Pyrogenic Black Carbon Suppresses Microbial Methane Production by Serving as a Terminal Electron Acceptor

Methane (CH₄) is the second most important greenhouse gas, 27 times as potent as CO₂ and responsible for >30% of the current anthropogenic warming. Globally, more than half of CH₄ is produced microbially through methanogenesis. Pyrogenic black carbon possesses a considerable electron storage capacity (ESC) and can be an electron donor or acceptor for abiotic and microbial redox transformation. Using wood-derived biochar as a model black carbon, we demonstrated that air-oxidized black carbon served as an electron acceptor to support anaerobic oxidation of organic substrates, thereby suppressing CH₄ production. Black carbon-respiring bacteria were immediately active and outcompeted methanogens. Significant CH₄ did not form until the bioavailable electron-accepting capacity of the biochar was exhausted. An experiment with labeled acetate (¹³CH₃COO^-) yielded 1:1 ¹³CH₄ and ¹²CO₂ without biochar and predominantly ¹³CO₂ with biochar, indicating that biochar enabled anaerobic acetate oxidation at the expense of methanogenesis. Methanogens were enriched following acetate fermentation but only in the absence of biochar. The electron balance shows that approximately half (∼2.4 mmol/g) of biochar's ESC was utilized by the culture, corresponding to the portion of the ESC > +0.173 V (vs SHE). These results provide a mechanistic basis for quantifying the climate impact of black carbon and developing ESC-based applications to reduce CH₄ emissions from biogenic sources.