Salinity as a key control on the diazotrophic community composition in the southern Baltic Sea

Over the next decade, the Baltic Sea is predicted to undergo severe changes including decreased salinity due to altered precipitation related to climate changes. This will likely impact the distribution and community composition of Baltic Sea dinitrogen-fixing (N-2-fixing) microbes, among which heterocystous cyanobacteria are especially adapted to low salinities and may expand to waters with currently higher salinity, including the Danish Strait and Kattegat, while other high-salinity-adapted N-2 fixers might decrease in abundance. In order to explore the impact of salinity on the distribution and activity of different diazotrophic clades, we followed the natural salinity gradient from the eastern Gotland and Bornholm basins through the Arkona Basin to the Kid Bight and combined N-2 fixation rate measurements with a molecular analysis of the diazotrophic community using the key functional marker gene for N-2 fixation nifH, as well as the key functional marker genes anfD and vnfD, encoding for the two alternative nitrogenases. We detected N-2 fixation rates between 0.7 and 6 nmol N L-1 d(-1), and the diazotrophic community was dominated by the cyanobacterium related to Nodularia spumigena and the small unicellular, cosmopolitan cyanobacterium UCYN-A. Nodularia was present in gene abundances between 8.07 x 10(5) and 1.6 x 10(7) copies L-1 in waters with salinities of 10 and below, while UCYN-A reached gene abundances of up to 4.5 x 10(7) copies L-1 in waters with salinity above 10. Besides those two cyanobacterial diazotrophs, we found several clades of proteobacterial N-2 fixers and alternative nitrogenase genes associated with Rhodopseudomonas palustris, a purple non-sulfur bacterium. Based on principal component analysis (PCA), salinity was identified as the primary parameter describing the diazotrophic distribution, while pH and temperature did not have a significant influence on the diazotrophic distribution. While this statistical analysis will need to be explored in direct experiments, it gives an indication for the future development of diazotrophy in a freshening Baltic Sea with UCYN-A retracting to more saline North Sea waters and heterocystous cyanobacteria expanding as salinity decreases.

Automated summary

In the next decade, it is predicted that the Baltic Sea will undergo significant changes, including decreased salinity, which will impact the distribution and composition of diazotrophic microbes. This study found that salinity is the primary factor influencing the distribution of these microbes, while pH and temperature have no significant influence.