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

Abstract

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.