Reverse electron transfer: Novel anaerobic methanogenesis pathway regulated through exogenous CO₂ synergized with...

Acid accumulation and carbon emission are two major challenges in anaerobic digestion. Syntrophic consortia can employ reverse electron transfer (RET) to facilitate thermodynamically unfavorable redox reactions during acetogenesis. However, the potential mechanisms and regulatory methods of RET remain unclear. This study examines the regulatory mechanisms by which exogenous CO₂ affects RET and demonstrates that biochar maximizes CO₂ solubility at 25.8 mmol/L to enhance effects further. CO₂ synergized with biochar significantly increases cumulative methane production and propionate degradation rate. From the bioenergetic perspective, CO₂ decreases energy level to a maximum of -87 kJ/mol, strengthening the thermodynamic viability. The underlying mechanism can be attributed to RET promotion, as indicated by increased formate dehydrogenase and enrichment of H₂/formate-producing bacteria with their partner Methanospirillum hungatei. Moreover, the 5 % ¹³CH₄ and methane contribution result show that CO₂ accomplishes directed methanogenesis. Overall, this investigation riches the roles of CO₂ and biochar in AD surrounding RET.