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Appl. Environ. Microbiol. doi:10.1128/AEM.01983-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The Native Bacterioplankton Community in the Central Baltic Sea is Influenced by Freshwater Bacterial Species

Natural Sciences, Kalmar University, 39182 Kalmar, Sweden; Department of Systems Ecology, Stockholm University, 10691 Stockholm, Sweden

^* To whom correspondence should be addressed. Email: Lasse.Riemann{at}hik.se.

	Abstract

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 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. Bacterial community composition was examined in surface water (3 m depth) at a single station throughout a full year. Denaturing gradient gel electrophoresis (DGGE) showed that community composition changed over the year. Further, it indicated that at the 4 extensive samplings (16S rRNA gene clone libraries and bacterial isolates from low and 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 by typical freshwater phylogenetic groups within Actinobacteria, Verrucomicrobia, and -Proteobacteria 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.