Abstract Non-hopanoid pentacyclic triterpenoids occur widely in lake sediments, but their biological sources and diagenetic pathways are not fully resolved. We tentatively identified a number of des-A-arborene isomers occurring in relatively high abundance in the 25,000 year (25 kyr) sedimentary record of Lake Chala, a deep crater lake in tropical East Africa. The mono-, di- and tri-unsaturated des-A-arborenes are transformation products of isoarborinol/arborinone. These precursors have an aquatic source and are most likely biosynthesized by algae or aerobic bacteria in the epilimnion. The relatively depleted δ13C values (on average −32.3‰ ± 1.3‰) of des-A-arbor-9(11)-ene are consistent with an aquatic source (algae or aerobic bacteria). In general, isoarborinol and its microbially induced transformation products are found in present and ancient tropical lacustrine settings (typically crater lakes) with permanently anoxic bottom waters and sediments. Based on molecular mechanics calculations of des-A-arborenes, it is clear that these transformation products are not in thermodynamic equilibrium, strongly indicating that their formation is microbially mediated. Subtle temporal and spatial differences in the microbial community might therefore not only dictate the variable relative contributions of different des-A-arborenes found in the sediments of Lake Chala over the last 25 kyr, but also explain the distribution of arborane derivatives in comparable crater-lake settings elsewhere.

中文翻译：

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东非火山口湖沉积物中des-A-arborenes的起源、形成和环境意义

摘要 非藿烷类五环三萜类化合物广泛存在于湖泊沉积物中，但其生物来源和成岩途径尚未完全阐明。我们初步确定了一些 des-A-arborene 异构体，这些异构体在 25,000 年 (25 kyr) 的沉积记录中以相对较高的丰度出现，Chala 湖是东非热带深火山口湖。单-、二-和三-不饱和des-A-arborenes是isoarborinol/arborinone的转化产物。这些前体具有水生来源，最有可能由表层中的藻类或需氧细菌生物合成。des-A-arbor-9(11​​)-ene 的相对贫乏的 δ13C 值（平均 -32.3‰±1.3‰）与水生来源（藻类或需氧细菌）一致。一般来说，isoarborinol 及其微生物诱导的转化产物存在于具有永久缺氧底水和沉积物的现代和古代热带湖泊环境（通常是火山口湖）中。基于 des-A-arborenes 的分子力学计算，很明显这些转化产物不处于热力学平衡，强烈表明它们的形成是微生物介导的。因此，微生物群落中微妙的时间和空间差异可能不仅决定了过去 25 kyr 查拉湖沉积物中发现的不同 des-A-arborenes 的可变相对贡献，而且还解释了可比陨石坑中 Arborane 衍生物的分布。其他地方的湖泊设置。基于 des-A-arborenes 的分子力学计算，很明显这些转化产物不处于热力学平衡，强烈表明它们的形成是微生物介导的。因此，微生物群落的细微时空差异可能不仅决定了过去 25 kyr 查拉湖沉积物中发现的不同 des-A-arborenes 的可变相对贡献，而且还解释了可比陨石坑中 Arborane 衍生物的分布其他地方的湖泊设置。基于 des-A-arborenes 的分子力学计算，很明显这些转化产物不处于热力学平衡，强烈表明它们的形成是微生物介导的。因此，微生物群落中微妙的时间和空间差异可能不仅决定了过去 25 kyr 查拉湖沉积物中发现的不同 des-A-arborenes 的可变相对贡献，而且还解释了可比陨石坑中 Arborane 衍生物的分布。其他地方的湖泊设置。