Theoretical estimates of equilibrium carbon and hydrogen isotope effects in microbial methane production and anaerobic oxidation of methane

Abstract

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (13 C/ 12 C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., 13 C-D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed 13 β and 2 β values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in 13 C and D. Using the computed β values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 • C. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during

Position-Specific 2H/H Equilibrium Isotopic Fractionation Factors in Alkane, Alkene, and Aromatic Molecules: A Density Functional Theory Approach

“…These include an α value determined for gas phase methane-water of 1.02 at 298 K; this value is one was obtained with theoretical calculations but does not agree well with our results of 0.83 at 298 K . Further, a recent paper provides α values for CH 3 2 H in H 2 O from a variety of computational, composited computational sources, and experimental sources . Their calculations were carried out at the M06-L/def2-TZVP level of theory; their reported α result was 0.83, as was their reported datum, which match our calculated result exactly.…”

“…Their calculations were carried out at the M06-L/def2-TZVP level of theory; their reported α result was 0.83, as was their reported datum, which match our calculated result exactly. In addition, our results match previous calculated results of 0.81 , and 0.86, as reported in Gropp et al…”

“…These include an α value determined for gas phase methane-water of 1.02 at 298 K; this value is one was obtained with theoretical calculations but does not agree well with our results of 0.83 at 298 K. 77 Further, a recent paper provides α values for CH 3 2 H in H 2 O from a variety of computational, composited computational sources, and experimental sources. 78 Their calculations were carried out at the M06-L/def2-TZVP level of theory; their reported α result was 0.83, as was their reported datum, which match our calculated result exactly. In addition, our results match previous calculated results of 0.81 76,79 and 0.86, 79 as reported in Gropp et al 78 Our results for B3LYP/6-311++(G(d,p) compared well with published results from studies using the B3LYP/6-311G(d,p) level of theory for methane, ethane, and propane.…”

“…78 Their calculations were carried out at the M06-L/def2-TZVP level of theory; their reported α result was 0.83, as was their reported datum, which match our calculated result exactly. In addition, our results match previous calculated results of 0.81 76,79 and 0.86, 79 as reported in Gropp et al 78 Our results for B3LYP/6-311++(G(d,p) compared well with published results from studies using the B3LYP/6-311G(d,p) level of theory for methane, ethane, and propane. 35 We note that this work did not use a frequency scaling factor, whereas we did; therefore, for these results, we compared α results from unscaled frequencies.…”

Modification of methane oxidation pathways during long-term incubations of methanic lake sediments

“…Due to the overall production of methane and the lack of intensive stimulation of AOM by any electron acceptor, the high increase in δ 13 CDIC could also theoretically result from carbon back flux during methanogenesis, which is feasible in environments close to thermodynamic equilibrium (Gropp et al, 2021). To determine whether back flux is feasible in the incubations, we assessed how much of methane is oxidized and converted to DIC using mass balance calculations.…”