Unicellular cyanobacteria fix N2 in the subtropical North Pacific Ocean

Fixed nitrogen (N) often limits the growth of organisms in terrestrial and aquatic biomes(1,2), and N availability has been important in controlling the CO2 balance of modern and ancient oceans(3,4). The fixation of atmospheric dinitrogen gas (N-2) to ammonia is catalysed by nitrogenase and provides a fixed N for N-limited environments(2,5). The filamentous cyanobacterium Trichodesmium has been assumed to be the predominant oceanic N-2-fixing microorganism since the discovery of N-2 fixation in Trichodesmium in 1961 (ref. 6). Attention has recently focused on oceanic N-2 fixation because nitrogen availability is generally limiting in many oceans, and attempts to constrain the global atmosphere-ocean fluxes of CO2 are based on basin-scale N balances(7-9). Biogeochemical studies and models have suggested that total N-2-fixation rates may be substantially greater than previously believed(7,8) but cannot be reconciled with observed Trichodesmium abundances(8,9). It is curious that there are so few known N-2-fixing microorganisms in oligotrophic oceans when it is clearly ecologically advantageous. Here we show that there are unicellular cyanobacteria in the open ocean that are expressing nitrogenase, and are abundant enough to potentially have a significant role in N dynamics.