MICROBIAL DIVERSITY AND COMMUNITY STRUCTURE IN METHANE-GENERATING SEDIMENTARY BASINS

The Antrim shale (Late Devonian, Michigan) and Forest City Basin (Carboniferous, Kansas) both form significant North American shale gas reserves. Isotopic analyses of methane, DIC and co-produced water establish that this gas is largely microbial in origin. Microscopic observations using fluorescent dyes that indicate metabolic activity reveal a morphologically diverse, active, and in part motile community in water from methane-producing wells, while no evidence of cell material is observed in adjacent non-producing wells. Molecular microbial analyses validate these findings. Two types of marker genes were investigated: species-specific marker genes (16S rRNA) to identify community members belonging to either the Bacteria or Archaea, and functional marker genes (mcrA) to determine the diversity of genes directly involved in methanogenesis. Efforts to date have determined a high diversity of methanogenic Archaea in formation waters from two Antrim wells, including an entirely new cluster of (mcrA) genotypes and novel 16S sequences within the Methanomicrobiales, Methanosarcinales, and Methanobacteriales. Bacterial diversity is more limited, confined to sequences closely related to acetate-producing Acetobacterium and within the Syntrophomonadaceae and Syntrophobacteraceae. Surprisingly, no DNA evidence for sulfate- or metal-reducing bacteria has yet been detected. Likewise enrichment cultures for sulfate-reducing bacteria and other anaerobic heterotrophs did not result in detectable growth. Thus rather than a suite of electron acceptors (i.e. ferric iron, sulfate, nitrate, manganese) followed by fermentation and methanogenesis such as found in modern sediments, the subsurface methanogenic environments of the Antrim Shale and Forest City Basin may more closely resemble documented syntrophic communities in which hydrocarbons are decomposed to acetate and H₂, a reaction that is energetically favored only when acetate and H₂ are in turn rapidly consumed by methanogens, and is possible only when the absence of other electron acceptors prevents anaerobic heterotrophs from outcompeting acetogens. Such results indicate severe thermodynamic constraints on required metabolic substrate and product activities necessary to yield energy in sedimentary basin environments.