Hydrogen and carbon isotope systematics in hydrogenotrophic methanogenesis under H2-limited and H2-enriched conditions: implications for the origin of methane and its isotopic diagnosis

摘要

Hydrogen and carbon isotope systematics of H~(2)O–H~(2)–CO~(2)–CH~(4) in hydrogenotrophic methanogenesis and their relation to H~(2) availability were investigated. Two H~(2)-syntrophic cocultures of fermentatively hydrogenogenic bacteria and hydrogenotrophic methanogens under conditions of 10_(2)?Pa-H~(2) and two pure cultures of hydrogenotrophic methanogens under conditions of ~10_(5)?Pa-H~(2) were tested. Carbon isotope fractionation between CH~(4) and CO~(2) during hydrogenotrophic methanogenesis was correlated with p H~(2), as indicated in previous studies. The hydrogen isotope ratio of CH~(4) produced during rapid growth of the thermophilic methanogen Methanothermococcus okinawensis under high p H~(2) conditions (~10_(5)?Pa) was affected by the isotopic composition of H~(2), as concluded in a previous study of Methanothermobacter thermautotrophicus . This “ δ D H 2 $$ {updelta mathrm{D}}_{{mathrm{H}}_2} $$ effect” is a possible cause of the diversity of previously reported values for hydrogen isotope fractionation between CH~(4) and H~(2)O examined in H~(2)-enriched culture experiments. Hydrogen isotope fractionation between CH~(4) and H~(2)O, defined by (1000?+? δ D C H 4 $$ {updelta mathrm{D}}_{{mathrm{CH}}_4} $$ )/(1000?+? δ D H 2 O $$ {updelta mathrm{D}}_{{mathrm{H}}_2mathrm{O}} $$ ), during hydrogenotrophic methanogenesis of the H~(2)-syntrophic cocultures was in the range 0.67–0.69. The hydrogen isotope fractionation of our H~(2)-syntrophic dataset overlaps with those obtained not only from low- p H~(2) 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 CH~(4) imprinted at the time of methanogenesis, as in the experiments and natural young methane, may be altered by diagenetic hydrogen isotope exchange between extracellular CH~(4) and H~(2)O through reversible reactions of the microbial methanogenic pathway in methanogenic region and/or geological methane reservoirs.