Publications by authors named "Richard D Pancost"

27 Publications

  • Page 1 of 1

Determination of the δ H values of high molecular weight lipids by high-temperature gas chromatography coupled to isotope ratio mass spectrometry.

Rapid Commun Mass Spectrom 2021 Feb;35(4):e8983

Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.

Rationale: The hydrogen isotopic composition of lipids (δ H ) is widely used in food science and as a proxy for past hydrological conditions. Determining the δ H values of large, well-preserved triacylglycerides and other microbial lipids, such as glycerol dialkyl glycerol tetraether (GDGT) lipids, is thus of widespread interest but has so far not been possible due to their low volatility which prohibits analysis by traditional gas chromatography/pyrolysis/isotope ratio mass spectrometry (GC/P/IRMS).

Methods: We determined the δ H values of large, polar molecules and applied high-temperature gas chromatography (HTGC) methods on a modified GC/P/IRMS system. The system used a high-temperature 7-m GC column, and a glass Y-splitter for low thermal mass. Methods were validated using authentic standards of large, functionalised molecules (triacylglycerides, TGs), purified standards of GDGTs. The results were compared with δ H values determined by high-temperature elemental analyser/pyrolysis/isotope ratio mass spectrometry (HTEA/P/IRMS), and subsequently applied to the analysis of GDGTs in a sample from a methane seep and a Welsh peat.

Results: The δ H values of TGs agreed within error between HTGC/P/IRMS and HTEA/IRMS, with HTGC/P/IRMS showing larger errors. Archaeal lipid GDGTs with up to three cyclisations could be analysed: the δ H values were not significantly different between methods with standard deviations of 5 to 6 ‰. When environmental samples were analysed, the δ H values of isoGDGTs were 50 ‰ more negative than those of terrestrial brGDGTs.

Conclusions: Our results indicate that the HTGC/P/IRMS method developed here is appropriate to determine the δ H values of TGs, GDGTs with up to two cyclisations, and potentially other high molecular weight compounds. The methodology will widen the current analytical window for biomarker and food light stable isotope analyses. Moreover, our initial measurements suggest that bacterial and archaeal GDGT δ H values can record environmental and ecological conditions.
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http://dx.doi.org/10.1002/rcm.8983DOI Listing
February 2021

Fundamentally different global marine nitrogen cycling in response to severe ocean deoxygenation.

Proc Natl Acad Sci U S A 2019 12 25;116(50):24979-24984. Epub 2019 Nov 25.

School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom.

The present-day marine nitrogen (N) cycle is strongly regulated by biology. Deficiencies in the availability of fixed and readily bioavailable nitrogen relative to phosphate (P) in the surface ocean are largely corrected by the activity of diazotrophs. This feedback system, termed the "nitrostat," is thought to have provided close regulation of fixed-N speciation and inventory relative to P since the Proterozoic. In contrast, during intervals of intense deoxygenation such as Cretaceous ocean anoxic event (OAE) 2, a few regional sedimentary δN records hint at the existence of a different mode of marine N cycling in which ammonium plays a major role in regulating export production. However, the global-scale dynamics during this time remain unknown. Here, using an Earth System model and taking the example of OAE 2, we provide insights into the global marine nitrogen cycle under severe ocean deoxygenation. Specifically, we find that the ocean can exhibit fundamental transitions in the species of nitrogen dominating the fixed-N inventory--from nitrate (NO) to ammonium (NH)--and that as this transition occurs, the inventory can partially collapse relative to P due to progressive spatial decoupling between the loci of NH oxidation, NO reduction, and nitrogen fixation. This finding is relatively independent of the specific state of ocean circulation and is consistent with nitrogen isotope and redox proxy data. The substantive reduction in the ocean fixed-N inventory at an intermediate state of deoxygenation may represent a biogeochemical vulnerability with potential implications for past and future (warmer) oceans.
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http://dx.doi.org/10.1073/pnas.1905553116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6911173PMC
December 2019

Past East Asian monsoon evolution controlled by paleogeography, not CO.

Sci Adv 2019 10 30;5(10):eaax1697. Epub 2019 Oct 30.

Cabot Institute, University of Bristol, Bristol BS8 1UJ, UK.

The East Asian monsoon plays an integral role in human society, yet its geological history and controlling processes are poorly understood. Using a general circulation model and geological data, we explore the drivers controlling the evolution of the monsoon system over the past 150 million years. In contrast to previous work, we find that the monsoon is controlled primarily by changes in paleogeography, with little influence from atmospheric CO. We associate increased precipitation since the Late Cretaceous with the gradual uplift of the Himalayan-Tibetan region, transitioning from an ITCZ-dominated monsoon to a sea breeze-dominated monsoon. The rising region acted as a mechanical barrier to cold and dry continental air advecting into the region, leading to increasing influence of moist air from the Indian Ocean/South China Sea. We show that, apart from a dry period in the middle Cretaceous, a monsoon system has existed in East Asia since at least the Early Cretaceous.
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http://dx.doi.org/10.1126/sciadv.aax1697DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6821471PMC
October 2019

Selective chemical degradation of silica sinters of the Taupo Volcanic Zone (New Zealand). Implications for early Earth and Astrobiology.

