Biological nitrogen fixation is catalyzed by the enzyme nitrogenase. Two forms of this metalloenzyme, the vanadium (V)- and iron (Fe)-only nitrogenases, were recently found to reduce small amounts of carbon dioxide (CO₂) into the potent greenhouse gas methane (CH₄). Here, we report carbon (¹³C/¹²C) and hydrogen (²H/¹H) stable isotopic compositions and fractionations of methane generated by V- and Fe-only nitrogenases in the metabolically versatile nitrogen fixer Rhodopseudomonas palustris. The stable carbon isotope fractionation imparted by both forms of alternative nitrogenase are within the range observed for hydrogenotrophic methanogenesis (¹³α_CO2/CH4 = 1.051 ± 0.002 for V-nitrogenase and 1.055 ± 0.001 for Fe-only nitrogenase; values are means ± standard errors). In contrast, the hydrogen isotope fractionations (²α_H2O/CH4 = 2.071 ± 0.014 for V-nitrogenase and 2.078 ± 0.018 for Fe-only nitrogenase) are the largest of any known biogenic or geogenic pathway. The large ²α_H2O/CH4 shows that the reaction pathway nitrogenases use to form methane strongly discriminates against ²H, and that ²α_H2O/CH4 distinguishes nitrogenase-derived methane from all other known biotic and abiotic sources. These findings on nitrogenase-derived methane will help constrain carbon and nitrogen flows in microbial communities and the role of the alternative nitrogenases in global biogeochemical cycles.

中文翻译：

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大氢同位素分馏可将固氮酶衍生的甲烷与其他甲烷来源区分开来。


生物固氮是由固氮酶催化的。最近发现这种金属酶的两种形式，即仅含钒 (V) 和仅含铁 (Fe) 的固氮酶，可以将少量二氧化碳 (CO ₂ ) 还原成强效温室气体甲烷 (CH ₄ )。在这里，我们报告了代谢多功能固氮剂沼泽红假单胞菌中仅含 V 和 Fe 的固氮酶产生的甲烷的碳 ( ¹³ C/ ¹² C) 和氢 ( ² H/ ¹ H) 稳定同位素组成和分馏。两种形式的替代固氮酶产生的稳定碳同位素分馏均在氢营养产甲烷作用观察到的范围内（V-固氮酶¹³ α _CO2/CH4 = 1.051 ± 0.002，纯铁固氮酶 1.055 ± 0.001；数值为平均值±标准误差） ）。相比之下，氢同位素分馏（V-固氮酶的² α _H2O/CH4 = 2.071 ± 0.014，仅 Fe 固氮酶的 2.078 ± 0.018）是任何已知的生物或地质途径中最大的。大的² α _H2O/CH4表明固氮酶用于形成甲烷的反应途径强烈地区分² H，并且² α _H2O/CH4将固氮酶衍生的甲烷与所有其他已知的生物和非生物来源区分开来。这些关于固氮酶衍生甲烷的发现将有助于限制微生物群落中的碳和氮流动以及替代固氮酶在全球生物地球化学循环中的作用。