Janet M. Hope

2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis

Oxygenic photosynthesis is widely accepted as the most important bioenergetic process happening in Earths surface environment. It is thought to have evolved within the cyanobacterial lineage, but it has been difficult to determine when it began. Evidence based on the occurrence and appearance of stromatolites and microfossils indicates that phototrophy occurred as long ago as 3,465 Myr although no definite physiological inferences can be made from these objects. Carbon isotopes and other geological phenomena, provide clues but are also equivocal. Biomarkers are potentially useful because the three domains of extant life—Bacteria, Archaea and Eukarya—have signature membrane lipids with recalcitrant carbon skeletons. These lipids turn into hydrocarbons in sediments and can be found wherever the recordis sufficiently well preserved. Here we show that 2-methylbacteriohopanepolyols occur in a high proportion of cultured cyanobacteria and cyanobacterial mats. Their 2-methylhopane hydrocarbon derivatives are abundant in organic-rich sediments as old as 2,500 Myr. These biomarkers may help constrain the age of the oldest cyanobacteria and the advent of oxygenic photosynthesis. They could also be used to quantify the ecological importance of cyanobacteria through geological time.

Signature lipids and stable carbon isotope analyses of Octopus Spring hyperthermophilic communities compared with those of Aquificales representatives.

ABSTRACT The molecular and isotopic compositions of lipid biomarkers of cultured Aquificales genera have been used to study the community and trophic structure of the hyperthermophilic pink streamers and vent biofilm from Octopus Spring. Thermocrinis ruber, Thermocrinis sp. strain HI 11/12,Hydrogenobacterthermophilus TK-6,Aquifex pyrophilus, and Aquifex aeolicusall contained glycerol-ether phospholipids as well as acyl glycerides. The n-C20:1 andcy-C21 fatty acids dominated all of theAquificales, while the alkyl glycerol ethers were mainly C18:0. These Aquificales biomarkers were major constituents of the lipid extracts of two Octopus Spring samples, a biofilm associated with the siliceous vent walls, and the well-known pink streamer community (PSC). Both the biofilm and the PSC contained mono- and dialkyl glycerol ethers in which C18 and C20 alkyl groups were prevalent. Phospholipid fatty acids included both the Aquificales n-C20:1 andcy-C21, plus a series ofiso-branched fatty acids (i-C15:0 toi-C21:0), indicating an additional bacterial component. Biomass and lipids from the PSC were depleted in13C relative to source water CO2 by 10.9 and 17.2‰, respectively. The C20–21 fatty acids of the PSC were less depleted than the iso-branched fatty acids, 18.4 and 22.6‰, respectively. The biomass of T. rubergrown on CO2 was depleted in 13C by only 3.3‰ relative to C source. In contrast, biomass was depleted by 19.7‰ when formate was the C source. Independent of carbon source, T. ruber lipids were heavier than biomass (+1.3‰). The depletion in the C20–21 fatty acids from the PSC indicates thatThermocrinis biomass must be similarly depleted and too light to be explained by growth on CO2. Accordingly,Thermocrinis in the PSC is likely to have utilized formate, presumably generated in the spring source region.

Carbon isotopic fractionation in lipids from methanotrophic bacteria II: the effects of physiology and environmental parameters on the biosynthesis and isotopic signatures of biomarkers.

Controls on the carbon isotopic signatures of methanotroph biomarkers have been further explored using cultured organisms. Growth under conditions which select for the membrane-bound particulate form of the methane monooxygenase enzyme (pMMO) leads to a significantly higher isotopic fractionation than does growth based on the soluble isozyme in both RuMP and serine pathway methanotrophs; in an RuMP type the delta delta 13Cbiomass equaled -23.9% for pMMO and -12.6% for sMMO. The distribution of biomarker lipids does not appear to be significantly affected by the dominance of one or the other MMO type and their isotopic compositions generally track those of the parent biomass. The 13C fractionation behaviour of serine pathway methanotrophs is very complex, reflecting the assimilation of both methane and carbon dioxide and concomitant dissimilation of methane-derived carbon. A limitation in CH4 availability leads to the production of biomass which is 13C-enriched with respect to both carbon substrates and this occurs irrespective of MMO type. This startling result indicates that there must be an additional fractionation step downstream from the MMO reaction which leads to incorporation of 13C-enriched carbon at the expense of dissimilation of 13C-depleted CO2. In these organisms, polyisoprenoid lipids are 13C-enriched compared to polymethylenic lipid which is the reverse of that found in the RuMP types. Serine cycle hopanoids, for example, can vary anywhere from 12% depleted to 10% enriched with respect to the CH4 substrate depending on its concentration. Decrease in growth temperature caused an overall increase in isotopic fractionation. In the total biomass, this effect tended to be masked by physiological factors associated with the type of organism and variation in the bulk composition. The effect was, however, clearly evident when monitoring the 13C signature of total lipid and individual biomarkers. Our results demonstrate that extreme carbon isotopic depletion in field samples and fossil biomarker lipids can be indicative of methanotrophy but the converse is not always true. For example, the hopanoids of a serine cycle methanotroph may be isotopically enriched by more than 10% compared to the substrate methane when the latter is limiting. In other words, hopanoids from some methanotrophs such as M. trichosporium would be indistinguishable from those of cyanobacteria or heterotrophic bacteria on the basis of either chemical structure or carbon isotopic signature.