Geobiology 2019 07 24;17(4):449-464. Epub 2019 Apr 24.

Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol, UK.

Most organic matter (OM) on Earth occurs as kerogen-like materials, that is naturally formed macromolecules insoluble with standard organic solvents. The formation of this insoluble organic matter (IOM) is a topic of much interest, especially when it limits the detection of compounds of geomicrobiological interest. For example, studies that search for biomarker evidence of life on early Earth or other planets usually use solvent-based extractions. This leaves behind a pool of OM as unexplored post-extraction residues, potentially containing diagnostic biomarkers. Since the IOM has an enhanced potential for preservation compared to soluble OM, analysing IOM-released biomarkers can also provide even deeper insights into the ecology of ancient settings, with implications for early Earth and Astrobiology investigations. Here, we analyse the prokaryotic lipid biosignature within soluble and IOM of the Taupo Volcanic Zone (TVZ) silica sinters, which are key analogues in the search for life. We apply sequential solvent extractions and a selective chemical degradation upon the post-solvent extraction residue. Moreover, we compare the IOM from TVZ sinters to analogous studies on peat and marine sediments to assess patterns in OM insolubilisation across the geosphere. Consistent with previous work, we find significant but variable proportions-1%-45% of the total prokaryotic lipids recovered-associated with IOM fractions. This occurs even in recently formed silica sinters, likely indicating inherent cell insolubility. Moreover, archaeal lipids seem more prone to insolubilisation as compared to the bacterial analogues, which might enhance their preservation and also bias overall biomarkers interpretation. These observations are similar to those observed in other settings, confirming that even in a setting where the OM derives predominantly from prokaryotic sources, patterns of IOM formation/occurrence are conserved. Differences with other settings, however, such as the occurrence of archaeol in IOM fractions, could be indicative of different mechanisms for IOM formation that merit further exploration.
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http://dx.doi.org/10.1111/gbi.12340DOI Listing
July 2019

Investigating the Origins of Membrane Phospholipid Biosynthesis Genes Using Outgroup-Free Rooting.

Genome Biol Evol 2019 03;11(3):883-898

School of Biological Sciences, University of Bristol, United Kingdom.

One of the key differences between Bacteria and Archaea is their canonical membrane phospholipids, which are synthesized by distinct biosynthetic pathways with nonhomologous enzymes. This "lipid divide" has important implications for the early evolution of cells and the type of membrane phospholipids present in the last universal common ancestor. One of the main challenges in studies of membrane evolution is that the key biosynthetic genes are ancient and their evolutionary histories are poorly resolved. This poses major challenges for traditional rooting methods because the only available outgroups are distantly related. Here, we address this issue by using the best available substitution models for single-gene trees, by expanding our analyses to the diversity of uncultivated prokaryotes recently revealed by environmental genomics, and by using two complementary approaches to rooting that do not depend on outgroups. Consistent with some previous analyses, our rooted gene trees support extensive interdomain horizontal transfer of membrane phospholipid biosynthetic genes, primarily from Archaea to Bacteria. They also suggest that the capacity to make archaeal-type membrane phospholipids was already present in last universal common ancestor.
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http://dx.doi.org/10.1093/gbe/evz034DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6431249PMC
March 2019

Response of carbon cycle to drier conditions in the mid-Holocene in central China.

Nat Commun 2018 04 10;9(1):1369. Epub 2018 Apr 10.

State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, 430074, P.R. China.

The nature and extent to which hydrological changes induced by the Asian summer monsoon affected key biogeochemical processes remain poorly defined. This study explores the relationship between peatland drying and carbon cycling on centennial timescales in central China using lipid biomarkers. The difference between peat n-alkane δH and a nearby stalagmite δO record reveals that intervals of prominent peatland drying occurred during the mid-Holocene. Synchronous with these drier conditions, leaf wax δC values show large negative excursions, with the utilization of CO respired from the peatland subsurface for plant photosynthesis being a possible mechanism. Crucially, successive drying events appear to have had a cumulative impact on the susceptibility of peat carbon stores to climate change. Concurrently, bacterially derived hopane δC values suggest the occurrence of enhanced methane oxidation during the drier periods. Collectively, these observations expand our understanding of how respiration and degradation of peat are enhanced during drying events.
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http://dx.doi.org/10.1038/s41467-018-03804-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5893629PMC
April 2018

Causes of ice age intensification across the Mid-Pleistocene Transition.

Proc Natl Acad Sci U S A 2017 12 27;114(50):13114-13119. Epub 2017 Nov 27.

Ocean and Earth Science, University of Southampton, National Oceanography Centre Southampton, Southampton SO14 3ZH, United Kingdom.

During the Mid-Pleistocene Transition (MPT; 1,200-800 kya), Earth's orbitally paced ice age cycles intensified, lengthened from ∼40,000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO data to show that the glacial to interglacial CO difference increased from ∼43 to ∼75 μatm across the MPT, mainly because of lower glacial CO levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.
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http://dx.doi.org/10.1073/pnas.1702143114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5740680PMC
December 2017

Changing atmospheric CO2 concentration was the primary driver of early Cenozoic climate.

Nature 2016 05 25;533(7603):380-4. Epub 2016 Apr 25.

