Abstract

Over the next decade, the Baltic Sea is predicted to undergo severe changes including a decrease in salinity due to altering precipitation. This will likely impact the distribution and community composition of Baltic Sea N2 fixing microbes, of which especially heterocystous cyanobacteria are adapted to low salinities and may expand to waters with currently higher salinity, including the Danish Strait and Kattegat, while other high-salinity adapted N2 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 Kiel Bight and combined N2 fixation rate measurements with a molecular analysis of the diazotrophic community using the key functional marker gene for N2 fixation nifH, as well as the key functional marker genes anf and vnf, encoding for the two alternative nitrogenases. We detected N2 fixation rates between 0.7 and 6 nmol N L-1 d-1, and the diazotrophic community was dominated by the cyanobacterium Nodularia and the small unicellular, cosmopolitan cyanobacterium UCYN-A. Nodularia was present in abundances between 8.07 x 105 and 1.6 x 107 copies L-1 in waters with salinities of 10 and below, while UCYN-A reached abundances of up to 4.5 x 107 copies L-1 in waters with salinity above 10. Besides those two cyanobacterial diazotrophs, we found several clades of proteobacterial N2 fixers and alternative nitrogenase genes associated with Rhodopseudomonas palustris, a purple non-sulfur bacterium. Based on statistical testing, salinity was identified as the primary parameter describing the diazotrophic distribution, while pH and temperature did not have a similarly significant influence on the diazotrophic distribution. While this statistical analysis will need to be explored in direct experiments, it gives an indication for a 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.

Highlights

  • The Baltic Sea (Fig. 1) is a marginal, brackish sea characterized by a natural salinity gradient increasing from the North-East 30 to the South-West

  • While this statistical analysis will need to be explored in direct experiments, it gives an indication for a future development of diazotrophy in a freshening Baltic Sea with UCYN-A retracting to more saline North Sea waters 25 and heterocystous cyanobacteria expanding as salinity decreases

  • N2 fixation rates were detectable but low for the Baltic Sea and sustained by a 325 diazotrophic community dominated by the heterocystous cyanobacterium Nodularia and the small unicellular cyanobacterium UCYN-A

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Summary

Introduction

The Baltic Sea (Fig. 1) is a marginal, brackish sea characterized by a natural salinity gradient increasing from the North-East 30 to the South-West. The resulting eutrophication affects severely sensitive coastal areas resulting in high pelagic production, frequent events of anoxia, and decreased biodiversity (Breitburg et al, 2018; Carstensen et al, 2014; Maar et al, 2016; Reusch et al, 2018; 40 Rutgersson et al, 2014). In this context, the Baltic Sea has been described as a "time machine" for how future oceans will respond to climate change (Reusch et al, 2018), making it an ideal environment to investigate biological fixation of dinitrogen gas (N2 fixation) in response to such changes

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