Reappraisal of hydrocarbon biomarkers in Archean rocks

Significance The advent of oxygenic photosynthesis set the stage for the evolution of complex life on an oxygenated planet, but it is unknown when this transformative biochemistry emerged. The existing hydrocarbon biomarker record requires that oxygenic photosynthesis and eukaryotes emerged more than 300 million years before the Great Oxidation Event [∼2.4 billion years ago (Ga)]. We report that hopane and sterane concentrations measured in new ultraclean Archean drill cores from Australia are comparable to blank concentrations, yet their concentrations in the exteriors of conventionally collected cores of stratigraphic equivalence exceed blank concentrations by more than an order of magnitude due to surficial contamination. Consequently, previous hydrocarbon biomarker reports no longer provide valid evidence for the advent of oxygenic photosynthesis and eukaryotes by ∼2.7 Ga. Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories.

Carbon isotope biogeochemistry of plant resins and derived hydrocarbons

Abstract Hydrocarbons derived from plant resins are major components of some terrigenous oils and bitumens. These compounds are structurally distinct and this makes them useful biomarkers applicable in petroleum exploration as well as sources of biogeochemical information about palaeoenvironment and palaeobotany. Although recent studies have elucidated the molecular structure of resinites, very little information has been available for the carbon isotope composition of resinites and no studies of resin-derived compounds in oils had been performed prior to the present study. Hence, carbon stable isotope analyses were carried out on a suite of modern and fossil resins of diverse origins, including compound specific isotope analysis of individual hydrocarbons produced during resin pyrolysis. Oils derived from resinitic source organic matter were also analysed. The results showed that “Class I” resinites derived from gymnosperms were enriched in the heavy carbon isotope compared with those from angiosperms (“Class II” resinites). Furthermore, both fossil resinites themselves and individual hydrocarbons derived from them were isotopically heavy compared with modern plant resins. The isotopic signatures of diterpanes and triterpanes in various early Tertiary oils from Australasia and Southeast Asia reflect their origins from gymnosperms and angiosperms, respectively.

Carbon isotopic composition of hydrocarbons in ocean-transported bitumens from the coastline of Australia

Abstract Bitumens stranding along the coastlines of the Northern Territory, Western Australia, South Australia, Victoria and Tasmania often have biomarker signatures which closely match those of oils of S.E. Asian origin. A suite of these bitumens was studied to determine the isotopic signatures of their alkanes and to compare them to similar oils from the Central Sumatra Basin. Saturated hydrocarbons were separated into a fraction containing n -alkanes and isoalkanes and one containing the multibranched/cyclic components using adduction into silicalite. Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) of the n -alkanes revealed δ 13 C signatures covering a wide range of values from −25.5‰ to −29‰ PDB but generally falling between those of the botryococcane-containing Minas and Duri oils from the Central Sumatra Basin. n -Alkanes of the Minas oil are 3–4‰ lighter than their Duri counterparts. The isotopic compositions of pristane and phytane cover a similar range, δ 13 C = −24‰ to −27‰. Botryococcane is consistently heavy with δ 13 C values in the range −11 to −14‰. One bitumen sample with no botryococcane but abundant bicadinanes showed n -alkane δ 13 C values in the range −28 to −29‰, at the light extreme for waxy bitumens in our sample suite. Sample to sample differences in carbon isotopic signatures of n -alkanes are attributed mainly to variations in primary source, with weathering and biodegradation as minor causes. Instances of low precision for replicate analyses (± 2‰) are caused by the relative abundance of co-eluting isoalkanes. Overall, the isotopic patterns of waxy bitumens from the Australian coastline provide independent confirmation of their similarity to Central Sumatran lacustrine oils. On the other hand, asphaltic bitumens from the southern Australian coastline are isotopically light with n -alkane δ 13 C values in the range −31.5 to −33‰. This distinctive feature may assist identification of their source.

