Significant contribution of the unicellular cyanobacterium UCYN-B to oceanic nitrogen fixation

Dinitrogen (N₂) fixation by diazotrophs forms a critical control on ocean productivity, yet our understanding of the distribution, abundance and activity of these microorganisms is informed by observations with limited geographical coverage. Here we show that the unicellular N₂-fixing cyanobacterium, UCYN-B, drives high N₂ fixation rates in the western North Pacific. Defining the ecological niche of UCYN-B using our new observations in combination with existing global diazotroph datasets allowed prediction of additional and previously unrecognized UCYN-B-dominated N₂ fixation hot spots throughout large, unsampled swathes of the global ocean. Collectively, these regions potentially contribute 10.8-15.0 Tg N yr^-1 to global marine N₂ fixation, suggesting a pivotal role for UCYN-B in the regulation of the marine nitrogen inventory and therefore ocean productivity.