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

Abstract:

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 CO<inf>2</inf> affects RET and demonstrates that biochar maximizes CO<inf>2</inf> solubility at 25.8 mmol/L to enhance effects further. CO<inf>2</inf> synergized with biochar significantly increases cumulative methane production and propionate degradation rate. From the bioenergetic perspective, CO<inf>2</inf> 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 H<inf>2</inf>/formate-producing bacteria with their partner Methanospirillum hungatei. Moreover, the 5 % <sup>13</sup>CH<inf>4</inf> and methane contribution result show that CO<inf>2</inf> accomplishes directed methanogenesis. Overall, this investigation riches the roles of CO<inf>2</inf> and biochar in AD surrounding RET.