Rhizobia-diatom symbiosis fixes missing nitrogen in the ocean

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.