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 43.