School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3AT, UK.

The Early Eocene Climate Optimum (EECO, which occurred about 51 to 53 million years ago), was the warmest interval of the past 65 million years, with mean annual surface air temperature over ten degrees Celsius warmer than during the pre-industrial period. Subsequent global cooling in the middle and late Eocene epoch, especially at high latitudes, eventually led to continental ice sheet development in Antarctica in the early Oligocene epoch (about 33.6 million years ago). However, existing estimates place atmospheric carbon dioxide (CO2) levels during the Eocene at 500-3,000 parts per million, and in the absence of tighter constraints carbon-climate interactions over this interval remain uncertain. Here we use recent analytical and methodological developments to generate a new high-fidelity record of CO2 concentrations using the boron isotope (δ(11)B) composition of well preserved planktonic foraminifera from the Tanzania Drilling Project, revising previous estimates. Although species-level uncertainties make absolute values difficult to constrain, CO2 concentrations during the EECO were around 1,400 parts per million. The relative decline in CO2 concentration through the Eocene is more robustly constrained at about fifty per cent, with a further decline into the Oligocene. Provided the latitudinal dependency of sea surface temperature change for a given climate forcing in the Eocene was similar to that of the late Quaternary period, this CO2 decline was sufficient to drive the well documented high- and low-latitude cooling that occurred through the Eocene. Once the change in global temperature between the pre-industrial period and the Eocene caused by the action of all known slow feedbacks (apart from those associated with the carbon cycle) is removed, both the EECO and the late Eocene exhibit an equilibrium climate sensitivity relative to the pre-industrial period of 2.1 to 4.6 degrees Celsius per CO2 doubling (66 per cent confidence), which is similar to the canonical range (1.5 to 4.5 degrees Celsius), indicating that a large fraction of the warmth of the early Eocene greenhouse was driven by increased CO2 concentrations, and that climate sensitivity was relatively constant throughout this period.
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http://dx.doi.org/10.1038/nature17423DOI Listing
May 2016

Uncertainty as knowledge.

Philos Trans A Math Phys Eng Sci 2015 Nov;373(2055)

School of Experimental Psychology and Cabot Institute, University of Bristol, 12a Priory Road, Bristol BS8 1TU, UK School of Chemistry and Cabot Institute, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.

This issue of Philosophical Transactions examines the relationship between scientific uncertainty about climate change and knowledge. Uncertainty is an inherent feature of the climate system. Considerable effort has therefore been devoted to understanding how to effectively respond to a changing, yet uncertain climate. Politicians and the public often appeal to uncertainty as an argument to delay mitigative action. We argue that the appropriate response to uncertainty is exactly the opposite: uncertainty provides an impetus to be concerned about climate change, because greater uncertainty increases the risks associated with climate change. We therefore suggest that uncertainty can be a source of actionable knowledge. We survey the papers in this issue, which address the relationship between uncertainty and knowledge from physical, economic and social perspectives. We also summarize the pervasive psychological effects of uncertainty, some of which may militate against a meaningful response to climate change, and we provide pointers to how those difficulties may be ameliorated.
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http://dx.doi.org/10.1098/rsta.2014.0462DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4608032PMC
November 2015

Source Apportionment of Polycyclic Aromatic Hydrocarbons in Central European Soils with Compound-Specific Triple Isotopes (δ(13)C, Δ(14)C, and δ(2)H).

Environ Sci Technol 2015 Jul 23;49(13):7657-65. Epub 2015 Jun 23.

†Department of Environmental Science and Analytical Chemistry and the Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden.

This paper reports the first study applying a triple-isotope approach for source apportionment of polycyclic aromatic hydrocarbons (PAHs). The (13)C/(12)C, (14)C/(12)C, and (2)H/(1)H isotope ratios of PAHs were determined in forest soils from mountainous areas of the Czech Republic, European Union. Statistical modeling applying a Bayesian Markov chain Monte Carlo (MCMC) framework to the environmental triple isotope PAH data and an end-member PAH isotope database allowed comprehensive accounting of uncertainties and quantitative constraints on the PAH sources among biomass combustion, liquid fossil fuel combustion, and coal combustion at low and high temperatures. The results suggest that PAHs in this central European region had a clear predominance of coal combustion sources (75 ± 6%; uncertainties represent 1 SD), mainly coal pyrolysis at low temperature (∼650 °C; 61 ± 8%). Combustion of liquid fossil fuels and biomass represented 16 ± 3 and 9 ± 3% of the total PAH burden (∑PAH14), respectively. Although some soils were located close to potential PAH point sources, the source distribution was within a narrow range throughout the region. These observation-based top-down constraints on sources of environmental PAHs provide a reference for both improved bottom-up emission inventories and guidance for efforts to mitigate PAH emissions.
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http://dx.doi.org/10.1021/acs.est.5b01190DOI Listing
July 2015

Temperature and pH control on lipid composition of silica sinters from diverse hot springs in the Taupo Volcanic Zone, New Zealand.

Extremophiles 2015 Mar 17;19(2):327-44. Epub 2014 Dec 17.

Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK,

Microbial adaptations to environmental extremes, including high temperature and low pH conditions typical of geothermal settings, are of interest in astrobiology and origin of life investigations. The lipid biomarkers preserved in silica deposits associated with six geothermal areas in the Taupo Volcanic Zone were investigated and variations in lipid composition as a function of temperature and pH were assessed. Lipid analyses reveal highly variable abundances and distributions, reflecting community composition as well as adaptations to extremes of pH and temperature. Biomarker profiles reveal three distinct microbial assemblages across the sites: the first in Champagne Pool and Loop Road, the second in Orakei Korako, Opaheke and Ngatamariki, and the third in Rotokawa. Similar lipid distributions are observed in sinters from physicochemically similar springs. Furthermore, correlation between lipid distributions and geothermal conditions is observed. The ratio of archaeol to bacterial diether abundance, bacterial diether average chain length, degree of GDGT cyclisation and C31 and C32 hopanoic acid indices typically increase with temperature. At lower pH, the ratio of archaeol to bacterial diethers, degree of GDGT cyclisation and C31 and C32 hopanoic acid indices are typically higher. No trends in fatty acid distributions with temperature or pH are evident, likely reflecting overprinting due to population influences.
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http://dx.doi.org/10.1007/s00792-014-0719-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4339782PMC
March 2015

Holocene variations in peatland methane cycling associated with the Asian summer monsoon system.

Nat Commun 2014 Aug 19;5:4631. Epub 2014 Aug 19.

Organic Geochemistry Unit, Cabot Institute and School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.

Atmospheric methane concentrations decreased during the early to middle Holocene; however, the governing mechanisms remain controversial. Although it has been suggested that the mid-Holocene minimum methane emissions are associated with hydrological change, direct evidence is lacking. Here we report a new independent approach, linking hydrological change in peat sediments from the Tibetan Plateau to changes in archaeal diether concentrations and diploptene δ(13)C values as tracers for methanogenesis and methanotrophy, respectively. A minimum in inferred methanogenesis occurred during the mid-Holocene, which, locally, corresponds with the driest conditions of the Holocene, reflecting a minimum in Asian monsoon precipitation. The close coupling between precipitation and methanogenesis is validated by climate simulations, which also suggest a regionally widespread impact. Importantly, the minimum in methanogenesis is associated with a maximum in methanotrophy. Therefore, methane emissions in the Tibetan Plateau region were apparently lower during the mid-Holocene and partially controlled by interactions of large-scale atmospheric circulation.
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http://dx.doi.org/10.1038/ncomms5631DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4143914PMC
August 2014

Microbial ecology of arsenic-mobilizing Cambodian sediments: lithological controls uncovered by stable-isotope probing.

Environ Microbiol 2015 Jun 5;17(6):1857-69. Epub 2014 Mar 5.

School of Earth, Atmospheric and Environmental Sciences, Williamson Research Centre for Molecular Environmental Science, University of Manchester, Manchester, UK.

Microbially mediated arsenic release from Holocene and Pleistocene Cambodian aquifer sediments was investigated using microcosm experiments and substrate amendments. In the Holocene sediment, the metabolically active bacteria, including arsenate-respiring bacteria, were determined by DNA stable-isotope probing. After incubation with (13) C-acetate and (13) C-lactate, active bacterial community in the Holocene sediment was dominated by different Geobacter spp.-related 16S rRNA sequences. Substrate addition also resulted in the enrichment of sequences related to the arsenate-respiring Sulfurospirillum spp. (13) C-acetate selected for ArrA related to Geobacter spp. whereas (13) C-lactate selected for ArrA which were not closely related to any cultivated organism. Incubation of the Pleistocene sediment with lactate favoured a 16S rRNA-phylotype related to the sulphate-reducing Desulfovibrio oxamicus DSM1925, whereas the ArrA sequences clustered with environmental sequences distinct from those identified in the Holocene sediment. Whereas limited As(III) release was observed in Pleistocene sediment after lactate addition, no arsenic mobilization occurred from Holocene sediments, probably because of the initial reduced state of As, as determined by X-ray Absorption Near Edge Structure. Our findings demonstrate that in the presence of reactive organic carbon, As(III) mobilization can occur in Pleistocene sediments, having implications for future strategies that aim to reduce arsenic contamination in drinking waters by using aquifers containing Pleistocene sediments.
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http://dx.doi.org/10.1111/1462-2920.12412DOI Listing
June 2015

High-resolution alkenone palaeobarometry indicates relatively stable pCO(2) during the Pliocene (3.3-2.8 Ma).

Philos Trans A Math Phys Eng Sci 2013 Oct 16;371(2001):20130094. Epub 2013 Sep 16.

Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, and The Cabot Institute, School of Chemistry, University of Bristol, , Cantock's Close, Bristol BS8 1TS, UK.

