Mine gas has come into the focus of the power industry and is being used increasingly for heat and power production worldwide (Thielemann et al. 2004). About 7% of the annual methane emissions originate from coal mining (Denman et al. 2007). In many coal deposits, stable carbon and hydrogen isotopic signatures of methane indicate a mixed thermogenic and biogenic origin (Thielemann et al. 2004).
The present study focused on the identification of recently produced coal-mine methane as well as on the diversity, abundance, and activity of the microorganisms linked directly to methanogenesis. We aimed to unravel the active methanogens being responsible for the methane release but also the active bacteria involved in the metabolic network. Therefore, weathered hard coal and mine timber were collected in two abandoned coal mines in the Ruhr Basin of Germany.
The combined geochemical and microbiological investigations identified microbial methanogenesis as a recent source of methane. Mine timber and hard coal showed an in situ production of methane with isotopic signatures similar to those of the methane in the mine atmosphere. Long-term incubations of coal and timber as sole carbon sources formed methane over a period of 9 months. Predominantly, acetoclastic methanogenesis was stimulated in enrichments containing acetate or H2+CO2. Highest methane formation rates were detected in the timber incubations (3-4 times higher than in the coal incubations). The methane-formation processes are complex. Wood-degrading fungi (Ascomycetes and Basidiomycetes) and a broad spectrum of facultative anaerobic bacteria (Proteobacteria, Bacteroidetes, Tenericutes, Actinobacteria, Chlorobi and Chloroflexi) were detected. Archaea were represented by members of Methanosarcinales and Crenarcheaota. Thus, the methanogenic community in the enrichments and unamended samples was dominated by Methanosarcinales closely related to Methanosarcina barkeri. The formation of methane was due to acetoclastic methanogenesis in the acetate but also in the H2+CO2 cultures of coal and timber. The H2+CO2 was mainly used by acetogens similar to Pelobacter acetylenicus and Clostridium species forming acetate as intermediate and providing it to the methanogens. Although, Methanosarcinales are known to use both, hydrogen as well as acetate, those identified rather utilized the easier accessible acetate than the thermodynamically more preferential hydrogen. The microbial communities appeared highly adapted to the low H2 concentrations in the coal mine with acetate as the main precursor of the biogenic methane. |