High rates of N2 fixation by unicellular diazotrophs in the oligotrophic Pacific Ocean

The availability of nitrogen is important in regulating biological productivity in marine environments. Deepwater nitrate has long been considered the major source of new nitrogen supporting primary production in oligotrophic regions of the open ocean, but recent studies have showed that biological N-2 fixation has a critical role in supporting oceanic new production(1-7). Large colonial cyanobacteria in the genus Trichodesmium and the heterocystous endosymbiont Richelia have traditionally been considered the dominant marine N-2 fixers, but unicellular diazotrophic cyanobacteria and bacterioplankton have recently been found in the picoplankton and nanoplankton community of the North Pacific central gyre, and a variety of molecular and isotopic evidence suggests that these unicells could make a major contribution to the oceanic N budget(8). Here we report rates of N-2 fixation by these small, previously overlooked diazotrophs that, although spatially variable, can equal or exceed the rate of N-2 fixation reported for larger, more obvious organisms. Direct measurements of N-15(2) fixation by small diazotrophs in various parts of the Pacific Ocean, including the waters off Hawaii where the unicellular diazotrophs were first characterized, show that N-2 fixation by unicellular diazotrophs can support a significant fraction of total new production in oligotrophic waters.