Isotope tracking of microbial methane formation and oxidation

Abstract

The diagenetic cycling of carbon within recent unconsolidated sediments can be followed by discerning changes in the dissolved constituents of the interstitial fluids. The major dissolved carbon species are represented by the two redox endmembers methane (CH₄) and carbon dioxide (CO₂). Microbial uptake by methanogens of either CO₂ or “preformed” reduced carbon substrates such as acetate, methanol or methylated amines can be tracked with the aid of carbon (C) and hydrogen (H) isotopes. The reduction of CO₂ to CH₄ is associated with a kinetic isotope effect (KIE) which discriminates against ¹³C, giving δ¹³C_CH4 values as negative as -109‰, resulting from fractionation factors (αc) exceeding 1.10. The KIE associated with fermentation of methylated substrates is lower (αc ca. 1.04 to 1.06, with δ¹³C_CH4 values of-50 to -60‰). Hydrogen isotope effects during methanogenesis of methylated substrates can lead to deuterium depletions as large as δD_CH4=-531‰, whereas microbial D/H discrimination for the CO₂-reduction pathway is significantly less (δD_CH4 ca. -170‰ to -250‰). These field observations have been confirmed by culture experiments with labelled isotopes. Microbial consumption of CH₄, both aerobic and anaerobic, is also associated with KIE_s for C and H isotopes which enrich the residual CH₄ in the heavier isotopes. Carbon fractionation factors related to CH₄ oxidation are generally less than αc = 1.01, although values >1.02 are known. The KIE for hydrogen (αD) during aerobic and anaerobic CH₄ oxidation is between 1.1 and 1.3. The differences in C and H isotope ratios of CH₄, in combination with the isotope ratios of the coexisting H₂O and CO₂ pairs, differentiate the various bacterial CH₄ generation and consumption pathways and elucidate the cycling of labile sedimentary carbon.