Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean

Bernhard Tschitschko, Mertcan Esti, Miriam Philippi, Abiel T. Kidane, Sten Littmann, Katharina Kitzinger, Daan R. Speth, Shengjie Li, Alexandra Kraberg, Daniela Tienken, Hannah K. Marchant, Boran Kartal, Jana Milucka, Wiebke Mohr and Marcel M. M. Kuypers ()
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Bernhard Tschitschko: Max Planck Institute for Marine Microbiology
Mertcan Esti: Max Planck Institute for Marine Microbiology
Miriam Philippi: Max Planck Institute for Marine Microbiology
Abiel T. Kidane: Max Planck Institute for Marine Microbiology
Sten Littmann: Max Planck Institute for Marine Microbiology
Katharina Kitzinger: Max Planck Institute for Marine Microbiology
Daan R. Speth: Max Planck Institute for Marine Microbiology
Shengjie Li: Max Planck Institute for Marine Microbiology
Alexandra Kraberg: Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research
Daniela Tienken: Max Planck Institute for Marine Microbiology
Hannah K. Marchant: Max Planck Institute for Marine Microbiology
Boran Kartal: Max Planck Institute for Marine Microbiology
Jana Milucka: Max Planck Institute for Marine Microbiology
Wiebke Mohr: Max Planck Institute for Marine Microbiology
Marcel M. M. Kuypers: Max Planck Institute for Marine Microbiology

Nature, 2024, vol. 630, issue 8018, 899-904

Abstract: Abstract Nitrogen (N2) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean1 and has a key role in fuelling the biological carbon pump2. Oceanic N2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N2 into ammonia, are widespread among marine bacteria and archaea3–5. Little is known about these non-cyanobacterial N2 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 N2-fixing symbiont, ‘Candidatus Tectiglobus diatomicola’, which provides its diatom host with fixed nitrogen 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 symbioses can contribute as much fixed nitrogen as can cyanobacterial N2 fixers in the tropical North Atlantic, and that they might be responsible for N2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.

Date: 2024
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DOI: 10.1038/s41586-024-07495-w

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