Temperature reconstructions indicate that the Pliocene was approximately 3(°)C warmer globally than today, and several recent reconstructions of Pliocene atmospheric CO2 indicate that it was above pre-industrial levels and similar to those likely to be seen this century. However, many of these reconstructions have been of relatively low temporal resolution, meaning that these records may have failed to capture variations associated with the 41 kyr glacial-interglacial cycles thought to have operated in the Pliocene. Here we present a new, high temporal resolution alkenone carbon isotope-based record of pCO2 spanning 3.3-2.8 Ma from Ocean Drilling Program Site 999. Our record is of high enough resolution (approx. 19 kyr) to resolve glacial-interglacial changes beyond the intrinsic uncertainty of the proxy method. The record suggests that Pliocene CO2 levels were relatively stable, exhibiting variation less than 55 ppm. We perform sensitivity studies to investigate the possible effect of changing sea surface temperature (SST), which highlights the importance of accurate and precise SST reconstructions for alkenone palaeobarometry, but demonstrate that these uncertainties do not affect our conclusions of relatively stable pCO2 levels during this interval.
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http://dx.doi.org/10.1098/rsta.2013.0094DOI Listing
October 2013

Carbon and chlorine isotope fractionation during microbial degradation of tetra- and trichloroethene.

Environ Sci Technol 2013 Jun 4;47(12):6449-56. Epub 2013 Jun 4.

Department of Applied Environmental Science, ITM, Stockholm University, 106 91 Stockholm, Sweden.

Two-dimensional compound-specific isotope analysis (2D-CSIA), combining stable carbon and chlorine isotopes, holds potential for monitoring of natural attenuation of chlorinated ethenes (CEs) in contaminated soil and groundwater. However, interpretation of 2D-CSIA data sets is challenged by a shortage of experimental Cl isotope enrichment factors. Here, isotope enrichments factors for C and Cl (i.e., εC and εCl) were determined for biodegradation of tetrachloroethene (PCE) and trichloroethene (TCE) using microbial enrichment cultures from a heavily CE-contaminated aquifer. The obtained values were εC = -5.6 ± 0.7‰ (95% CI) and εCl = -2.0 ± 0.5‰ for PCE degradation and εC = -8.8 ± 0.2‰ and εCl = -3.5 ± 0.5‰ for TCE degradation. Combining the values for both εC and εCl yielded mechanism-diagnostic εCl/εC ratios of 0.35 ± 0.11 and 0.37 ± 0.11 for the degradation of PCE and TCE, respectively. Application of the obtained εC and εCl values to a previously investigated field site gave similar estimates for the fraction of degraded contaminant as in the previous study, but with a reduced uncertainty in assessment of the natural attenuation. Furthermore, 16S rRNA gene clone library analyses were performed on three samples from the PCE degradation experiments. A species closely related to Desulfitobacterium aromaticivorans UKTL dominated the reductive dechlorination process. This study contributes to the development of 2D-CSIA as a tool for evaluating remediation strategies of CEs at contaminated sites.
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http://dx.doi.org/10.1021/es305236yDOI Listing
June 2013

Archaeol: an indicator of methanogenesis in water-saturated soils.

Archaea 2012 22;2012:896727. Epub 2012 Nov 22.

Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre and The Cabot Institute, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.

Oxic soils typically are a sink for methane due to the presence of high-affinity methanotrophic Bacteria capable of oxidising methane. However, soils experiencing water saturation are able to host significant methanogenic archaeal communities, potentially affecting the capacity of the soil to act as a methane sink. In order to provide insight into methanogenic populations in such soils, the distribution of archaeol in free and conjugated forms was investigated as an indicator of fossilised and living methanogenic biomass using gas chromatography-mass spectrometry with selected ion monitoring. Of three soils studied, only one organic matter-rich site contained archaeol in quantifiable amounts. Assessment of the subsurface profile revealed a dominance of archaeol bound by glycosidic headgroups over phospholipids implying derivation from fossilised biomass. Moisture content, through control of organic carbon and anoxia, seemed to govern trends in methanogen biomass. Archaeol and crenarchaeol profiles differed, implying the former was not of thaumarcheotal origin. Based on these results, we propose the use of intact archaeol as a useful biomarker for methanogen biomass in soil and to track changes in moisture status and aeration related to climate change.
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http://dx.doi.org/10.1155/2012/896727DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3512251PMC
February 2013

Dual carbon-chlorine stable isotope investigation of sources and fate of chlorinated ethenes in contaminated groundwater.

Environ Sci Technol 2012 Oct 3;46(20):10918-25. Epub 2012 Oct 3.

Department of Applied Environmental Science (ITM), Stockholm University, Svante Arrhenius väg 8c, SE-106 91 Stockholm, Sweden.

Chlorinated ethenes (CEs) are ubiquitous groundwater contaminants, yet there remains a need for a method to efficiently monitor their in situ degradation. We report here the first field application of combined stable carbon and chlorine isotope analysis of tetrachloroethene (PCE) and trichloroethene (TCE) to investigate their biodegradation in a heavily contaminated aquifer. The two-dimensional Compound Specific Isotope Analysis (2D-CSIA) approach was facilitated by a recently developed gas chromatography-quadrupole mass spectrometry (GCqMS) method for δ(37)Cl determination. Both C and Cl isotopes showed evidence of ongoing PCE transformation. Applying published C isotope enrichment factors (ε(C)) enabled evaluation of the extent of in situ PCE degradation (11-78%). We interpreted C and Cl isotopes using a numerical reactive transport model along a 60-m flow path. It revealed that combined PCE and TCE mass load was dechlorinated by less than 10%, and that cis-dichloroethene was not further dechlorinated. Furthermore, the 2D-CSIA approach allowed estimation of Cl isotope enrichment factors ε(Cl) (-7.8 to -0.8‰) and characteristic ε(Cl)/ε(C) values (0.42-1.12) for reductive PCE dechlorination at this field site. This investigation demonstrates the benefit of 2D-CSIA to assess in situ degradation of CEs and the applicability of Cl isotope fractionation to evaluate PCE and TCE dechlorination.
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http://dx.doi.org/10.1021/es3016843DOI Listing
October 2012

The discovery of new deep-sea hydrothermal vent communities in the southern ocean and implications for biogeography.

