Quantifying nitrogen fixation by heterotrophic bacteria in sinking marine particles

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Nitrogen (N 2) fixation by heterotrophic bacteria associated with sinking particles contributes to marine N cycling, but a mechanistic understanding of its regulation and significance are not available. Here we develop a mathematical model for unicellular heterotrophic bacteria growing on sinking marine particles. These bacteria can fix N 2 under suitable environmental conditions. We find that the interactive effects of polysaccharide and polypeptide concentrations, sinking speed of particles, and surrounding O 2 and NO3− concentrations determine the N 2 fixation rate inside particles. N 2 fixation inside sinking particles is mainly fueled by SO42− respiration rather than NO3− respiration. Our model suggests that anaerobic processes, including heterotrophic N 2 fixation, can take place in anoxic microenvironments inside sinking particles even in fully oxygenated marine waters. The modelled N 2 fixation rates are similar to bulk rates measured in the aphotic ocean, and our study consequently suggests that particle-associated heterotrophic N 2 fixation contributes significantly to oceanic N 2 fixation.

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Nature Communications