Quantifying nitrogen fixation by heterotrophic bacteria in sinking marine particles

Nitrogen ( $${{\rm{N}}}_{2}$$ 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 $${{\rm{N}}}_{2}$$ N 2 under suitable environmental conditions. We find that the interactive effects of polysaccharide and polypeptide concentrations, sinking speed of particles, and surrounding $${{\rm{O}}}_{2}$$ O 2 and $${{{\rm{NO}}}_{3}}^{-}$$ NO 3 − concentrations determine the $${{\rm{N}}}_{2}$$ N 2 fixation rate inside particles. $${{\rm{N}}}_{2}$$ N 2 fixation inside sinking particles is mainly fueled by $${{{\rm{SO}}}_{4}}^{2-}$$ SO 4 2 − respiration rather than $${{{\rm{NO}}}_{3}}^{-}$$ NO 3 − respiration. Our model suggests that anaerobic processes, including heterotrophic $${{\rm{N}}}_{2}$$ N 2 fixation, can take place in anoxic microenvironments inside sinking particles even in fully oxygenated marine waters. The modelled $${{\rm{N}}}_{2}$$ N 2 fixation rates are similar to bulk rates measured in the aphotic ocean, and our study consequently suggests that particle-associated heterotrophic $${{\rm{N}}}_{2}$$ N 2 fixation contributes significantly to oceanic $${{\rm{N}}}_{2}$$ N 2 fixation.