10 Sep 2021

10 Sep 2021

Review status: this preprint is currently under review for the journal OS.

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

Christian Furbo Reeder1, Ina Stoltenberg1, Jamileh Javidpour1, and Carolin Regina Löscher1,2 Christian Furbo Reeder et al.
  • 1Department of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M, DK
  • 2Danish Institute for Advanced Study, University of Southern Denmark, Campusvej 55, 5230 Odense M, DK

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.

Christian Furbo Reeder et al.

Status: open (until 14 Dec 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2021-82', Anonymous Referee #1, 20 Oct 2021 reply
  • RC2: 'Comment on os-2021-82', Harri Kuosa, 26 Nov 2021 reply

Christian Furbo Reeder et al.

Christian Furbo Reeder et al.


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Short summary
The Baltic Sea is predicted to freshen in the future. To explore the effect of decreasing salinity on N2 fixers, we followed the natural salinity gradient in the Baltic Sea from the Kiel Fjord to the Gotland Basin and identified an N2 fixer community dominated by Nodularia and UCYN-A. A salinity threshold was identified at a salinity of 10, with Nodularia dominating at low and UCYN-A dominating at higher salinity, suggesting a future expansion of Nodularia N2 fixers and a retraction of UCYN-A.