Salinity as a Determinant Structuring Microbial Communities in Coastal Lakes.

The response of bacterioplankton structure to salinity level in coastal lakes ( n = 9) along the southern Baltic Sea coastline was studied. In terms of mean salinity levels (0.2-5.2 PSU), the lakes represented freshwater, transitional, and brackish types. Results showed that salinity determines the spatial and seasonal distribution patterns of microorganisms in costal lakes. Increased salinity contributed to a significant decline in total bacterial numbers (TBN). The TBN was lowest in brackish lakes in autumn (4 × 10 6 cells/mL) and highest in freshwater lakes in summer (7.11 × 10 6 cells/mL). The groups of Proteobacteria are appropriate bioindicators in any classifications of coastal ecosystems, particularly at low-haline stress. Alpha - and Gamma - subclasses of Proteobacteria are identifiers for brackish habitats, while Betaproteobacteria , due to their intolerance to haline stress, prefer freshwater habitats. Counts of euryhaline Actinobacteria , the dominant group of bacterioplankton (31.8%), decreased significantly with increased salinity. Actinobacteria and Deltaproteobacteria were identifiers of transitional lakes. C ytophaga-Flavobacteria showed affinity with freshwater ecosystems, but this relation was not statistically significant ( p > 0.05). The bacteria groups correlated with other physico-chemical parameters of water, such as oxygenation ( Actinobacteria ) or organic carbon ( Betaproteobacteria , Deltaproteobacteria ). The impact of hydrological connectivity and salt-water interference on the microbiota structure and biogeochemistry of coastal waters should be considered in the assessment of the ecological status of coastal lakes.