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 (N2) from the atmosphere. Despite this, they occur in low abundance in N-limited coastal waters, which represents an ecological paradox1–3. One hypothesis is that this is partly due to elevated salinity (> 10 g/L NaCl), which inhibits cyanobacterial N2fixation2,3. Here we show that N2fixation in a unicellular coastal cyanobacterium is not inhibited but rather exclusively dependent on sodium (Na+) ions. In N-deficient environments, both N2fixation 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 N2fixation, as Na+deficiency resulted in insufficient ATP supply for N2fixation. 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 N2fixers 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 N2-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 N2fixation and population growth, likely resulting in dense cyanobacterial blooms.