The nitrogenase gene pool in surface waters of the global ocean is dominated by genes of non-cyanobacteria

Biological nitrogen (N2) fixation plays a critical role for primary production and carbon and nitrogen fluxes in the oceans. Cyanobacteria are thought to be the main N2 fixing organism (diazotrophs) in marine pelagic waters, but recent molecular analyses indicate that non-cyanobacterial diazotrophs are also present and active. Existing data are restricted geographically, however, and it has not been clear whether previous reports on the composition and diversity of diazotrophs could be reasonably extrapolated to the global ocean. Using 454- pyrosequencing, we obtained a total of 117,440 nitrogenase (nifH) sequences encoding 10,099 unique proteins from twelve surface samples (ten DNA samples and two RNA samples) collected at ten marine locations world-wide. The analyses provide a unique overview of composition and diversity of diazotrophic communities in the global ocean and document a striking dominance of nifH related to non-cyanobacteria, especially Proteobacteria. Although the two transcript samples were dominated by unicellular cyanobacteria, significant transcription of proteobacterial nifH was also shown. We conclude that non-cyanobacteria account for a substantial part of the global marine nifH gene pool and that these genes are at least occasionally expressed. The contribution of these diazotrophs to the global N2 fixation budget cannot be inferred from sequence data alone, but the prevalence and wide distribution of non-cyanobacterial nifH genes and transcripts suggest that these bacteria are ecologically significant. We conclude that a proper accounting of N2 fixation in the ocean is going to require a concerted effort to quantify the fixation rates and ecological interactions of the non-cyanobacterial diazotrophs.