Abstract Aerobic methanotrophic bacteria are known to synthesize a variety of cyclic terpenoids which are typified by 13C-depleted, methane-derived carbon. This peculiarity facilitates identification of methanotroph biomarkers in natural samples. However, the current biomarker database does not always allow biomarker patterns of marine samples to be assigned to the different types of aerobic methanotrophs. To overcome this shortcoming, the carbon stable isotope composition of cyclic terpenoids of two strains of the Type I methanotroph genus Methylomicrobium was analyzed. Other than aerobic methanotrophs used for biomarker studies in the past, these two strains deriving from soda lake environments are able to tolerate the conditions typifying marine environments including high alkalinity and salinity. The cyclic terpenoid inventory of the two strains comprises 4-methyl steroids, 3-methyl- and desmethyl bacteriohopanepolyols (aminotetrol and aminotriol), and tetrahymanol, all of which are 13C-depleted. The average carbon isotope fractionation between methane and the respective lipid (Δδ13Cterpenoid-methane) is found to be −25‰ for M. kenyense and −16‰ for M. alcaliphilum. These data shed new light on the previously reported compound and carbon stable isotope patterns of cyclic terpenoids from methane-seep environments. Particularly, 13C-depleted tetrahymanol and gammacerane are reinterpreted as biomarkers of aerobic methanotrophic bacteria based on their occurrence in methane-seep deposits in association with other biomarkers of aerobic methanotrophs. The use of δ13C values of anaerobic methane-oxidizing archaea (ANME) lipids for the reconstruction of the isotopic composition of parent methane allows us to calculate the Δδ13Cterpenoid-methane even for ancient seep environments. With this calculation, Type I and Type II methanotrophs can be discriminated, representing a new approach to better characterize past methanotrophy at seeps and possibly other marine environments.

中文翻译：

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环状萜类化合物的碳稳定同位素模式：培养的嗜碱好氧甲烷氧化菌与甲烷渗漏环境的比较

摘要 已知需氧甲烷营养菌可合成多种以 13C 耗尽、甲烷衍生的碳为代表的环状萜类化合物。这种特性有助于鉴定天然样品中的甲烷氧化生物标志物。然而，当前的生物标志物数据库并不总是允许将海洋样品的生物标志物模式分配给不同类型的需氧甲烷氧化菌。为了克服这个缺点，分析了两种 I 型甲烷氧化菌属 Methylomicrobium 的环状萜类化合物的碳稳定同位素组成。除了过去用于生物标志物研究的需氧甲烷氧化菌外，这两种源自苏打湖环境的菌株能够耐受典型的海洋环境条件，包括高碱度和盐度。这两个菌株的环状萜类化合物清单包括 4-甲基类固醇、3-甲基和去甲基细菌霍烷多元醇（氨基四醇和氨基三醇）和四氢甘醇，所有这些都是 13C 耗尽的。发现甲烷和相应脂质（Δδ13Cterpenoid-methane）之间的平均碳同位素分馏对于 M. kenyense 为 -25‰，对于 M. alcaliphilum 为 -16‰。这些数据为先前报道的来自甲烷渗漏环境的环状萜类化合物的化合物和碳稳定同位素模式提供了新的线索。特别是，13C 耗尽的四氢乙醇和伽马蜡烷被重新解释为好氧甲烷氧化菌的生物标志物，因为它们与其他好氧甲烷氧化菌的生物标志物一起出现在甲烷渗出沉积物中。使用厌氧甲烷氧化古细菌 (ANME) 脂质的 δ13C 值重建母体甲烷的同位素组成，使我们能够计算 Δδ13Cterpenoid-methane，即使是在古代渗漏环境中。通过这种计算，可以区分 I 型和 II 型甲烷氧化菌，这代表了一种新方法，可以更好地表征渗漏处和可能的其他海洋环境中过去的甲烷氧化菌。