Cenozoic evolution of deep ocean temperature from clumped isotope thermometry

Characterizing past climate states is crucial for understanding the future consequences of ongoing greenhouse gas emissions. Here, we revisit the benchmark time series for deep ocean temperature across the past 65 million years using clumped isotope thermometry. Our temperature estimates from the deep Atlantic Ocean are overall much warmer compared with oxygen isotope-based reconstructions, highlighting the likely influence of changes in deep ocean pH and/or seawater oxygen isotope composition on classical oxygen isotope records of the Cenozoic. In addition, our data reveal previously unrecognized large swings in deep ocean temperature during early Eocene acute greenhouse warmth. Our results call for a reassessment of the Cenozoic history of ocean temperatures to achieve a more accurate understanding of the nature of climatic responses to tectonic events and variable greenhouse forcing.

InterCarb: A Community Effort to Improve Interlaboratory Standardization of the Carbonate Clumped Isotope Thermometer Using Carbonate Standards

Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth-system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Δ47) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ47 and Δ47 values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ47 (I-CDES) values for Intercarb-Carbon Dioxide Equilibrium Scale.

Geochemistry and Microtextures of Vein Calcites Pervading the Izu-Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351

International Ocean Discovery Program Expeditions 352 and 351 drilled into the Western Pacific Izu-Bonin forearc and rear arc. The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites and the rear arc consists of FAB-like rocks. These rocks are pervaded by calcite veins. Blocky vein microtextures enclosing host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Significant diffusion-fed and crystallization pressure-driven antitaxial veining is restricted to the rear arc. The lack of faults and presence of an Eocene sedimentary cover in the rear arc facilitated antitaxial veining. Rare earth element and isotopic (δ18O, δ13C, 87Sr/86Sr, and Δ47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB-hosted low-T (<30 °C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35-33 and ~22 Ma. These intersections are interpreted as precipitation ages, which concur with Pacific slab rollback. Boninite-hosted low-T (<30 °C) vein calcites precipitated from seawater that was modified by fluid-rock interactions. Mixing calculations yield a mixture of >95% seawater and <5% basaltic 87Sr/86Sr. In the rear arc, low-T rock alteration lowered the circulating seawater in δ18O and 87Sr/86Sr. Thus, vein calcites precipitated from modified seawater with up to 20-30% basaltic 87Sr/86Sr at temperatures up to 74 ± 12 °C. These results show how the local geology and vein growth dynamics affect microtextures and geochemical compositions of vein precipitates.

Penultimate deglacial warming across the Mediterranean Sea revealed by clumped isotopes in foraminifera

The variability of seawater temperature through time is a critical measure of climate change, yet its reconstruction remains problematic in many regions. Mg/Ca and oxygen isotope (δ¹⁸O_C) measurements in foraminiferal carbonate shells can be combined to reconstruct seawater temperature and δ¹⁸O (δ¹⁸O_SW). The latter is a measure of changes in local hydrology (e.g., precipitation/evaporation, freshwater inputs) and global ice volume. But diagenetic processes may affect foraminiferal Mg/Ca. This restricts its potential in many places, including the Mediterranean Sea, a strategic region for deciphering global climate and sea-level changes. High alkalinity/salinity conditions especially bias Mg/Ca temperatures in the eastern Mediterranean (eMed). Here we advance the understanding of both western Mediterranean (wMed) and eMed hydrographic variability through the penultimate glacial termination (TII) and last interglacial, by applying the clumped isotope (Δ₄₇) paleothermometer to planktic foraminifera with a novel data-processing approach. Results suggest that North Atlantic cooling during Heinrich stadial 11 (HS11) affected surface-water temperatures much more in the wMed (during winter/spring) than in the eMed (during summer). The method’s paired Δ₄₇ and δ¹⁸O_C data also portray δ¹⁸O_SW. These records reveal a clear HS11 freshwater signal, which attenuated toward the eMed, and also that last interglacial surface warming in the eMed was strongly amplified by water-column stratification during the deposition of the organic-rich (sapropel) interval known as S5.

PALEOCEANOGRAPHY. A warm and poorly ventilated deep Arctic Mediterranean during the last glacial period

Changes in the formation of dense water in the Arctic Ocean and Nordic Seas [the "Arctic Mediterranean" (AM)] probably contributed to the altered climate of the last glacial period. We examined past changes in AM circulation by reconstructing radiocarbon ventilation ages of the deep Nordic Seas over the past 30,000 years. Our results show that the glacial deep AM was extremely poorly ventilated (ventilation ages of up to 10,000 years). Subsequent episodic overflow of aged water into the mid-depth North Atlantic occurred during deglaciation. Proxy data also suggest that the deep glacial AM was ~2° to 3°C warmer than modern temperatures; deglacial mixing of the deep AM with the upper ocean thus potentially contributed to the melting of sea ice, icebergs, and terminal ice-sheet margins.

