Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean

Nitrogen (N₂) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean¹ and has a key role in fuelling the biological carbon pump². Oceanic N₂ fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N₂ into ammonia, are widespread among marine bacteria and archaea^3-5. Little is known about these non-cyanobacterial N₂ 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 N₂-fixing symbiont, 'Candidatus Tectiglobus diatomicola', which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N₂-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 N₂-fixing rhizobia-legume symbioses on land⁶. Our results show that the rhizobia-diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N₂ fixers in the tropical North Atlantic, and that they might be responsible for N₂ fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.