Monochloro- and dichloroacetic acids as carbon and energy sources for a stable, methanogenic mixed culture

Abstract

A stable methanogenic mixed culture was enriched from an industrial environment to utilize chloroacetate as sole carbon and energy source for growth. It immobilized spontaneously on activated charcoal and grew reproducibly on this carrier in a fluidized bed reactor when supplied with an anaerobic mineral salts medium. Substrate disappearance was complete. Methane, CO₂ and chloride ions were conclusively identified as the metabolic products and quantified. The growth yield from chloroacetate was about 1 g of protein/mol of carbon. The calculated degradation rate in the fluidized bed reactor was 0.2 to 0.8 mmol/l·h. The first metabolic intermediate from [2−¹³C]monochloroacetate in portions of biofilm-coated carrier was shown by ¹³C-NMR to be glycolate, from which ¹³CO₂ and ¹³CH₄ were formed. Glycolate was formed in an oxygen-insensitive hydrolysis, but its conversion to CO₂ and CH₄ was strictly anaerobic and sensitive to inhibition by bromoethanesulfonate. Degradation of [1-¹⁴C]-and [2-¹⁴C]-chloroacetate each yielded the same amount of [¹⁴C]-methane. We thus presume glycolate to be cleaved to CO₂ and H₂, which were the substrates for methanogenesis. Dehalogenation was limited to chlorobromo-, iodo- and dichloroacetate. These four compounds and glycolate were utilized as the sole carbon and energy sources by the methanogenic mixed culture.