Oceanic Nitrogen Fixation Nitrogen fixation in the oceans is important in sustaining global marine productivity and balances carbon dioxide export to the deep ocean. It was previously believed that marine nitrogen fixation was due to a single genus of filamentous cyanobacteria, Trichodesmium . The recent discovery of unicellular open-ocean cyanobacteria has raised the question of how they contribute to global ocean nitrogen fixation and how they compare in distribution and activity to Trichodesmium . Using data collected from the southwest Pacific Ocean, Moisander et al. (p. 1512 , published online 25 February) show that the unicellular nitrogen-fixing cyanobacteria (UCYN-A and Crocosphaera watsonii ) have distinct ecophysiologies and distinct oceanic distributions from each other, and from Trichodesmium . These data can be incorporated into models to retune estimates of the global rates of oceanic nitrogen fixation and carbon sequestration.