Non-aceticlastic methanogenesis from acetate: acetate oxidation by a thermophilic syntrophic coculture

TLDR: Results support a mechanism for methanogenesis from acetate by the coculture in which acetate was oxidized to CO2 and H2 by one organism, while H2 was subsequently used by a second organism to reduce CO2 to CH4.

Abstract: Methanogenesis from acetate by a rod-shaped enrichment culture grown at 60° C was found to require the presence of two organisms rather than a single aceticlastic methanogen. A thermophilic Methanobacterium which grew on H2/CO2 or formate was isolated from the enrichment. Lawns of this methanogen were used to co-isolate an “acetate oxidizer” in roll tubes containing acetate agar. The rod-shaped acetate oxidizer was morphologically distinct from the methanogen and did not show F420 autofluorescence. The coculture completely degraded 40 μmol/ml acetate, and produced nearly equal quantities of methane, and methanogenesis was coupled with growth. The doubling time for the coculture at 60°C was 30–40 h and the yield was 2.7±0.3 g dry wt/mol CH4. Studies with 14C-labelled substrates showed that the methyl group and the carboxyl group of acetate were both converted primarily to CO2 by the coculture and that CO2 was concurrently reduced to CH4. During growth, there was significant isotopic exchange between CO2 and acetate, especially with thecarboxyl position of acetate. These results support a mechanism for methanogenesis from acetate by the coculture in which acetate was oxidized to CO2 and H2 by one organism, while H2 was subsequently used by a second organism to reduce CO2 to CH4. Since the H2 partial pressure must be maintained below 10-4 atm by the methanogen for acetate oxidation to be thermodynamically feasible, this is an example of obligate interspecies hydrogen transfer. This mechanism was originally proposed for a single organism by Barker in 1936.