Pathway-specific isotopic approach to assess methane biostimulation in coalbeds

The fundamental objective of this research has been to apply a novel isotopic technique to delineate the biogeochemical pathways that methanogenic archaea utilize during the final step of coal biodegradation when converting simple energy sources (primarily thought to be H 2 and acetate; Figure 1) into methane and CO 2. This long-standing problem in subsurface biogeochemistry has relevance to both natural and engineered systems. For example, academic and industry researchers have pursued techniques to hasten or " biostimulate " the biodegradation of coal to low-molecular-weight organic intermediates and/or the uptake of intermediate products by methanogenic archaea. Yet, existing techniques of delineating biogeochemical pathways can lead to inconsistent results. Microbial community characterization does not always confidently identify the most active, environmentally relevant organisms in the subsurface. Isotopic approaches have conventionally analyzed accumulated end products (typically the methane and CO 2 in a gas sample) and are not specific to individual metabolic pathways of methanogenesis. In contrast, compound-specific carbon isotope analysis of acetate directly records acetate uptake by methanogens because the residual (unreacted) acetate is enriched in the heavier isotope 13 C, relative to the lighter isotope 12 C. Therefore, the compound-specific technique is a complement to established but sometimes inconsistent techniques. In this investigation, we implemented a method for analyzing the natural abundance 13 C/ 12 C ratio of acetate in a natural setting characterized by lower acetate concentrations than in previous investigations, typically <20 µmol/L. The results presented here are in the " delta notation, " representing the carbon-13/carbon-12 ratio relative to the 13 C/ 12 C ratio of a standard reference material. As δ 13 C becomes more positive, a compound is enriched in the heavier isotope 13 C. δ 13 C is reported in units of per mil (‰). Therefore, a variation of 10‰ means that the isotope ratio changed by 1%. Sample preconcentration Because of the low concentrations of acetate in most of the waters in this study (<20 uM), it was necessary to preconcentrate large volumes of water (typically 500-1000 mL). Samples were preconcentrated in Teflon beakers in a warm vacuum oven after adjusting pH to ≥10. Saline samples were vacuum distilled to remove acetate from the nonvolatile salts. Acetate extraction Dried samples were redissolved and the salt content adjusted to ~33% NaCl and pH to 2. The headspace above 3 mL of the sample was extracted from a 4 mL vial by headspace solid-phase microextraction …