New Insights into Nano Magnetite Enhancing Anaerobic Digestion: Regulating the Intracellular Electron Bifurcation and Storing Energy for Methanogens to Respond to Unfavorable Conditions

Nano magnetite has been reported to enhance the syntrophic metabolism of anaerobic digestion by serving as an electron bridge between microorganisms. However, the underlying mechanism is not fully understood. We hypothesized that nano magnetite might regulate intracellular electron bifurcation to drive the reduction of ferredoxin (E = −500 mV) via a thermodynamically favorable reaction to store energy that makes anaerobic systems more effective. Results of this study show that the addition of nano magnetite to an anaerobic digester improved methane production under the high influent COD concentration. Metagenomic analysis revealed that the abundance of subunits of the electron bifurcation enzymes MvhADG-HdrABC and HdrA2B2C2 was higher than that without nano magnetite. Metaproteomic tests showed that the contents of HS–CoM and HS–CoB associated with electron bifurcation during methanogenesis increased by 30.2% and ferredoxin increased by 156.5% with the addition of nano magnetite. As a result, the ATP production was indirectly driven by the electron bifurcation increase by 41.2%, and the total abundance of hydrogenotrophic methanogens containing MvhADG-HdrABC and aceticlastic methanogens containing HdrA2B2C2 also increased. These findings suggest that nano magnetite might improve intracellular electron bifurcation to methanogens to respond to load shock by storing energy.