Background
Some bacteria found in marine waters can convert nitrogen gas (N2) to ammonia (known as N2 fixation), and thereby supply the marine food web with nitrogen. It has puzzled scientists for years how bacteria, that live from dissolved organic matter in marine waters, can carry out N2 fixation. It was assumed that the high levels of oxygen combined with the low amount of dissolved organic matter in the marine water column would prevent the anaerobic and energy-consuming N2 fixation. It was suggested that aggregates, so-called “marine snow particles”, could possibly be suitable sites for N2 fixation, 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 N2 under suitable environmental conditions.
Results
We find that the interactive effects of polysaccharide and polypeptide concentrations, sinking speed of particles, and surrounding O2 and NO3- concentrations determine the N2 fixation rate inside particles. N2 fixation inside sinking particles is mainly fueled by SO4- respiration rather than NO3- respiration. 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. The modelled N2 fixation rates are similar to bulk rates measured in the aphotic ocean, and our study consequently suggests that particle-associated heterotrophic N2 fixation contributes significantly to oceanic N2 fixation.
The paper was published in Nature Communications on July the 2nd 2021.