PLoS Biol 2012 Jan 3;10(1):e1001234. Epub 2012 Jan 3.

Department of Zoology, University of Oxford, Oxford, United Kingdom.

Since the first discovery of deep-sea hydrothermal vents along the Galápagos Rift in 1977, numerous vent sites and endemic faunal assemblages have been found along mid-ocean ridges and back-arc basins at low to mid latitudes. These discoveries have suggested the existence of separate biogeographic provinces in the Atlantic and the North West Pacific, the existence of a province including the South West Pacific and Indian Ocean, and a separation of the North East Pacific, North East Pacific Rise, and South East Pacific Rise. The Southern Ocean is known to be a region of high deep-sea species diversity and centre of origin for the global deep-sea fauna. It has also been proposed as a gateway connecting hydrothermal vents in different oceans but is little explored because of extreme conditions. Since 2009 we have explored two segments of the East Scotia Ridge (ESR) in the Southern Ocean using a remotely operated vehicle. In each segment we located deep-sea hydrothermal vents hosting high-temperature black smokers up to 382.8°C and diffuse venting. The chemosynthetic ecosystems hosted by these vents are dominated by a new yeti crab (Kiwa n. sp.), stalked barnacles, limpets, peltospiroid gastropods, anemones, and a predatory sea star. Taxa abundant in vent ecosystems in other oceans, including polychaete worms (Siboglinidae), bathymodiolid mussels, and alvinocaridid shrimps, are absent from the ESR vents. These groups, except the Siboglinidae, possess planktotrophic larvae, rare in Antarctic marine invertebrates, suggesting that the environmental conditions of the Southern Ocean may act as a dispersal filter for vent taxa. Evidence from the distinctive fauna, the unique community structure, and multivariate analyses suggest that the Antarctic vent ecosystems represent a new vent biogeographic province. However, multivariate analyses of species present at the ESR and at other deep-sea hydrothermal vents globally indicate that vent biogeography is more complex than previously recognised.
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http://dx.doi.org/10.1371/journal.pbio.1001234DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3250512PMC
January 2012

Preservation of microbial lipids in geothermal sinters.

Astrobiology 2011 Apr 10;11(3):259-74. Epub 2011 Apr 10.

Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, UK.

Lipid biomarkers are widely used to study the earliest life on Earth and have been invoked as potential astrobiological markers, but few studies have assessed their survival and persistence in geothermal settings. Here, we investigate lipid preservation in active and inactive geothermal silica sinters, with ages of up to 900 years, from Champagne Pool, Waiotapu, New Zealand. Analyses revealed a wide range of bacterial biomarkers, including free and bound fatty acids, 1,2-di-O-alkylglycerols (diethers), and various hopanoids. Dominant archaeal lipids include archaeol and glycerol dialkyl glycerol tetraethers (GDGTs). The predominance of generally similar biomarker groups in all sinters suggests a stable microbial community throughout Champagne Pool's history and indicates that incorporated lipids can be well preserved. Moreover, subtle differences in lipid distributions suggest that past changes in environmental conditions can be elucidated. In this case, higher archaeol abundances relative to the bacterial diethers, a greater proportion of cyclic GDGTs, the high average chain length of the bacterial diethers, and greater concentrations of hopanoic acids in the older sinters all suggest hotter conditions at Champagne Pool in the past.
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http://dx.doi.org/10.1089/ast.2010.0540DOI Listing
April 2011

Functional diversity of bacteria in a ferruginous hydrothermal sediment.

ISME J 2010 Sep 22;4(9):1193-205. Epub 2010 Apr 22.

School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, UK.

A microbial community showing diverse respiratory processes was identified within an arsenic-rich, ferruginous shallow marine hydrothermal sediment (20-40 degrees C, pH 6.0-6.3) in Santorini, Greece. Analyses showed that ferric iron reduction with depth was broadly accompanied by manganese and arsenic reduction and FeS accumulation. Clone library analyses indicated the suboxic-anoxic transition zone sediment contained abundant Fe(III)- and sulfate-reducing Deltaproteobacteria, whereas the overlying surface sediment was dominated by clones related to the Fe(II)-oxidizing zetaproteobacterium, Mariprofundus ferroxydans. Cultures obtained from the transition zone were enriched in bacteria that reduced Fe(III), nitrate, sulfate and As(V) using acetate or lactate as electron donors. In the absence of added organic carbon, bacteria were enriched that oxidized Fe(II) anaerobically or microaerobically, sulfide microaerobically and aerobically and As(III) aerobically. According to 16S rRNA gene analyses, enriched bacteria represented a phylogenetically wide distribution. Most probable number counts indicated an abundance of nitrate-, As(V)- and Fe(III)((s,aq))-reducers, and dissolved sulfide-oxidizers over sulfate-reducers, and FeS-, As(III)- and nitrate-dependent Fe(II)-oxidisers in the transition zone. It is noteworthy that the combined community and geochemical data imply near-surface microbial iron and arsenic redox cycling were dominant biogeochemical processes.
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http://dx.doi.org/10.1038/ismej.2010.38DOI Listing
September 2010

Increased terrestrial methane cycling at the Palaeocene-Eocene thermal maximum.

