Anaerobic degradation of isobutyrate by methanogenic enrichment cultures and by a Desuifococcus multivorans strain

Abstract

Methanogenic enrichment cultures with isobuty-rate as sole source of carbon and energy were inoculated with sediment and sludge samples from freshwater and marine origin. Over more than 20 transfers, these cultures fermented 2 mol isobutyrate with 1 mol CO2 via an intermediate for-marion of n-butyrate to 4 mol acetate and 1 mol CH4. The primary isobutyrate-fermenting bacteria could not be purified. From one of the marine enrichment cultures, a sulfate-reducing bacterium was isolated which oxidized isobutyrate with sulfate completely to COz. Based on its physiological and morphological properties, this strain was assigned to the known species Desulfococcus multivorans. It also oxidized many other fatty acids without significant release of short-chain intermediates. The enzymes involved in isobutyrate degradation by this bacterium were assayed in cell-free extracts. The results indicate that isobutyrate is activated to its CoA derivative and oxidized via methyl-malonate semialdehyde to propionyl-CoA. Propionyl-CoA is further converted via the methylmalonyl-CoA pathway to acetyl-CoA which is finally cleaved by the CO-dehydrogen-ase system. It is evident that this is not the pathway used by the fermenting bacteria prevailing in the methanogenic enrichment cultures. These results are discussed on the basis of energetical considerations.