Rhizobia-diatom symbiosis fixes missing nitrogen in the ocean.

Nitrogen (N 2 ) fixation in oligotrophic surface waters is the main source of new nitrogen (N) to the ocean 1 and plays a key role in fueling the biological carbon pump 2 . Oceanic N 2 fixation is almost exclusively attributed to cyanobacteria, even though genes encoding nitrogenase, the enzyme fixing 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 N in the ocean is missing. Here we report the discovery of a non-cyanobacterial N 2 -fixing symbiont, Candidatus Tectiglobus diatomicola, which provides its diatom host with fixed-N 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 symbiosis can contribute as much fixed-N as cyanobacterial N 2 -fixers in the tropical North Atlantic, and that they may be responsible for N 2 fixation in the vast regions of the ocean where cyanobacteria are too rare to account for the measured rates.