Thermodynamic constraints on syntrophic acetate oxidation.

Syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis has been identified as a significant anaerobic pathway in high-temperature (55°C) oil fields (1, 2). In a paper on acetate production from oil under sulfate-reducing conditions, Callbeck et al. (3) took note of that observation, only to emphatically state that “syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis is not thermodynamically feasible at lower temperatures, as in the Medicine Hat Glauconitic C field” (MHGC field). This is not necessarily true. Syntrophic associations are known to metabolize at close to thermodynamic equilibrium (Gibbs free energy [ G] 0) (4). At the temperature of the MHGC field (22°C) (5) and under environmentally realistic conditions (acetate 5 mM, CO2 and CH4 at 1 atm, and pH 7) the amount of G available from acetate oxidation coupled to H2driven methanogenesis is 22.0 kJ/mol, which allows for a substantial window of opportunity in which both reactions are exergonic (Fig. 1A). Molecular hydrogen is not the only electron shuttle compound potentially involved in syntrophic acetate oxidation: (i) formate-