Extended 3β-alkyl steranes and 3-alkyl triaromatic steroids in crude oils and rock extracts

Abstract In oils and Precambrian- to Miocene-age source rocks from varying depositional environments, we have conclusively identified several novel 3-alkyl sterane and triaromatic steroid series, including (1) 3β-n-pentyl steranes, (2) 3β-isopentyl steranes, (3) 3β-n-hexyl steranes, (4) 3β-n-heptyl steranes, (5) 3,4-dimethyl steranes, (6) 3β-butyl,4-methyl steranes, (7) triaromatic 3-n-pentyl steroids, and (8) triaromatic 3-isopentyl steroids. We have also tentatively identified additional homologs with 3-alkyl substituents as large as C11. The relative abundances of these compounds vary substantially between samples, as indicated by (1) the ratio of 3β-n-pentyl steranes to 3β-isopentyl steranes and (2) the ratio of 3-n-pentyl triaromatic steroids to 3-isopentyl triaromatic steroids. These data suggest possible utility for these parameters as tools for oil-source rock correlations and reconstruction of depositional environments. Although no 3-alkyl steroid natural products are currently known, several lines of evidence suggest that 3β-alkyl steroids result from bacterial side-chain additions to diagenetic Δ2-sterenes.

Tris- and pentakis-dialkyldithiocarbamates of ruthenium, [Ru(S2CNR2)3]n and [Ru2(S2CNR2)5]n (n = +1, 0, and –1): chemical and electrochemical interrelations

The electron-transfer characteristics of many ruthenium(III) dithiocarbamates have been studied in acetone at a platinum electrode. The tris complexes undergo a relatively facile, reversible, one-electron reduction step to yield the ruthenium(II) complexes [Ru(S2CNRR′)3]–(R,R′= Me, Et, Pri, Bui, Ph, PhCH2, C6H11; RR′= piperidinyl, 2,6-dimethylpiperidinyl, pyrrolrdinyl, or morpholinyl). The corresponding one-electron oxidation step to the formally ruthenium(IV) complex [Ru(S2CNRR′)3]– is not reversible, the degree of irreversibility being markedly dependent on the substituents R and R′. Substituent effects on the redox potentials parallel those reported previously for many first-row transition-metal dithiacarbamates. Chemical and electrolytic oxidation of [Ru(S2CNR2)3] yield the dimeric ruthenium(III) cations [Ru2(S2CNR2)5]+ which are obtained in two structural isomeric forms, α and β. Both series (α and β) undergo successive one-electron reductions to the corresponding [Ru2(S2CNR2)5] and [Ru2(S2CNR2)5]– complexes. The rates of isomerisations (i) and (ii) have been measured. β-[Ru2(S2CNMe2)5]–→α-[Ru2(S2CNMe2)5]–(i), α-[Ru2(S2CNMe2)5]+→β-[Ru2(S2CNMe2)5]+(ii) The combined redox-potential data and equilibrium measurements afford the relative thermodynamic stabilities of the six dimeric species. Many of the new complexes have been characterised by spectroscopic techniques including i.r., visible–u.v., and 1H n.m.r. Paramagnetic shift reagents have been employed and, in the case of β-[Ru2(S2CNMe2)5]+, enantiomers have been distinguished with an optically active shift reagent.

Tailing of chromatographic peaks in GC-MS caused by interaction of halogenated solvents with the ion source.

The injection of analytes into a gas chromatography-mass spectrometry (GC-MS) system using dichloromethane (DCM) as solvent led to gradual deterioration of chromatographic signals, with significant tailing and loss of sensitivity for C17+ hydrocarbons. The injector, gas chromatograph and transfer line were excluded as causes. Normal peak shape could only be restored by the insertion of a cleaned MS ion source. To elucidate potential surficial contaminants, the ion source was heated from 260 to 320°C, leading to the release of increasing concentrations of ferrous chloride [FeCl2(g)]. The ferrous chloride probably formed through the decomposition of DCM on metal surfaces in the ion source. We posit that the tailing was caused by the adsorption of analytes to sub-µm layers of FeCl2 at crystal defect sites in the metal, followed by the slow release of molecules back into the gas phase. There are at least two other cases in the literature in which tailing is specifically associated with the use of halogenated solvents. However, it is possible that the problem is relatively common, albeit rarely diagnosed and reported. The tailing of chromatographic peaks caused by the formation of ferrous chloride in the mass spectrometer can be diagnosed by scanning the MS background signal for the diagnostic isotopic pattern of FeCl2(+). The problem is easily solved by cleaning the MS ion source and avoiding halogenated solvents.

Sterically hindered co-ordination sites in metal complexes. Part 1. The synthesis and properties of ‘capped’ four-co-ordinate metal Schiff-base complexes

The synthesis of several sterically hindered N2O2 and N4 ligands is described in which a quadridentate Schiff base is ‘capped’ by condensation of salicylaldehyde or pyrrole-2-carbaldehyde with a series of bis(8-aminonaphthyl) n-alkyl diethers. A number of representative complexes have been prepared and characterized with the metal ions FeII, CoII, NiII, CuII, and ZnII. The n.m.r. spectrum of the neutral monomeric nickel(II) derivative is consistent with the proposed trans planar ‘capped’ structure, a structure designed to provide for a study of the steric requirements of bonding to hindered metal-ion sites.