Nature 2007 Sep;449(7160):332-5

Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.

The Palaeocene-Eocene thermal maximum (PETM), a period of intense, global warming about 55 million years ago, has been attributed to a rapid rise in greenhouse gas levels, with dissociation of methane hydrates being the most commonly invoked explanation. It has been suggested previously that high-latitude methane emissions from terrestrial environments could have enhanced the warming effect, but direct evidence for an increased methane flux from wetlands is lacking. The Cobham Lignite, a recently characterized expanded lacustrine/mire deposit in England, spans the onset of the PETM and therefore provides an opportunity to examine the biogeochemical response of wetland-type ecosystems at that time. Here we report the occurrence of hopanoids, biomarkers derived from bacteria, in the mire sediments from Cobham. We measure a decrease in the carbon isotope values of the hopanoids at the onset of the PETM interval, which suggests an increase in the methanotroph population. We propose that this reflects an increase in methane production potentially driven by changes to a warmer and wetter climate. Our data suggest that the release of methane from the terrestrial biosphere increased and possibly acted as a positive feedback mechanism to global warming.
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http://dx.doi.org/10.1038/nature06012DOI Listing
September 2007

Biogeochemistry and biodiversity of methane cycling in subsurface marine sediments (Skagerrak, Denmark).

Environ Microbiol 2007 May;9(5):1146-61

School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff CF103YE, UK.

This biogeochemical, molecular genetic and lipid biomarker study of sediments ( approximately 4 m cores) from the Skagerrak (Denmark) investigated methane cycling in a sediment with a clear sulfate-methane-transition zone (SMTZ) and where CH(4) supply was by diffusion, rather than by advection, as in more commonly studied seep sites. Sulfate reduction removed sulfate by 0.7 m and CH(4) accumulated below. (14)C-radiotracer measurements demonstrated active H(2)/CO(2) and acetate methanogenesis and anaerobic oxidation of CH(4) (AOM). Maximum AOM rates occurred near the SMTZ ( approximately 3 nmol cm(-3) day(-1) at 0.75 m) but also continued deeper, overall, at much lower rates. Maximum rates of H(2)/CO(2) and acetate methanogenesis occurred below the SMTZ but H(2)/CO(2) methanogenesis rates were x 10 those of acetate methanogenesis, and this was consistent with initial values of (13)C-depleted CH(4) (delta(13)C c.-80 per thousand). Areal AOM and methanogenic rates were similar ( approximately 1.7 mmol m(-2) day(-1)), hence, CH(4) flux is finely balanced. A 16S rRNA gene library from 1.39 m combined with methanogen (T-RFLP), bacterial (16S rRNA DGGE) and lipid biomarker depth profiles showed the presence of populations similar to some seep sites: ANME-2a (dominant), ANME-3, Methanomicrobiales, Methanosaeta Archaea, with abundance changes with depth corresponding to changes in activities and sulfate-reducing bacteria (SRB). Below the SMTZ to approximately 1.7 m CH(4) became progressively more (13)C depleted (delta(13)C -82 per thousand) indicating a zone of CH(4) recycling which was consistent with the presence of (13)C-depleted archaeol (delta(13)C -55 per thousand). Pore water acetate concentrations decreased in this zone (to approximately 5 microM), suggesting that H(2), not acetate, was an important CH(4) cycling intermediate. The potential biomarkers for AOM-associated SRB, non-isoprenoidal ether lipids, increased below the SMTZ but this distribution reflected 16S rRNA gene sequences for JS1 and OP8 bacteria rather than those of SRB. At this site peak rates of methane production and consumption are spatially separated and seem to be conducted by different archaeal groups. Also AOM is predominantly coupled to sulfate reduction, unlike recent reports from some seep and gassy sediment sites.
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http://dx.doi.org/10.1111/j.1462-2920.2006.01237.xDOI Listing
May 2007

The organic preservation of fossil arthropods: an experimental study.

Proc Biol Sci 2006 Nov;273(1602):2777-83

Organic Geochemistry Unit, Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, UK.

