Methane-producing microbial community in a coal bed of the Illinois basin

Abstract

A series of molecular and geochemical studies were performed to study microbial, coal bed methane formation in the eastern Illinois Basin. Results suggest that organic matter is biodegraded to simple molecules, such as H(2) and CO(2), which fuel methanogenesis and the generation of large coal bed methane reserves. Small-subunit rRNA analysis of both the in situ microbial community and highly purified, methanogenic enrichments indicated that Methanocorpusculum is the dominant genus. Additionally, we characterized this methanogenic microorganism using scanning electron microscopy and distribution of intact polar cell membrane lipids. Phylogenetic studies of coal water samples helped us develop a model of methanogenic biodegradation of macromolecular coal and coal-derived oil by a complex microbial community. Based on enrichments, phylogenetic analyses, and calculated free energies at in situ subsurface conditions for relevant metabolisms (H(2)-utilizing methanogenesis, acetoclastic methanogenesis, and homoacetogenesis), H(2)-utilizing methanogenesis appears to be the dominant terminal process of biodegradation of coal organic matter at this location.

FIG. 1.

Map showing the extent of the Seelyville Coal formation in the Illinois Basin. The sampling site is located in the eastern marginal zone of the basin. The dotted and dashed lines represent the southernmost extents of the most recent Pleistocene glaciations. The arrows indicate the inferred direction of melt water influxes during inter- and postglacial periods.

FIG. 2.

SEM pictures of the methanogenic enrichment on a 0.22-μm membrane filter. (a) Culture was predominantly one morphotype, consisting of spherical cells with diameters of ≤0.5 μm. (b) Higher magnification of three typical spherical cells, most likely Methanocorpusculum parvum.

FIG. 3.

Microbial diversity in water from coal gas-producing well INS-P11, Seelyville Coal, depth of 105 m. The tree was generated using MEGA 3.1 software (29) with neighbor joining, a p distance substitution method, and 3,000 bootstrap replicates. Archaeal coal bed water clones (#) and methanogenic enrichment clones (*) were almost identical and are represented as clades. The number of clones in each clade is shown. Designations in parentheses are NCBI accession numbers.

FIG. 4.

Distribution of archaeal IPLs in the methanogenic enrichment sample dominated by close relatives of Methanocorpusculum parvum. Core lipids were GDGT and AR. Polar head groups were mono- and diglycosyl, PG, TMAPT, Gly-GlyA (tentatively identified glycosyl-glycuronic acid), and TMAPT(tentative) (tentatively identified derivative of TMAPT with a similar fragmentation pattern and a 14-Da-higher mass). misc., miscellaneous.

FIG. 5.

Proposed mechanisms of stepwise biodegradation of OM in coal, annotated with microbes related to those found in the clone library and potentially capable of performing the indicated processes: (i) defragmentation of coal geomacromolecular structure predominately by fermentation targeted at oxygen-linked moieties and oxygen-containing functional groups (this process detaches some of the oxygen-linked aromatic rings and generates some short organic acids); (ii) anaerobic oxidation of available aromatic and aliphatic moieties, derived from coal defragmentation or from dispersed oil; (iii) fermentation of products available from step i described above to H₂, CO₂, and acetate; and (iv) methanogenesis utilizing H₂ and CO₂ likely predominating over homoacetogenesis and acetoclastic methanogenesis. The dark area represents a droplet of oil. The molecular model of coal is adapted from reference .

FIG. 6.

Free energy dependence on the substrate and product availability of microbial reactions calculated for typical measured in situ conditions (salinity of 3.11 g/liter, temperature of 17°C, pressure of 10.5 atm, H⁺ of 10⁻⁸ M, H_2,aq of 3.8·10⁻⁸ M, CH₃COO⁻ of 4.3·10⁻⁶ M, CH_4,aq of 1.5·10⁻² M, and HCO₃⁻ of 1.7·10⁻² M). The data points represent in situ conditions in three, coal gas-producing wells within 5 km of each other. The arrows indicate conditions under which the given reaction is more exergonic than the required minimum (15 or 0 kJ), based on the reactions shown in Table 1.