Microbial acetate conversion to methane: kinetics,yields and pathways in a two-step digestion process

Summary Organic waste is converted in a two-stage process to methane and carbon dioxide by mixed cultures of microorganisms. Acetate, a product of acidogenic and acetogenic bacteria and the main substrate for methanogenic bacteria, is an important intermediate of the anaerobic degradation process, which results in the generation of methane. It was shown by labelling experiments using (U-¹⁴C) acetate that as much as 65%–96% of the total methane produced came from the acetate. The first order utilization rate for acetate in the methanogenic stages of a two-stage digestion process was between 0.17 h^-1 and 0.5 h^-1. The kinetics as well as the mass flow and yields of acetate and the methyl group of acetate were determined by pulse-labelling experiments with (U-¹⁴C) acetate and (2-¹⁴C) acetate without a significant rise of the total concentrations. Up to 58% of the acetate carbon was transformed to methane, and about 30% to carbon dioxide; only 4%–15% was incorporated into the biomass. There are at least two parallel degradation mechanisms in the metabolic transformation of acetate to methane: acetate is cleaved either to form methane and carbon dioxide or to form hydrogen and carbon dioxide, which can be transformed by an additional reaction to methane. Labelling experiments with (2-¹⁴C) acetate show that both mechanisms took place at similar order.