Microbial production of long-chain n-alkanes: Implication for interpreting sedimentary leaf wax signals

Relative distributions as well as compound-specific carbon and hydrogen isotope ratios of long-chain C-25 to C-33 n-alkanes in sediments provide important paleoclimate and paleoenvironmental information. These compounds in aquatic sediments are generally attributed to leaf waxes produced by higher plants. However, whether microbes, such as fungi and bacteria, can make a significant contribution to sedimentary long-chain n-alkanes is uncertain, with only scattered reports in the early 1960s to 1970s that microbes can produce long-chain n-alkanes. Given the rapidly expanding importance of leaf waxes in paleoclimate and paleoenvironmental studies, the impact of microbial contribution to long-chain n-alkanes in sediments must be fully addressed. In this study, we performed laboratory incubation of peat-land soils under both anaerobic and aerobic conditions in the absence of light with deuterium-enriched water over 1.5 years and analyzed compound-specific hydrogen isotopic ratios of n-alkanes. Under aerobic conditions, we find n-alkanes of different chain length display variable degrees of hydrogen isotopic enrichments, with short-chain (C-18-C-21) n-alkanes showing the greatest enrichment, followed by long-chain "leaf wax" (C-27-C-31) n-alkanes, and minimal or no enrichment for mid-chain (C-22-C-25) n-alkanes. In contrast, only the shorter chain (C-18 and C-19) n-alkanes display appreciable isotopic enrichment under anaerobic conditions. The degrees of isotopic enrichment for individual n-alkanes allow for a quantitative assessment of microbial contributions to n-alkanes. Overall our results show the microbial contribution to long-chain n-alkanes can reach up to 0.1% per year in aerobic conditions. For shorter chain n-alkanes, up to 2.5% per year could be produced by microbes in aerobic and anaerobic conditions respectively. Our results indicate that prolonged exposure to aerobic conditions can lead to substantial accumulation of microbially derived long-chain n-alkanes in sediments while original n-alkanes of leaf wax origin are degraded; hence caution must be exercised when interpreting sedimentary records of long-chain n-alkanes, including chain length distributions and isotopic ratios. (c) 2017 Elsevier Ltd. All rights reserved

Origin and Evolution of Prebiotic Organic Matter as Inferred from the Tagish Lake Meteorite

The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites' asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration and at least some molecules of pre-biotic importance formed during the alteration

Pyrolytic formation of alkylsteranes - Assigning geological orphans to their biological parents

Steranes alkylated at position C-3 occur in significant concentrations in many geological samples (e.g. Fig. 1). However, biological equivalents are not known from any living organisms and the formation pathway remains equally enigmatic, rendering them some of the most prominent orphan biomarkers. In some geological samples, the presence of sulphur functionalities indicated that the 3-alkyl group was originally functionalised, which together with a dominance of pentyl-derivatives pointed towards origins form C5 sugars. 3-alkylsteranes were therefore inferred to represent an entirely new class of natural products. Classified as putative ‘bacteriosteroids’ they were thought to reflect the bacterial fusion of eukaryotic (dietary) steroids with sugars to yield steroids with hopanepolyol-like side-chains (Dahl al., 1992; 1995). Other hypotheses encompass the likely bacterially mediated alkylation of stereneintermediates (Summons and Capon, 1988) or algal sources (Schaeffer et al., 1993). We simulated the geological maturation of regular 3-hydroxylated sterols by laboratory-based thermolysis and pyrolysis in the presence of carbon-catalysts and observed the formation of significant quantities of C-3 alkylated products that exhibit alkyl chain lengths of up to eight carbon units—similar to distributions in many geological extracts. Co-elution with an extract of the Ediacaran Araras group, previously shown to contain a series of 3β-n-alkyl steranes (Sousa Jr. et al., 2016), reveals that the lower members correspond to geological αααR isomers of steranes that have a straight hydrocarbon chain added to the 3β-position (Fig. 1). Our results show that 3-alkylsteranes readily form via carbon-catalysed geological process acting on regular (3-OH) sterol precursors. Considering that 3-alkylated steroids have never been identified in any living organism, there is thus no reason to assume that any of the geological 3-alkyl steroids have direct biosynthetic origins. Instead, regular sterols or their diagenetic intermediates are likely abiogenically alkylated in proportions that may be related to the diagenetic and catagenetic conditions, as well as the composition of the bitumen, kerogen and mineral matrix