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

Abstract. Over the next decade, Baltic Sea is predicted to undergo severe changes including decreased salinity due altered precipitation related climate changes. This will likely impact distribution and community composition of dinitrogen-fixing (N2-fixing) microbes, among which heterocystous cyanobacteria are especially adapted low salinities may expand waters with currently higher salinity, Danish Strait Kattegat, while other high-salinity-adapted N2 fixers might decrease in abundance. In order explore on activity different diazotrophic clades, we followed natural gradient from eastern Gotland Bornholm basins through Arkona Basin Kiel Bight combined fixation rate measurements a molecular analysis using key functional marker gene for nifH, as well genes anfD vnfD, encoding two alternative nitrogenases. We detected rates between 0.7 6 nmol N L−1 d−1, was dominated by cyanobacterium Nodularia spumigena small unicellular, cosmopolitan UCYN-A. present abundances 8.07 × 105 1.6 107 copies 10 below, UCYN-A reached up 4.5 above 10. Besides those cyanobacterial diazotrophs, found several clades proteobacterial nitrogenase associated Rhodopseudomonas palustris, purple non-sulfur bacterium. Based principal component (PCA), identified primary parameter describing distribution, pH temperature did not have significant influence distribution. While this statistical need be explored direct experiments, it gives an indication future development diazotrophy freshening retracting more saline North expanding decreases.