Miller L G, Kalin R M, McCauley S E, Hamilton J T, Harper D B, Millet D B, Oremland R S, Goldstein A H
United States Geological Survey, Menlo Park, CA 94025, USA.
Proc Natl Acad Sci U S A. 2001 May 8;98(10):5833-7. doi: 10.1073/pnas.101129798.
The largest biological fractionations of stable carbon isotopes observed in nature occur during production of methane by methanogenic archaea. These fractionations result in substantial (as much as approximately 70 per thousand) shifts in delta(13)C relative to the initial substrate. We now report that a stable carbon isotopic fractionation of comparable magnitude (up to 70 per thousand) occurs during oxidation of methyl halides by methylotrophic bacteria. We have demonstrated biological fractionation with whole cells of three methylotrophs (strain IMB-1, strain CC495, and strain MB2) and, to a lesser extent, with the purified cobalamin-dependent methyltransferase enzyme obtained from strain CC495. Thus, the genetic similarities recently reported between methylotrophs, and methanogens with respect to their pathways for C(1)-unit metabolism are also reflected in the carbon isotopic fractionations achieved by these organisms. We found that only part of the observed fractionation of carbon isotopes could be accounted for by the activity of the corrinoid methyltransferase enzyme, suggesting fractionation by enzymes further along the degradation pathway. These observations are of potential biogeochemical significance in the application of stable carbon isotope ratios to constrain the tropospheric budgets for the ozone-depleting halocarbons, methyl bromide and methyl chloride.
自然界中观察到的稳定碳同位素最大生物分馏发生在产甲烷古菌产生甲烷的过程中。这些分馏导致相对于初始底物，δ(13)C 发生显著（高达约千分之70）的变化。我们现在报告，在甲基营养细菌氧化卤代甲烷的过程中，发生了幅度相当（高达千分之70）的稳定碳同位素分馏。我们已经用三种甲基营养菌（IMB-1菌株、CC495菌株和MB2菌株）的全细胞证明了生物分馏，并且在较小程度上，用从CC495菌株获得的纯化的钴胺素依赖性甲基转移酶也证明了生物分馏。因此，最近报道的甲基营养菌和产甲烷菌在其C(1)单元代谢途径方面的遗传相似性，也反映在这些生物体实现的碳同位素分馏中。我们发现，观察到的碳同位素分馏中只有一部分可以由类咕啉甲基转移酶的活性来解释，这表明沿着降解途径的其他酶也会导致分馏。这些观察结果在应用稳定碳同位素比率来限制对流层中消耗臭氧层的卤代烃、甲基溴和甲基氯的预算方面具有潜在的生物地球化学意义。
