Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean

Abstract

Nitrogen (N₂) fixation in oligotrophic surface waters is the main source of new nitrogen (N) to the ocean¹ and plays a key role in fueling the biological carbon pump². Oceanic N₂ fixation is almost exclusively attributed to cyanobacteria, even though genes encoding nitrogenase, the enzyme fixing 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 N in the ocean is missing. Here we report the discovery of a non-cyanobacterial N₂-fixing symbiont, Candidatus Tectiglobus diatomicola, which provides its diatom host with fixed-N 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 symbiosis can contribute as much fixed-N as cyanobacterial N₂-fixers in the tropical North Atlantic, and that they may be responsible for N₂ fixation in the vast regions of the ocean where cyanobacteria are too rare to account for the measured rates.