Anaerobic degradation of benzoate to methane by a microbial consortium

A stabilized consortium of microbes which anaerobically degraded benzoate and produced CH4 was established by inoculation of a benzoate-mineral salts medium with sewage sludge; the consortium was routinely subcultured anaerobically in this medium for 3 years. Acetate, formate, H2 and CO2 were identified as intermediates in the overall conversion of benzoate to CH4 by the culture. Radioactivity was equally divided between the CH4 and CO2 from the degradation of uniformly ring-labeled [14C]benzoate. The methyl group of acetate was stoichiometrically converted to CH4. Acetate, cyclohexanecarboxylate, 2-hydroxycyclohexanecarboxylate, o-hydroxybenzoic acid and pimelic acid were converted to CH4 without a lag suggesting that benzoate was degraded by a reductive pathway. Addition of o-chlorobenzoate inhibited benzoate degradation but not acetate degradation or methane formation. Two methanogenic organisms were isolated from the mixed culture; neither organism was able to degrade benzoate, showing that the methanogenic bacteria served as terminal organisms of a metabolic food chain composed of several organisms. Removal of intermediates by the methanogenic bacteria provided thermodynamically favorable conditions for benzoate degradation.