Pathways of methanol conversion in a thermophilic anaerobic (55 degrees C) sludge consortium.

Abstract

The pathway of methanol conversion by a thermophilic anaerobic consortium was elucidated by recording the fate of carbon in the presence and absence of bicarbonate and specific inhibitors. Results indicated that about 50% of methanol was directly converted to methane by the methylotrophic methanogens and 50% via the intermediates H(2)/CO(2) and acetate. The deprivation of inorganic carbon species [Sigma(HCO(3)(-)+CO(2))] in a phosphate-buffered system reduced the rate of methanol conversion. This suggests that bicarbonate is required as an electron (H(2)) sink and as a co-substrate for the efficient and complete removal of the chemical oxygen demand. Nuclear magnetic resonance spectroscopy was used to investigate the route of methanol conversion to acetate in bicarbonate-sufficient and bicarbonate-depleted environments. The proportions of [1,2-(13)C]acetate, [1-(13)C]acetate and [2-(13)C]acetate were determined. Methanol was preferentially incorporated into the methyl group of acetate, whereas HCO(3)(-) was the preferred source of the carboxyl group. A small amount of the added H(13)CO(3)(-) was reduced to form the methyl group of acetate and a small amount of the added (13)CH(3)OH was oxidised and found in the carboxyl group of acetate when (13)CH(3)OH was converted. The recovery of [(13)C]carboxyl groups in acetate from (13)CH(3)OH was enhanced in bicarbonate-deprived medium. The small amount of label incorporated in the carboxyl group of acetate when (13)CH(3)OH was converted in the presence of bromoethanesulfonic acid indicates that methanol can be oxidised to CO(2 )prior to acetate formation. These results indicate that methanol is converted through a common pathway (acetyl-CoA), being on the one hand reduced to the methyl group of acetate and on the other hand oxidised to CO(2), with CO(2) being incorporated into the carboxyl group of acetate.