Quantification of methanogenic pathways using stable carbon isotopic signatures: a review and a proposal

In most environments, CH4 is produced from acetate (i.e. the methyl group) and H2/CO2 as precursors. The relative contribution of these two methanogenic pathways to total CH4 production can be quantified when the stable carbon isotopic signatures of CO2, CH4 and acetate methyl are measured for the CH4 production site and the isotopic fractionation factors are known for the conversion of CO2 and of acetate methyl to CH4. Literature review shows that the fractionation factors are not constants but differ, sometimes substantially, from site to site and from condition to condition. Large differences are also evident from studies of microbial cultures. More data, in particular explicit determination of fractionation factors under various conditions and environments are required. Experimental determination of fractionation factors under environmental conditions may be done by stimulation or inhibition experiments that allow the measurement of stable carbon fractionation in distinct reactions. Isotopic fractionation during acetate turnover is a particular challenge, since acetate methyl may be produced and consumed by several competing pathways. Acetate may be produced from organic carbon or from CO2 and may be consumed by conversion to CH4, CO2 or biomass. Use of isotopic signatures in CH4 emitted from a production site (e.g. a wetland) requires even more complex models, since isotopic discrimination in addition occurs during transport and oxidation of the CH4 produced.