Abstract
Hydrogen and carbon isotope systematics of H2O–H2–CO2–CH4 in hydrogenotrophic methanogenesis and their relation to H2 availability were investigated. Two H2-syntrophic cocultures of fermentatively hydrogenogenic bacteria and hydrogenotrophic methanogens under conditions of <102 Pa-H2 and two pure cultures of hydrogenotrophic methanogens under conditions of ~105 Pa-H2 were tested. Carbon isotope fractionation between CH4 and CO2 during hydrogenotrophic methanogenesis was correlated with pH2, as indicated in previous studies. The hydrogen isotope ratio of CH4 produced during rapid growth of the thermophilic methanogen Methanothermococcus okinawensis under high pH2 conditions (~105 Pa) was affected by the isotopic composition of H2, as concluded in a previous study of Methanothermobacter thermautotrophicus. This “ $$ {\updelta \mathrm{D}}_{{\mathrm{H}}_2} $$ effect” is a possible cause of the diversity of previously reported values for hydrogen isotope fractionation between CH4 and H2O examined in H2-enriched culture experiments. Hydrogen isotope fractionation between CH4 and H2O, defined by (1000 + $$ {\updelta \mathrm{D}}_{{\mathrm{CH}}_4} $$ )/(1000 + $$ {\updelta \mathrm{D}}_{{\mathrm{H}}_2\mathrm{O}} $$ ), during hydrogenotrophic methanogenesis of the H2-syntrophic cocultures was in the range 0.67–0.69. The hydrogen isotope fractionation of our H2-syntrophic dataset overlaps with those obtained not only from low-pH2 experiments reported so far but also from natural samples of “young” methane reservoirs (0.66–0.74). Conversely, such hydrogen isotope fractionation is not consistent with that of “aged” methane in geological samples (≥0.79), which has been regarded as methane produced via hydrogenotrophic methanogenesis from the carbon isotope fractionation. As a possible process inducing the inconsistency in hydrogen isotope signatures between experiments and geological samples, we hypothesize that the hydrogen isotope signature of CH4 imprinted at the time of methanogenesis, as in the experiments and natural young methane, may be altered by diagenetic hydrogen isotope exchange between extracellular CH4 and H2O through reversible reactions of the microbial methanogenic pathway in methanogenic region and/or geological methane reservoirs.
Highlights
Methane is a greenhouse gas, a primary product of methanogenic archaea, a useful fuel for human activity, and the simplest organic molecular
Note that the observed pH2 values were close to the threshold at which hydrogenotrophic methanogens cease H2 consumption (100–101 Pa; Lovley 1985; Thauer et al 2008), suggesting that our experiment covered the lower end of H2 availability for microbial methanogenesis
The δDH2 values of Mesophilic Cocultures (MC) batches were obtained from only three samples; B1-11 h, B3-12 h, and B4-12 h, because the pH2 values of the other samples were below the detection limits of δDH2 measurement (Fig. 1i)
Summary
Methane is a greenhouse gas, a primary product of methanogenic archaea, a useful fuel for human activity, and the simplest organic molecular. The usefulness of the stable isotope tracers for methane origins have been long debated (Martini et al 1996; Waldron et al 1999; Tang et al 2000), the stable isotope diagnosis becomes more accurate by enhancing the understanding of stable isotope systematics with respect to each of the methanogenic processes in addition to subsequent alteration of the imprinted isotope signature. For this purpose, carbon isotope fractionation between CH4 and CO2 in hydrogenotrophic methanogenesis has been well studied by laboratory incubations (e.g., Valentine et al 2004; Penning et al 2005; Takai et al 2008). This relationship has been explained by “differential reversibility” in the multistep pathway of methanogenesis from H2 and CO2 (Valentine et al 2004): higher reversibility exhibits at a lower H2 availability
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