Typical freshwater and marine bacterial lineages dynamics at salinity between 0 and 4 in the Vistula Lagoon.

The Vistula Lagoon is an almost closed basin at the southern coast of the Baltic Sea. Its mild salinity gradient provides a unique environment for the coexistence of freshwater and marine bacteria. This study employs catalysed reporter deposition-fluorescence in situ hybridisation (CARD-FISH) to investigate seasonal dynamics of marine (SAR11 clade I/II) and freshwater (SAR11 clade IIIb (LD12), Limnohabitans clades B, C, D, Burkholderiales (former Betaproteobacteria)) bacterial groups. Samples were collected from below the ice in February, and then monthly from April to October 2011 from three stations with different salinity. The abundance of SAR11 clade I/II strongly positively correlated with salinity, with the highest abundance in autumn at time of inflows of saline waters from the Baltic Sea. Two groups (spring-summer and autumn-winter) were distinguished within this clade, based on a scatter chart between SAR11-I/II abundance and salinity. Salinity explained 69% of the variability of the spring-summer group and 77% of the autumn-winter group. This suggests that the presence of marine SAR11-I/II in the Vistula Lagoon was caused by passive inflow with waters from the open Baltic Sea, and this environment is not suitable for its regular existence. The abundance of the freshwater SAR11 clade IIIb was similar to that of SAR11-I/II. However, it depended on temperature, organic and inorganic phosphorous, and not on salinity, suggesting that SAR11-IIIb found a regular niche in the eutrophic Vistula Lagoon. Burkholderiales positively correlated with temperature, chlorophyll-a, organic and inorganic phosphorous, and heterotrophic nanoflagellates, suggesting that their role in the Vistula Lagoon may be similar to that in lakes. Interestingly, Burkholderiales had the highest relative abundance under ice in February. Finally, the abundance of Limnohabitans clades B, C, D positively depended on temperature, chlorophyll-a and negatively on salinity. This together with positive correlation with Cryptophytes and with heterotrophic nanoflagellates emphasis its importance in this ecosystem. These results show that the conditions in the Vistula Lagoon are rather inappropriate for typical marine bacteria, while freshwater bacteria may find their niches despite slightly elevated salinity. • Marine SAR11-I/II was passively brought to the Lagoon with waters inflowing from the Baltic Sea. • Freshwater SAR11-IIIb and Limnohabitans are active members of the bacterial community in the Vistula Lagoon. • Burkholderiales had the highest relative abundance under ice cover in winter. [ABSTRACT FROM AUTHOR]