Transitions in bacterial communities along the 2000 km salinity gradient of the Baltic Sea

Salinity is a major factor controlling the distribution of biota in aquatic systems, and most aquatic multicellular organisms are either adapted to life in saltwater or freshwater conditions. Consequently, the saltwater-freshwater mixing zones in coastal or estuarine areas are characterized by a limited faunal and floral diversity. Whereas changes in diversity and decline in species richness in brackish waters is well documented in aquatic ecology, it is unknown to what extent this applies to bacterial communities. Here we report a first detailed bacterial inventory from vertical profiles of 60 sampling stations distributed along the salinity gradient of the Baltic Sea, one of worldâ??s largest brackish water environments, generated using 454 pyrosequencing of partial (400 bp) 16S rRNA genes. Within the sampled salinity gradient, bacterial community composition altered at broad and at fine-scale phylogenetic levels. Analogous to observed faunal community composition within brackish conditions, we identified a distinct bacterial brackish water community comprising a diverse combination of freshwater and marine groups, along with populations unique to this environment. Since water residence times in the Baltic Sea exceed five years, the brackish bacterial community cannot be the result of conservative mixing of freshwater and saltwater, but reveals the first detailed description of an autochthonous brackish microbiome. In stark contrast to the decline in the diversity of multicellular organisms, reduced bacterial diversity was not observed at brackish conditions. Hence, the rapid adaptation of bacteria has enabled a variety of lineages to fill what for higher organisms remains a challenging, relatively unoccupied ecological niche.