Patrick Wehrung

Highly isotopically depleted isoprenoids: molecular markers for ancient methane venting

Abstract We propose that organic compounds found in a Miocene limestone from Marmorito (Northern Italy) are source markers for organic matter present in ancient methane vent systems (cold seeps). The limestone contains high concentrations of the tail-to-tail linked, acyclic C20 isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), a C25 homolog 2,6,10,15,19-pentamethylicosane (PME), and a distinctive glycerol ether lipid containing 3,7,11,15-tetramethylhexadecyl (phytanyl-) moieties. The chemical structures of these biomarkers indicate a common origin from archaea. Their extremely 13C-depleted isotope compositions (δ13C ≈ −108 to −115.6‰ PDB) suggest that the respective archaea have directly or indirectly introduced isotopically depleted, methane-derived carbon into their biomass. We postulate that a second major cluster of biomarkers showing heavier isotope values (δ13C ≈ −88‰) is derived from sulfate-reducing bacteria (SRB). The observed biomarkers sustain the idea that methanogenic bacteria, in a syntrophic community with SRB, are responsible for the anaerobic oxidation of methane in marine sediments. Marmorito may thus represent a conceivable ancient scenario for methane consumption performed by a defined, two-membered bacterial consortium: (1) archaea that perform reversed methanogenesis by oxidizing methane and producing CO2 and H2; and (2) SRB that consume the resulting H2. Furthermore, the respective organic molecules are, unlike other compounds, tightly bound to the crystalline carbonate phase. The Marmorito carbonates can thus be regarded as “cold seep microbialites” rather than mere “authigenic” carbonates.

A novel pathway of soil organic matter formation by selective preservation of resistant straight-chain biopolymers: chemical and isotope evidence

A resistant soil organic residue, “humin”, has been analyzed by solid-state 13C-NMR and pyrolysis followed by molecular and 13C determination of the pyrolysate alkane–alkene fraction. The results show the occurrence of highly aliphatic, straight-chain biopolymer material as a substantial part of soil organic matter. They confirm the hypothesis by which a part of soil organic matter can be formed by selective preservation of resistant highly aliphatic microbial polymers. This pathway is additional to the longstanding neogenesis hypothesis involving recondensation of small polar molecules such as amino acids, carbohydrates and phenols.

Novel triterpene-derived hydrocarbons of the arborane/fernane series in sediments: Part II

Two ring-B aromatized triterpenes related to the arborane/fernane series were isolated from an Italian Triassic black shale (4 and 7) and a French Jurassic laminated bituminous limestone (7). They were identified by MS and NMR spectroscopy. These compounds, 25-norferna-5,7,9-triene 7 (C29) and 22,25,29,30-tetranor-18β-ferna-5,7,9-triene 4 (C26), in the case of the Italian sediment, belong to a family which extends to at least C32 as shown by GC-MS. The presence of higher homologs and ring-A methylated structures in the Italian Triassic shale is analogous with the hopane series, which together with carbon isotopic data obtained by gas chromatography-isotope ratio mass spectrometry (GC-IRMS), strongly support a bacterial origin for most of these compounds. This suggests that 4 and 7 may result from a yet unrecognized rearrangement of sedimentary hopenes which have subsequently undergone ring-B aromatization. As an alternative, the C29 compound 7 could arise from C30 precursors not necessarily of bacterial origin (fernenes?). This hypothesis would apply particularly to the case of the Jurassic limestone, which does not show any trace of 4 nor of compounds with an extended side chain or a methyl group on ring A. Related structures also bearing an isopropyl group could derive by microbiological transformations from the corresponding fernen-3β-ols or isoarborinol as can be deduced from the occurrence of ring-A degraded monoaromatic hydrocarbons 3 and 5.

Novel triterpene-derived hydrocarbons of arborane/fernane series in sediments. Part I.

