The Baltic Sea is one of the largest brackish environments on Earth. Despite extensive knowledge about food web interactions and pelagic ecosystem functioning, information about the bacterial community composition in the Baltic Sea is scarce. We hypothesized that due to the eutrophic low-salinity environment and the long water residence time (>5 years), the bacterioplankton community from the Baltic proper shows a native "brackish" composition influenced by both freshwater and marine phylotypes. The bacterial community composition in surface water (3-m depth) was examined at a single station throughout a full year. Denaturing gradient gel electrophoresis (DGGE) showed that the community composition changed over the year. Further, it indicated that at the four extensive samplings (16S rRNA gene clone libraries and bacterial isolates from lowand high-nutrient agar plates and seawater cultures), different bacterial assemblages associated with different environmental conditions were present. Overall, the sequencing of 26 DGGE bands, 160 clones, 209 plate isolates, and 9 dilution culture isolates showed that the bacterial assemblage in surface waters of the central Baltic Sea was dominated by Bacteroidetes but exhibited a pronounced influence of typical freshwater phylogenetic groups within Actinobacteria, Verrucomicrobia, and Betaproteobacteria and a lack of typical marine taxa. This first comprehensive analysis of bacterial community composition in the central Baltic Sea points to the existence of an autochthonous estuarine community uniquely adapted to the environmental conditions prevailing in this brackish environment.

Plasmids and phages in£uence bacterial phenotype and may serve as vectors for transferring genes between bacteria. In the present study, we examined 130 marine bacterioplankton isolates for the presence of plasmids and prophages. Samples were obtained in spring, summer and autumn in the Baltic Sea proper. Plasmids and inducible prophages were found in 19% and 28% of the isolates, respectively. During spring, plasmids and prophages were 41^55% and 30% more common compared to the summer and autumn measurements and prevalence varied up to ¢ve-fold between bacterial phylogenetic groups, with the highest plasmid prevalence found in Bacteriodetes (41%), and lysogeny being common in a-, b-, and g-Proteobacteria (32^50%). Plasmid genome sizes ranged from 1.5^15 kb with most in the 2.1^4.0 kb size-range. No plasmids showed identity to the broad-host-range incompatibility groups N and P. Phage genomes ranged in size from 8^87 kb, with 57% being 35^45 kb in size. Strain typing of phages with similar genome sizes by means of DP-RAPD (degenerated primer randomly ampli¢ed polymorphic DNA) showed that all were di¡erent (except two that were not resolved). In PFGE (pulsed-¢eld gel electrophoresis) 34% of the lysates produced multiple bands. Transmission electron microscopy suggested that these originated from several phage morphotypes indicating that polylysogeny is common. The widespread distribution of small cryptic plasmids as well as of lysogeny and polylysogeny in Baltic Sea bacterioplankton may have important implications for bacterial phenotype and for lateral gene transfer; hence, the ecological signi¢cance of these vectors in marine environments requires further study.