Quantifying nitrogen fixation by heterotrophic bacteria in sinking marine particles.

ABSTRACT

Nitrogen ([Formula see text]) 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 [Formula see text] under suitable environmental conditions. We find that the interactive effects of polysaccharide and polypeptide concentrations, sinking speed of particles, and surrounding [Formula see text] and [Formula see text] concentrations determine the [Formula see text] fixation rate inside particles. [Formula see text] fixation inside sinking particles is mainly fueled by [Formula see text] respiration rather than [Formula see text] respiration. Our model suggests that anaerobic processes, including heterotrophic [Formula see text] fixation, can take place in anoxic microenvironments inside sinking particles even in fully oxygenated marine waters. The modelled [Formula see text] fixation rates are similar to bulk rates measured in the aphotic ocean, and our study consequently suggests that particle-associated heterotrophic [Formula see text] fixation contributes significantly to oceanic [Formula see text] fixation.