Abstract Three aromatic polycyclic hydrocarbons were isolated from Eocene shales and the Permian Kupferschiefer, and their molecular structures established by 1D and 2D NMR spectroscopy. These compounds (des-A-arbora-5,7,9- triene; 24,25-dinorarbora-1,3,5(10),9(11)-tetraene; 24,25-dinorarbora-1,3,5,7,9-pentaene) derive from microbial transformations of pentacyclic triterpenes oxygenated at C-3 belonging to the arborane or fernane families.

Novel aromatic carotenoid derivatives from sulfur photosynthetic bacteria in sediments

Novel aromatic carotenoid derivatives 7 and 8 have been identified by synthesis of reference compounds in a H2/PtO2-hydrogenated fraction from the organic extract of a lake sediment (lake Cadagno, Switzerland). They derive from unreported carotenoids presumably biosynthesized by purple sulfur photosynthetic bacteria (Chromatiaceae) and represent potential biomarkers to establish the occurrence of photic zone anoxia in ancient ecosystems.

Analysis of 13C labeling enrichment in microbial culture applying metabolic tracer experiments using gas chromatography–combustion–isotope ratio mass spectrometry

The applicability of gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) for the quantification of 13C enrichment of proteinogenic amino acids in metabolic tracer experiments was evaluated. Measurement of the 13C enrichment of proteinogenic amino acids from cell hydrolyzates of Corynebacterium glutamicum growing on different mixtures containing between 0.5 and 10% [1-13C]glucose shows the significance of kinetic isotope effects in metabolic flux studies at low degree of labeling. We developed a method to calculate the 13C enrichment. The approach to correct for these effects in metabolic flux studies using delta13C measurement by GC-C-IRMS uses two parallel experiments applying substrate with natural abundance and 13C-enriched tracer substrate, respectively. The fractional enrichment obtained in natural substrate is subtracted from that of the enriched one. Tracer studies with C. glutamicum resulted in a statistically identical relative fractional enrichment of 13C in proteinogenic amino acids over the whole range of applied concentrations of [1-13C]glucose. The current findings indicate a great potential of GC-C-IRMS for labeling quantification in 13C metabolic flux analysis with low labeling degree of tracer substrate directly in larger scale bioreactors.

Biogeochemical aromatic markers in the sediments from Mahakam Delta (Indonesia)

Abstract Analysis of the tri-, tetraand pentaaromatic hydrocarbon fractions of several sediments (0–600 m) from the Mahakam Delta (Indonesia), indicate the predominance of polycyclic aromatic hydrocarbons (PAH) diagenetically related to triterpenoid natural precursors. High resolution Shpolskii spectrofluorometry (HRS) proved to be a valuable analytical tool, in conjunction with high performance liquid chromatography (HPLC) and capillary gas chromatography coupled with mass spectrometry (CGC/MS), for the identification of individual components in Chromatographic fractions obtained after a high resolution reverse phase liquid chromatography.

C(14a)-homo-26-nor-17α-hopanes, a novel and unexpected series of molecular fossils in biodegraded petroleum

A series (C27–C35) of pentacyclic triterpene hydrocarbons, the C(14a)-homo-26-nor-17α-hopanes, has been characterized in a biodegraded petroleum by gas chromatography–mass spectrometry and NMR structural determination of the C29 and C30 members; these biological markers reveal an as yet unknown transposition of the hopane skeleton.

Rearranged des-E-hopanoid hydrocarbons in sediments and petroleum

Abstract Two tetracyclic triterpene-derived hydrocarbons, des-E-D:C-friedo-25-norhopa-5,7,9-triene 1 and des-E-D:C-friedohop-8-ene 8, have been isolated from an Italian Triassic black shale and identified by MS and NMR spectroscopy. Their structure, the occurrence in the sediment of their ring A methylated counterparts, as well as carbon isotopic data indicate that these compounds originate from bacterial hopanoids. Their formation most probably involves a sedimentary rearrangement of a functionalised des-E-hopane intermediate. This hypothesis was further substantiated by experiments in which 8 was formed from (18αH)-des-E-hopan-17α-ol 10 by treatment with K-10 montmorillonite clay. In addition, some minor unsaturated hydrocarbons formed as by-products are also present in sediments.