Thermodynamic Constraints on Syntrophic Acetate Oxidation

摘要

Syntrophic acetate oxidation coupled to hydrogenotrophic methanogenesis has been identified as a significant anaerobic pathway in high-temperature (55C) 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 (22C) (5) and under environmentally realistic conditions (acetate 5 mM, CO_(2) and CH_(4) at 1 atm, and pH 7) the amount of G available from acetate oxidation coupled to H_(2)-driven methanogenesis is 22.0 kJ/mol, which allows for a substantial window of opportunity in which both reactions are exergonic (Fig. 1A).Thermodynamic constraints on syntrophic processes involved in hydrocarbon degradation under sulfate-reducing and methanogenic conditions at 22C under standard conditions (solutes at 1 M, gases at 1 atm, unless otherwise indicated), with hexadecane as the model hydrocarbon and hydrogen (A) or formate (B) as the interspecies electron carrier. (A) Acetate oxidation is exergonic to the right of the dotted line; hydrogenotrophic methanogenesis is exergonic at hydrogen partial pressure ( P _(H2)) 10~(5.84) atm (open circles); hydrogenotrophic sulfate reduction is exergonic at P _(H2) 10~(6.73) atm (closed circles). (B) Acetate oxidation is exergonic to the right of the dotted line; formate-driven methanogenesis is exergonic at [formate] 10~(5.65) M (open circles); formate-driven sulfate reduction is exergonic at [formate] 10~(6.54) M (closed circles); calculations after Dolfing et al. (17).