Acetogenic methane-carbon monoxide comproportionation: an exergonic but unobserved microbial metabolism

Microbial metabolism relies on redox reactions that exploit chemical disequilibria. While aerobic carbon oxidation, carbon fixation, and fermentation are well studied, the broader space of anaerobic carbon redox reactions remains underexplored. In this study, carbon comproportionation, or reverse fermentation, reactions are identified as a previously unrecognized and potentially favorable class of microbial carbon redox transformations. Particular attention is given to the reaction between methane (CH 4 ) and carbon monoxide (CO) to form acetate, a reaction that has not previously been evaluated despite the widespread occurrence of CH 4 and CO in anoxic systems. Gibbs energies ( ΔG r ) for this reaction were calculated across broad ranges of temperature, pH, and dissolved CH 4 and CO concentrations using measured physicochemical data from a wide variety of environmental systems. We show that acetogenic CH 4 -CO comproportionation is exergonic in all environments where both substrates were detected. The most favorable energetic conditions occur at high pH, low temperature, and high reactant concentrations, consistent with cool serpentinizing systems. In several settings, the calculated Gibbs energy yields and energy densities overlap or exceed known anaerobic metabolisms involving CH 4 , CO, and acetate. These results demonstrate that acetogenic CH 4 -CO comproportionation can support microbial energy conservation in a variety of settings. To determine if this metabolism could have operated on early Earth or Mars, modeled fluid compositions show that this reaction is also exergonic under plausible physicochemical regimes. This work broadens the suite of possible microbial energy metabolisms and provides testable criteria for evaluating carbon-based catabolic reactions on Earth and on other planetary bodies.