The Baltic Sea: A Brackish Environment

Many different environmental factors influence the composition of communities in marine environments, and this paper expands on previous research which has shown that salinity has a particularly important effect on the distribution of both pelagic and benthic organisms. Although research has been carried out to investigate the effect of salinity on micro-organisms, this study is the first to investigate the distribution and community composition of micro-organisms found in brackish environments with the use of 454 pyrosequencing of partial 16S rRNA genes.

Brackish environments occur where freshwater mixes with saltwater, as happens in estuaries. Typically a reduction in diversity is seen in these environments due to the adaptations of multicellular organisms to either freshwater or saltwater conditions, but less is known about the diversity of micro-organisms within these environments. Previous studies of microbial communities have been carried out in areas such as river outlets where conditions change rapidly and communities of micro-organisms adapted to brackish environments are not able to establish.

The Baltic Sea differs from these types of sites because it is comparatively more stable, with a water retention time between 3 and 30 years. The Baltic Sea is one of the largest brackish environments in the world, with salinity gradients maintained by the discharge of freshwater and a limited exchange with the North Sea.

A total of 213 samples were collected from 60 sampling stations along the salinity gradient of the Baltic Sea during June and July 2008. DNA was extracted, amplified and sequenced, to produce a ‘bacterial inventory’ of the sea.

Results revealed three clusters of samples at salinities of 0-3, 5-8, and 10-31, providing evidence of three bacterial communities, which were described as freshwater-brackish, brackish and brackish-marine. A shift in community composition was observed across the salinity gradient. It was predicted that at intermediate salinities there would be minimum species richness but results showed no significant difference in diversity between the three salinity clusters. The authors conclude that freshwater and marine clades that have apparently adapted to brackish conditions exist within the Baltic Sea and maintain this diversity at intermediate salinities.

This paper details the first taxonomic study of a brackish microbiome, and highlights the differences between bacteria and multicellular organisms in their ability to adapt to these conditions. Further research is needed to understand more about the micro-organisms within these communities and how they contribute to the brackish environment, and their functioning within ecosystems.

A review of: Herlemann, D.P.R., Labrenz, M., Jürgens, K., Bertilsson, S., Waniek, J.J. and Andersson, A.F. (2011) Transitions in bacterial communities along the 2000km salinity gradient of the Baltic Sea, The ISME Journal 5, 1571-1579.