Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean

Abstract Nitrogen (N 2 ) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean 1 and has a key role in fuelling the biological carbon pump 2 . Oceanic N 2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N 2 into ammonia, are widespread among marine bacteria and archaea 3–5 . Little is known about these non-cyanobacterial N 2 fixers, and direct proof that they can fix nitrogen in the ocean has so far been lacking. Here we report the discovery of a non-cyanobacterial N 2 -fixing symbiont, ‘ Candidatus Tectiglobus diatomicola’, which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N 2 -fixing symbiont belongs to the order Rhizobiales and its association with a unicellular diatom expands the known hosts for this order beyond the well-known N 2 -fixing rhizobia–legume symbioses on land 6 . Our results show that the rhizobia–diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N 2 fixers in the tropical North Atlantic, and that they might be responsible for N 2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.