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.

贫营养地表水中的氮 (N ₂ ) 固定是海洋¹新氮 (N) 的主要来源，并且在为生物碳泵提供燃料²方面发挥着关键作用。海洋的 N ₂固定几乎完全归因于蓝藻，尽管编码固氮酶（将 N ₂固定为氨的酶）的基因在海洋细菌和古细菌中广泛存在^3-5。人们对这些非蓝藻 N ₂固定剂知之甚少，并且缺乏它们可以固定海洋中 N 的直接证据。在这里，我们报告了一种非蓝藻_固氮共生体Candidatus Tectiglobus diatomicola的发现，它为其硅藻宿主提供固定氮以换取光合碳。 N ₂固定共生体属于根瘤菌目，其与单细胞硅藻的关联将该目的已知宿主扩展到陆地上众所周知的N ₂固定根瘤菌-豆科植物共生体之外⁶。我们的结果表明，在北大西洋热带地区，根瘤菌-硅藻共生体可以提供与蓝藻 N ₂固定剂一样多的固定氮，并且它们可能负责蓝细菌太稀有的广阔海洋区域的N ₂固定。考虑测量的速率。