Nitrogen Fixation and Anoxygenic Photosynthesis in Filamentous Non-Heterocystous Cyanobacterium of the Genus Sodalinema Isolated from Soda Lake

Saline and highly alkaline soda lakes are often characterized by a persistent nitrogen loss and high sulfide levels. Cyanobacteria are key aerobic diazotrophs in soda lakes, where light-dependent nitrogen fixation (NF) is crucial for sustaining ecosystem functioning. While sulfide is a well-known inhibitor of oxygenic photosynthesis, some cyanobacteria may tolerate it and utilize it via anoxygenic photosynthesis. In this study, we investigated the NF and anoxygenic photosynthesis in the genus <i>Sodalinema</i>, including non-heterocystous cyanobacteria widely distributed in soda and saline environments around the world and possessing an anaerobe-like nitrogenase. Our data suggest that their <i>nif</i>-operon could have been more likely acquired in soda or saline-alkaline lakes from natronophilic sulfate-reducing bacteria of the family <i>Desulfonatronovibrionaceae</i> than in the marine environment. It was shown that <i>Sodalinema</i> sp. P-1104, isolated from a southwestern Siberian soda lake, is capable of NF only in a light/dark switching mode, both in oxic and anoxic conditions. Sulfide did not suppress photosynthesis and stimulated NF up to threefold in oxygenic conditions. Anaerobic NF was obligately sulfide-dependent and supported by anoxygenic photosynthesis. However, removal of photosynthetic oxygen due to the high reducing potential of sulfide stimulated NF to a greater extent than does the use of sulfide through anoxygenic photosynthesis.