Modern arthropod cuticles consist of chitin fibres in a protein matrix, but those of fossil arthropods with an organic exoskeleton, particularly older than Tertiary, contain a dominant aliphatic component. This apparent contradiction was examined by subjecting modern cockroach, scorpion and shrimp cuticle to artificial maturation (350 degrees C/700 bars/24 h) following various chemical treatments, and analysing the products with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Analysis of artificially matured untreated cuticle yielded moieties related to phenols and alkylated substituents, pyridines, pyrroles and possibly indenes (derived from chitin). n-Alkyl amides, C16 and C18 fatty acids and alkane/alk-1-ene homologues ranging from C9 to C19 were also generated, the last indicating the presence of an n-alkyl component, similar in composition to that encountered in fossil arthropods. Similar pyrolysates were obtained from matured pure C16 and C18 fatty acids. Py-GC/MS of cuticles matured after lipid extraction and hydrolysis did not yield any aliphatic polymer. This provides direct experimental evidence that lipids incorporated from the cuticle were the source of aliphatic polymer. This process of in situ polymerization appears to account for most of the fossil record of terrestrial arthropods as well as marine arthropods that lacked a biomineralized exoskeleton.
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http://dx.doi.org/10.1098/rspb.2006.3646DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1635499PMC
November 2006

Lipid biomarkers and carbon isotope signatures of a microbial (Beggiatoa) mat associated with gas hydrates in the gulf of Mexico.

Appl Environ Microbiol 2005 Apr;71(4):2106-12

Department of Marine Sciences and Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, SC 29802, USA.

White and orange mats are ubiquitous on surface sediments associated with gas hydrates and cold seeps in the Gulf of Mexico. The goal of this study was to determine the predominant pathways for carbon cycling within an orange mat in Green Canyon (GC) block GC 234 in the Gulf of Mexico. Our approach incorporated laser-scanning confocal microscopy, lipid biomarkers, stable carbon isotopes, and 16S rRNA gene sequencing. Confocal microscopy showed the predominance of filamentous microorganisms (4 to 5 mum in diameter) in the mat sample, which are characteristic of Beggiatoa. The phospholipid fatty acids extracted from the mat sample were dominated by 16:1omega7c/t (67%), 18:1omega7c (17%), and 16:0 (8%), which are consistent with lipid profiles of known sulfur-oxidizing bacteria, including Beggiatoa. These results are supported by the 16S rRNA gene analysis of the mat material, which yielded sequences that are all related to the vacuolated sulfur-oxidizing bacteria, including Beggiatoa, Thioploca, and Thiomargarita. The delta13C value of total biomass was -28.6 per thousand; those of individual fatty acids were -29.4 to -33.7 per thousand. These values suggested heterotrophic growth of Beggiatoa on organic substrates that may have delta13C values characteristic of crude oil or on their by-products from microbial degradation. This study demonstrated that integrating lipid biomarkers, stable isotopes, and molecular DNA could enhance our understanding of the metabolic functions of Beggiatoa mats in sulfide-rich marine sediments associated with gas hydrates in the Gulf of Mexico and other locations.
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http://dx.doi.org/10.1128/AEM.71.4.2106-2112.2005DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1082508PMC
April 2005

Two episodes of microbial change coupled with Permo/Triassic faunal mass extinction.

Nature 2005 Mar;434(7032):494-7

Organic Geochemistry Unit, Bristol Biogeochemistry Center, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.

Microbial expansion following faunal mass extinctions in Earth history can be studied by petrographic examination of microbialites (microbial crusts) or well-preserved organic-walled microbes. However, where preservation is poor, quantification of microbial communities can be problematic. We have circumvented this problem by adopting a lipid biomarker-based approach to evaluate microbial community changes across the Permo/Triassic (P/Tr) boundary at Meishan in South China. We present here a biomarker stratigraphic record showing episodic microbial changes coupled with a high-resolution record of invertebrate mass extinction. Variation in the microbial community structure is characterized by the 2-methylhopane (2-MHP) index (a ratio of the abundance of cyanobacterial biomarkers to more general bacterial biomarkers). Two episodes of faunal mass extinction were each preceded by minima in the 2-MHP index, followed by strong maxima, likely reflecting microbial responses to the catastrophic events that caused the extinction and initiated ecosystem changes. Hence, both cyanobacterial biomarker and invertebrate fossil records provide evidence for two episodes of biotic crisis across the P/Tr boundary.
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http://dx.doi.org/10.1038/nature03396DOI Listing
March 2005

Lipid biomolecules in silica sinters: indicators of microbial biodiversity.

Environ Microbiol 2005 Jan;7(1):66-77

Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.

To explore further the diversity of the microorganisms and their relationship with geothermal sinters, we examined the lipids preserved in six sinters associated with four different hot spring (58-82 degrees C) areas of the Taupo Volcanic Zone (TVZ), New Zealand. These sinters contain microbial remains, but the process of mineralization has rendered them largely unidentifiable. Dominant lipids include free fatty acids, 1,2-diacylglycerophospholipids, 1,2-di-O-alkylglycerols, glycerol dialkylglycerol tetraethers and 1-O-alkylglycerols. These confirm the presence and, in some cases, high abundances of bacteria in all six sinters and archaea in four of the six sinter samples; in addition, the presence of novel macrocyclic diethers and unusual distributions of monoethers and diethers suggest the presence of previously uncharacterized bacteria. The lipid distributions are also markedly dissimilar among the four sites; for example, novel macrocyclic diethers are restricted to the Rotokawa samples while particularly abundant monoethers occur only in the Orakei Korako sample. Thus, biomarkers can provide crucial insight into the complex community structure of thermophilic microorganisms, including both archaea and bacteria, involved with biogenic silica sinter formation.
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http://dx.doi.org/10.1111/j.1462-2920.2004.00686.xDOI Listing
January 2005