Record of archaeal activity at the serpentinite-hosted Lost City Hydrothermal Field

Samples of young, outer surfaces of brucite-carbonate deposits from the ultramafic-hosted Lost City hydrothermal field were analyzed for DNA and lipid biomarker distributions and for carbon and hydrogen stable isotope compositions of the lipids. Methane-cycling archaeal communities, notably the Lost City Methanosarcinales (LCMS) phylotype, are specifically addressed. Lost City is unlike all other hydrothermal systems known to date and is characterized by metal- and CO2 -poor, high pH fluids with high H2 and CH4 contents resulting from serpentinization processes at depth. The archaeal fraction of the microbial community varies widely within the Lost City chimneys, from 1-81% and covaries with concentrations of hydrogen within the fluids. Archaeal lipids include isoprenoid glycerol di- and tetraethers and C25 and C30 isoprenoid hydrocarbons (pentamethylicosane derivatives - PMIs - and squalenoids). In particular, unsaturated PMIs and squalenoids, attributed to the LCMS archaea, were identified for the first time in the carbonate deposits at Lost City and probably record processes exclusively occurring at the surface of the chimneys. The carbon isotope compositions of PMIs and squalenoids are remarkably heterogeneous across samples and show highly (13) C-enriched signatures reaching δ(13) C values of up to +24.6‰. Unlike other environments in which similar structural and isotopic lipid heterogeneity has been observed and attributed to diversity in the archaeal assemblage, the lipids here appear to be synthesized solely by the LCMS. Some of the variations in lipid isotope signatures may, in part, be due to unusual isotopic fractionation during biosynthesis under extreme conditions. However, we argue that the diversity in archaeal abundances, lipid structure and carbon isotope composition rather reflects the ability of the LCMS archaeal biofilms to adapt to chemical gradients in the hydrothermal chimneys and possibly to perform either methanotrophy or methanogenesis using dissolved inorganic carbon, methane or formate as a function of the prevailing environmental conditions.

Sources of organic nitrogen at the serpentinite-hosted Lost City hydrothermal field

The reaction of ultramafic rocks with water during serpentinization at moderate temperatures results in alkaline fluids with high concentrations of reduced chemical compounds such as hydrogen and methane. Such environments provide unique habitats for microbial communities capable of utilizing these reduced compounds in present-day and, possibly, early Earth environments. However, these systems present challenges to microbial communities as well, particularly due to high fluid pH and possibly the availability of essential nutrients such as nitrogen. Here we investigate the source and cycling of organic nitrogen at an oceanic serpentinizing environment, the Lost City hydrothermal field (30°N, Mid-Atlantic Ridge). Total hydrolizable amino acid (THAA) concentrations in the fluids range from 736 to 2300 nm and constitute a large fraction of the dissolved organic carbon (2.5-15.1%). The amino acid distributions, and the relative concentrations of these compounds across the hydrothermal field, indicate they most likely derived from chemolithoautotrophic production. Previous studies have identified the presence of numerous nitrogen fixation genes in the fluids and the chimneys. Organic nitrogen in actively venting chimneys has δ(15) N values as low as 0.1‰ which is compatible with biological nitrogen fixation. Total hydrolizable amino acids in the chimneys are enriched in (13) C by 2-7‰ compared to bulk organic matter. The distribution and absolute δ(13) C(THAA) values are compatible with a chemolithoautotrophic source, an attribution also supported by molar organic C/N ratios in most active chimneys (4.1-5.5) which are similar to those expected for microbial communities. In total, these data indicate nitrogen is readily available to microbial communities at Lost City.

Microbial diversity of Loki's Castle black smokers at the Arctic Mid-Ocean Ridge

Hydrothermal vent systems harbor rich microbial communities ranging from aerobic mesophiles to anaerobic hyperthermophiles. Among these, members of the archaeal domain are prevalent in microbial communities in the most extreme environments, partly because of their temperature-resistant and robust membrane lipids. In this study, we use geochemical and molecular microbiological methods to investigate the microbial diversity in black smoker chimneys from the newly discovered Loki's Castle hydrothermal vent field on the Arctic Mid-Ocean Ridge (AMOR) with vent fluid temperatures of 310-320 °C and pH of 5.5. Archaeal glycerol dialkyl glycerol tetraether lipids (GDGTs) and H-shaped GDGTs with 0-4 cyclopentane moieties were dominant in all sulfide samples and are most likely derived from both (hyper)thermophilic Euryarchaeota and Crenarchaeota. Crenarchaeol has been detected in low abundances in samples derived from the chimney exterior indicating the presence of Thaumarchaeota at lower ambient temperatures. Aquificales and members of the Epsilonproteobacteria were the dominant bacterial groups detected. Our observations based on the analysis of 16S rRNA genes and biomarker lipid analysis provide insight into microbial communities thriving within the porous sulfide structures of active and inactive deep-sea hydrothermal vents. Microbial cycling of sulfur, hydrogen, and methane by archaea in the chimney interior and bacteria in the chimney exterior may be the prevailing biogeochemical processes in this system.

