Low Δ 12 CH 2 D 2 values in microbialgenic methane result from combinatorial isotope effects

Lina Taenzer , Jabrane Labidi (1) , Andrew L Masterson , Xiahong Feng , Douglas Rumble , Edward D Young , William D Leavitt

1 EPSS - Department of Earth, Planetary and Space Sciences [Los Angeles]

Methane generated by microorganisms is most often depleted in the doubly substituted isotopologue 12 CH 2 D 2 relative to the stochastic reference distribution. To constrain the controls on depleted ∆ 12 CH 2 D 2 values, we experimentally isolated the root cause with microorganisms that produce methane from methylphosphonate via the C-P lyase pathway. This mechanism of methane production preserves the three hydrogens from methylphosphonate and adds one hydrogen from water. When maintaining the same methylphosphonate source, but varying the D/H composition of growth medium water, we observed significant shifts in methane ∆ 12 CH 2 D 2 values, but little to no change in ∆ 13 CH 3 D values. We reproduced these observations with a model that considers only the combinatorial isotope effect. The variation in ∆ 12 CH 2 D 2 values of product methane resulted from the differences in D/H between reactants water and methylphosphonate. This work validates the hypothesis that combinatorial effects can strongly influence methane ∆ 12 CH 2 D 2 values, and must be considered for low temperature, abiotic or biotic systems where methane hydrogen is derived from multiple reservoirs.

Mots clés: methane; clumped isotopes; methanogenesis; C-P lyase

Domaines: Sciences de l'environnement

Lina Taenzer, Jabrane Labidi, Andrew L Masterson, Xiahong Feng, Douglas Rumble, et al.. Low Δ 12 CH 2 D 2 values in microbialgenic methane result from combinatorial isotope effects. Geochimica et Cosmochimica Acta, 2020, 285, pp.225-236. ⟨10.1016/j.gca.2020.06.026⟩. ⟨hal-03091028⟩