and investigated bacterial community composition (BCC) and dynamics over seven consecutive years between 2003 and 2009 in mesotrophic Lake Tiefwaren (NE Germany). We separated between free-living (FL, > 0.2, < 5.0 mu m) and particle-associated (PA, > 5.0 mu m) bacteria to account for different bacterial lifestyles and to obtain a higher resolution of the microbial diversity. Changes in BCC were studied by DGGE based on PCR-amplified 16S rRNA gene fragments. Sequencing of DGGE bands revealed that ca. 70 % of all FL bacteria belonged to the Actinobacteria, whereas PA bacteria were dominated by Cyanobacteria (43 %). FL communities were generally less diverse and rather stable over time compared to their PA counterpart. Annual changes in reoccurring seasonal patterns of dominant freshwater bacteria were supported by statistical analyses, which revealed several significant correlations between DGGE profiles and various environmental variables, e.g. temperature and nutrients. Overall, FL bacteria were generally less affected by environmental changes than members of the PA fraction. Close association of PA bacteria with phytoplankton and zooplankton suggests a tight coupling of PA bacteria to organisms of higher trophic levels. Our results indicate substantial differences in bacterial lifestyle of pelagic freshwater bacteria, which are reflected by contrasting seasonal dynamics and relationships to a number of environmental variables.