A unicellular cyanobacterium relies on sodium energetics to fix N.

Diazotrophic cyanobacteria can fix nitrogen gas (N) but are usually scarce in nitrogen-limited coastal waters, which poses an apparent ecological paradox. One hypothesis is that high salinities (> 10 g/L NaCl) may inhibit cyanobacterial N fixation. However, here we show that N fixation in a unicellular coastal cyanobacterium exclusively depends on sodium ions and is inhibited at low NaCl concentrations (< 4 g/L). In the absence of Na, cells of Cyanothece sp. ATCC 51142 (recently reclassified as Crocosphaera subtropica) upregulate the expression of nifHDK genes and synthesise a higher amount of nitrogenase, but do not fix N and do not grow. We find that the loss of N-fixing ability in the absence of Na is due to insufficient ATP supply. Additional experiments suggest that N fixation in this organism is driven by sodium energetics and mixed-acid fermentation, rather than proton energetics and aerobic respiration, even though cells were cultured aerobically. Further work is needed to clarify the underlying mechanisms and whether our findings are relevant to other coastal cyanobacteria.