Kinetics of two complementary hydrogen sink reactions in a defined 3‐chlorobenzoate degrading methanogenic co‐culture

Abstract The interrelationships between an obligate hydrogen‐producing and two different hydrogen‐scavenging populations grown as synthrophic members of a 3‐chlorobenzoate degrading methanogenic consortium were studied. The hydrogen producer was a benzoate degrader (strain BZ‐2), and the hydrogen consumers were a 3‐chlorobenzoate dechlorinating bacterium (Desulfomonile tiedjei) and a hydrogenotropic methanogen (Methanospirillum strain PM‐1). When a mixture of 3‐chlorobenzoate plus benzoate was added to this consortium, the rate of benzoate degradation was 50% higher, at slightly lower H₂ concentrations, than when benzoate alone was added. The enhanced benzoate degradation rate was apparantly triggered by the lower H₂ concentration, as the rate of benzoate degradation was shown to be a function of the H₂ concentration. By offering a hydrogen sink, in addition to methanogenesis, the dechlorinating hydrogen‐scavenging population stimulated the rate of benzoate degradation. The lowering of the H₂ concentration was very small, which was in agreement with the observation that the rate of methanogenesis was hardly affected by this lower hydrogen concentration. Thus there was no significant competition for H₂ between the two hydrogen‐scavenging populations in the consortium, as they practically complemented each other's hydrogen‐scavenging potential at in situ hydrogen concentrations during the degradation of 3‐chlorobenzoate. The H₂ concentrations at which hydrogen driven methanogenesis by Methanospirillum occurred in the consortium were well below the threshold concentration extrapolated for this methanogen after growth at high H₂ concentrations.