Seasonal dynamics and phylogenetic diversity of free-living and particle-associated bacterial communities in four lakes in northeastern Germany

The phylogenetic diversity and seasonal dynamics of free-living and particle-associated bacterial communities were investigated in the epilimnion of 4 lakes of the Mecklenburg Lake District, northeastern Germany. All lakes differed in their limnological features, ranging from oligotrophic to eutrophic and dystrophic. Bacterial community structure and seasonal dynamics were analyzed by denaturing gradient gel electrophoresis (DGGE) and clone libraries of 16S rRNA gene fragments. Communities of free-living and particle-associated bacteria greatly differed among the lakes. In addition, significant differences occurred between both bacterial fractions within each lake. Seasonal changes were more pronounced in free-living than in particle-associated bacterial communities. Non-metric multidimensional scaling (NMDS) analyses revealed several strong correlations between bacterial communities (both free-living and particle-associated) and environmental variables such as pH, dissolved organic carbon (DOC), phytoplankton biomasses, and primary production. Phylogenetically, all cloned and sequenced 16S rRNA gene fragments belonged to already known freshwater clusters. Clone libraries of free-living bacteria were dominated by sequences of Actinobacteria, Bacteroidetes, and Betaproteobacteria, whereas those of particle-associated bacteria predominantly consisted of Cyanobacteria and Bacteroidetes sequences. Other freshwater phyla such as Alpha- and Gammaproteobacteria, Verrucomicrobia, Planctomycetes, and members of Candidate Division OP10 were found in low proportions. These differences may indicate an adaptation of distinct bacterioplankton communities to the respective environmental conditions of each lake.