CO2 Incorporation and 4-Hydroxy-2-Methylbenzoic Acid Formation during Anaerobic Metabolism of m-Cresol by a Methanogenic Consortium

The metabolism of m-cresol by methanogenic cultures enriched from domestic sewage sludge was investigated. In the initial studies, bromoethanesulfonic acid was used to inhibit methane production. This led to the accumulation of 4.0 ± 0.8 mol of acetate per mol of m-cresol metabolized. These results suggested that CO₂ incorporation occurred because each molecule of m-cresol contained seven carbon atoms, whereas four molecules of acetate product contained a total of eight carbon atoms. To verify this, [¹⁴C]bicarbonate was added to bromoethanesulfonic acid-inhibited cultures, and those cultures yielded [¹⁴C]acetate. Of the label recovered as acetate, 89% was found in the carboxyl position. Similar cultures fed [methyl-¹⁴C]m-cresol yielded methyl-labeled acetate. A ¹⁴C-labeled transient intermediate was detected in cultures given either m-cresol and [¹⁴C]bicarbonate or bicarbonate and [methyl-¹⁴C]m-cresol. The intermediate was identified as 4-hydroxy-2-methylbenzoic acid. In addition, another metabolite was detected and identified as 2-methylbenzoic acid. This compound appeared to be produced only sporadically, and it accumulated in the medium, suggesting that the dehydroxylation of 4-hydroxy-2-methylbenzoic acid led to an apparent dead-end product.