Microbial methane formation from different lithotypes of Miocene lignites from the Konin Basin, Poland: Geochemistry of the gases and composition of the microbial communities

The present research compares the yield of microbial methane production from different lithotypes of Miocene lignite from the Konin Basin in Poland and attempts to establish an understanding of the processes responsible for methanogenesis. A series of batch experiments were carried out with detritic and xylitic lignites inoculated with microorganisms from an external source (the anaerobic chamber of a wastewater treatment plant in a sugar factory). Biogas volume, concentration and stable carbon isotopes of microbial methane were measured. It was found that detritic lignites are a slightly more suitable raw material for microbial methane production than xylites. Methane yield for detritic lignites equalled 14.3 μmol CH4/g of total organic carbon (TOC); for xylites, 13.7 μmol CH4/g of TOC. The mean δ13C(CH4) value in experiments with detritic coal from Konin equalled −36.3‰; for fossil wood fragments, −47.3‰ and −42.7‰. We suppose that differences in mean δ13C(CH4) values from the biodegradation of different lithotypes of lignite from the Konin Basin most probably depend on cellulose and lignin content (%). The holocellulose content in xylites decreased over time, suggesting the important role of the enzymatic hydrolysis of cellulose to glucose. Methane from detritic lignite was formed due to lignin decomposition. Methane-producing microbial communities were dominated by Bacteria mainly from the phyla Proteobacteria (Alpha-, Beta- and Gamma- or Deltaproteobacteria), Firmicutes (Clostridia and Bacilli), Actinobacteria, and Bacteroidetes. Archaea constituted only 2–6% of the microbial community, including Methanosarcinales, Methanomicrobiales, and Methanobacteriales. The data presented here show no clear correlations between lignite type and specific bacterial or archaeal taxa. However, certain tendencies were observed. Exiguobacterium and Pseudomonas are more abundant in xylites, Rhizobacteriaceae in detritic lignites. This indicates the complexity and diversity of lignite material and processes leading to lignite degradation. In the presented research model, anaerobic oxidation of methane may occur.