Rhizobia-diatom symbiosis fixes missing nitrogen in the ocean.

Nitrogen (N) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean and has a key role in fuelling the biological carbon pump. Oceanic N fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N into ammonia, are widespread among marine bacteria and archaea. Little is known about these non-cyanobacterial N 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-fixing symbiont, 'Candidatus Tectiglobus diatomicola', which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N-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-fixing rhizobia-legume symbioses on land. Our results show that the rhizobia-diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N fixers in the tropical North Atlantic, and that they might be responsible for N fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.