Unicellular cyanobacteria rely on sodium energetics to fix N2

Abstract Diazotrophic cyanobacteria can thrive in combined nitrogen (N)-limited environments due to their ability to fix nitrogen gas (N 2 ) from the atmosphere. Despite this, they occur in low abundance in N-limited coastal waters, which represents an ecological paradox 1–3 . One hypothesis is that this is partly due to elevated salinity (> 10 g/L NaCl), which inhibits cyanobacterial N 2 fixation 2,3 . Here we show that N 2 fixation in a unicellular coastal cyanobacterium is not inhibited but rather exclusively dependent on sodium (Na + ) ions. In N-deficient environments, both N 2 fixation and population growth were significantly inhibited at low NaCl concentrations (< 4 g/L). Additional experiments indicated that sodium energetics, rather than proton energetics, is necessary for N 2 fixation, as Na + deficiency resulted in insufficient ATP supply for N 2 fixation. We show that this is due to the non-functioning Na + -coupled ATP synthase, which we found to be likely coupled to anaerobic rather than aerobic respiration. Sequence alignment analysis of the ion-coupling site of the ATP synthase revealed a high prevalence of Na + energetics in cyanobacteria, with all unicellular N 2 fixers capable of Na + energetics. This suggests a critical role for sodium energetics in cyanobacteria. It also raises the possibility that sodium energetics is not as rare as thought, but that we may have underestimated the prevalence and importance of sodium energetics in other organisms. Finally, the low abundance of diazotrophic unicellular cyanobacteria in coastal waters may be due to insufficient NaCl levels to support N 2 -fixation during periods of growth-supporting high temperatures. This provides another perspective on the regulation of the oceanic N cycle that needs to be considered in times of global climate change. Changes in current patterns could lead to an overlap of periods optimal for N 2 fixation and population growth, likely resulting in dense cyanobacterial blooms.