Ecology

Ocean gyre

Mesopelagic zone

Nitrogen fixation

Pelagic zone

Nitrogenase

Upwelling

Geology

Subtropics

Diazotroph

Bacteria

Biology

Oceanography

Genetics

Cyanobacteria

Limnology and Oceanography

Recent observations of N 2 fixation rates (NFR) and the presence of nitrogenase ( nifH ) genes from heterotrophic N 2 ‐fixing (diazotrophic) prokaryotes in unusual habitats challenge the paradigm that pelagic marine N 2 fixation is constrained to cyanobacteria in warm, oligotrophic, surface waters. Here, we compare NFR and diazotrophic diversity (assessed via high‐throughput nifH sequencing) from a region known to be dominated by cyanobacterial diazotrophs (the North Pacific Subtropical Gyre, NPSG) to two regions dominated by heterotrophic diazotrophs: the Eastern South Pacific (ESP, from the Chilean upwelling system to the subtropical gyre) and the Pacific Northwest coastal upwelling system (PNW). We observed distinct biogeographical patterns among the three regions. Diazotrophic community structure differed strongly between the NPSG, dominated by cyanobacterium UCYN‐A, and the ESP, dominated by heterotrophic nifH group 1J/1K, yet surface NFR were similar in magnitude (up to 5.1 nmol N L −1 d −1 ). However, ...

Diversity and activity of nitrogen‐fixing communities across ocean basins