The study reports the discovery of a non-cyanobacterial nitrogen-fixing symbiont, 'Candidatus Tectiglobus diatomica,' which provides fixed nitrogen to its diatom host in return for photosynthetic carbon. This symbiosis expands the known hosts for the Rhizobiales order beyond the well-studied cyanobacterial nitrogen fixers. The symbiont, belonging to the Rhizobiales order, was found to fix nitrogen at rates comparable to those of cyanobacterial fixers in the tropical North Atlantic, suggesting it contributes significantly to nitrogen fixation in oligotrophic waters. The study also highlights the global distribution of 'Ca. Tectiglobus' species and their potential role in nitrogen fixation in various oceanic regions, where cyanobacterial fixers are rare. The findings indicate that non-cyanobacterial nitrogen fixers, such as 'Ca. Tectiglobus,' play a crucial role in sustaining marine productivity and global carbon sequestration.The study reports the discovery of a non-cyanobacterial nitrogen-fixing symbiont, 'Candidatus Tectiglobus diatomica,' which provides fixed nitrogen to its diatom host in return for photosynthetic carbon. This symbiosis expands the known hosts for the Rhizobiales order beyond the well-studied cyanobacterial nitrogen fixers. The symbiont, belonging to the Rhizobiales order, was found to fix nitrogen at rates comparable to those of cyanobacterial fixers in the tropical North Atlantic, suggesting it contributes significantly to nitrogen fixation in oligotrophic waters. The study also highlights the global distribution of 'Ca. Tectiglobus' species and their potential role in nitrogen fixation in various oceanic regions, where cyanobacterial fixers are rare. The findings indicate that non-cyanobacterial nitrogen fixers, such as 'Ca. Tectiglobus,' play a crucial role in sustaining marine productivity and global carbon sequestration.