Fractionation of the methane isotopologues 13CH4, 12CH3D, and 13CH3D during aerobic oxidation of methane by Methylococcus capsulatus (Bath)
Journal Article
Aerobic oxidation of methane plays a major role in reducing the amount of methane emitted to the atmosphere from freshwater and marine settings. We cultured an aerobic methanotroph, Methylococcus capsulatus
(Bath) at 30 and 37 °C, and determined the relative abundance of 12
CH4
, 13
CH4
, 12
CH3
D, and 13
CH3
D (a doubly-substituted, or “clumped” isotopologue of methane) to characterize the clumped isotopologue effect associated with aerobic methane oxidation. In batch culture, the residual methane became enriched in 13
C and D relative to starting methane, with D/H fractionation a factor of 9.14 (D
ε/13
ε) larger than that of 13
C/12
C. As oxidation progressed, the Δ13
CH3
D value (a measure of the excess in abundance of 13
CH3
D relative to a random distribution of isotopes among isotopologues) of residual methane decreased. The isotopologue fractionation factor for 13
CH3
D/12
CH4
was found to closely approximate the product of the measured fractionation factors for 13
CH4
/12
CH4
and 12
CH3
D/12
CH4
(i.e., 13
C/12
C and D/H). The results give insight into enzymatic reversibility in the aerobic methane oxidation pathway. Based on the experimental data, a mathematical model was developed to predict isotopologue signatures expected for methane in the environment that has been partially-oxidized by aerobic methanotrophy. Measurement of methane clumped isotopologue abundances can be used to distinguish between aerobic methane oxidation and alternative methane-cycling processes.
