Particle-associated N2 fixation by heterotrophic bacteria in the global ocean

N2-fixing microorganisms (diazotrophs) sustain life on our planet by providing biologically available nitrogen to plants. In the oceans, cyanobacterial diazotrophs, mostly prevalent in warm tropical and subtropical waters, were traditionally considered the sole contributors to marine N2 fixation. Recently, an almost ubiquitous distribution of N2-fixing heterotrophic bacteria has been discovered in the pelagic ocean. However, the mechanisms enabling heterotrophic diazotrophs to thrive in cold high-latitude waters and their contribution to the global nitrogen budget are unknown. Using a data-driven cell-based metabolic model, we show that heterotrophic bacteria inside sinking particles can fix N2 over a wide range of temperatures, explaining their ubiquitous presence in the oceans. We estimate that heterotrophic diazotrophs account for about 10% of global marine N2 fixation, with the highest contribution in oxygen minimum zones. These findings call for a reassessment of the N2 fixation patterns and the biogeochemical cycling of nitrogen in the global ocean.