Abstract
Nitrogen ({{rm{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} under suitable environmental conditions. We find that the interactive effects of polysaccharide and polypeptide concentrations, sinking speed of particles, and surrounding {{rm{O}}}_{2} and {{{rm{NO}}}_{3}}^{-} concentrations determine the {{rm{N}}}_{2} fixation rate inside particles. {{rm{N}}}_{2} fixation inside sinking particles is mainly fueled by {{{rm{SO}}}_{4}}^{2-} respiration rather than {{{rm{NO}}}_{3}}^{-} respiration. Our model suggests that anaerobic processes, including heterotrophic {{rm{N}}}_{2} fixation, can take place in anoxic microenvironments inside sinking particles even in fully oxygenated marine waters. The modelled {{rm{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} fixation contributes significantly to oceanic {{rm{N}}}_{2} fixation.
Highlights
Nitrogen (N2) 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
N2 fixation occurs on sinking particles, we present a trait-based model of heterotrophic bacteria associated with sinking particles
Our model suggests that particle-associated heterotrophic N2 fixation is viable and reasonable based on the known properties and physics of marine particles and reveals a significant contribution to the oceanic biological N2 fixation
Summary
Nitrogen (N2) 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. Our model suggests that anaerobic processes, including heterotrophic N2 fixation, can take place in anoxic microenvironments inside sinking particles even in fully oxygenated marine waters. Molecular dinitrogen gas (N2) is highly abundant in the marine water column, only specific prokaryotes that can fix N2 (diazotrophs) using the nitrogenase enzyme complex[1] assimilate this form of nitrogen. Heterotrophic conditions surround cells diazotrophs may with extracellular under rich culture polymers[13] to lower the permeability to extracellular O2 to protect the nitrogenase, but since this is highly energy-demanding[14,15] it is an unlikely strategy in the relatively nutrient-poor marine water column. While synthesizing ATP during respiration, O2 is used as the most common and favorable form of electron acceptor by pro-
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