Corrigendum to: Experimental validation of the Intergovernmental Panel on Climate Change default values for ruminant-derived methane and its carbon-isotope signature

Two aspects regarding the ruminant's contribution to global methane (CH4) emissions were investigated: (i) testing the accuracy of the Intergovernmental Panel on Climate Change default values for dairy cows fed different diet types and differing slurry storage temperatures; and (ii) providing carbon-isotope (C-isotope) signature data to contribute information on the characteristics of ruminant-derived CH4 as global source. The experimental diets, fed to 18 dairy cows, were separated into forage-only (hay, C3 plant) and forage-concentrate diets (barley, C3 plant; maize, C4 plant). Accumulated slurry was stored at either 14 or 27�C. The hay diet had the highest CH4 conversion rate (Ym 7.9%). Negligible amounts of CH4 were emitted from slurries stored at low temperature. No diet effect was found at 27�C (~33 L/kg volatile solids). The isotope ratios of enteric CH4 averaged -67.7‰ (C3 plants) and -57.4‰ (C4; maize). High temperature slurry storage resulted in different enrichment factors εCO2-CH4 for maize (33.2‰) and hay (35.9‰). Compared with the Intergovernmental Panel on Climate Change default values for Ym and slurry CH4 emission the results gained in the present experiment were higher and lower, respectively. Slurry-derived CH4 was less depleted in 13C than enteric CH4, which decreases the usefulness of this signature for global ruminant-derived CH4.

Experimental validation of the Intergovernmental Panel on Climate Change default values for ruminant-derived methane and its carbon-isotope signature

Two aspects regarding the ruminant’s contribution to global methane (CH4) emissions were investigated: (i) testing the accuracy of the Intergovernmental Panel on Climate Change default values for dairy cows fed different diet types and differing slurry storage temperatures; and (ii) providing carbon-isotope (C-isotope) signature data to contribute information on the characteristics of ruminant-derived CH4 as global source. The experimental diets, fed to 18 dairy cows, were separated into forage-only (hay, C3 plant) and forage-concentrate diets (barley, C3 plant; maize, C4 plant). Accumulated slurry was stored at either 14 or 27°C. The hay diet had the highest CH4 conversion rate (Ym 7.9%). Negligible amounts of CH4 were emitted from slurries stored at low temperature. No diet effect was found at 27°C (~33 L/kg volatile solids). The isotope ratios of enteric CH4 averaged −67.7‰ (C3 plants) and −57.4‰ (C4; maize). High temperature slurry storage resulted in different enrichment factors ϵCO2-CH4 for maize (33.2‰) and hay (35.9‰). Compared with the Intergovernmental Panel on Climate Change default values for Ym and slurry CH4 emission the results gained in the present experiment were higher and lower, respectively. Slurry-derived CH4 was less depleted in 13C than enteric CH4, which decreases the usefulness of this signature for global ruminant-derived CH4.

The methanogenic potential and C-isotope fractionation of different diet types represented by either C3 or C4 plants as evaluated in vitro and in dairy cows

The methanogenic potential of two major feed production systems, grassland and arable land, was tested in vitro and in vivo. Diets were differentiated into C3 (grass, cereals) and C4 plants (maize) to determine whether C-isotope fractionation during methane formation differed between the C3 and C4 plant based diets. In the in vitro experiment, four diets consisting of hay, maize, wheat (the latter two straw and grain) and a 1 : 1 mixture of the maize and wheat diet were investigated using the Rusitec system (n = 4 per diet). In the in vivo experiment with 12 lactating cows, diets were basically similar, but barley was used instead of wheat and no maize–cereal mixture was tested (n = 4 per diet). Diets were always iso-energetic and iso-nitrogenous. Methane was either measured by gas chromatography from complete daily collections (in vitro) or by putting cows into respiratory chambers. In vitro, the methanogenic potential of the diets was similar. Methane related to apparently degraded neutral detergent fibre (NDF) was 50% lower in the hay than in the wheat diet, while the other diets ranged in between. The largest C-isotope fractionation between fermentation gases (αCO2 – CH4) was found in the wheat diet. In vivo, methane formation was highest with the barley diet followed by the hay and the maize diet (580, 461 and 453 L/day). Relating methane emission to dry matter intake reduced differences among diets (average 34 L/kg). On average, 118 L of methane were produced per kg digested NDF. Methane conversion rate was higher than the IPCC default value of 6.5% given for diets with less than 90% concentrate. No differences in the C-isotope fractionation were found in vivo. It was shown that forage-only diets, containing higher levels of fermentable fibre, do not necessarily have a higher methanogenic potential than mixed forage-concentrate diets.

Changes in oxygen uptake, shoulder muscles activity, and propulsion cycle timing during strenuous wheelchair exercise

STUDY DESIGN

Cross-over study.

OBJECTIVE

To determine the effect of strenuous wheelchair exercise on oxygen uptake (VO2 ), muscle activity and propulsion cycle timing (including the push time and recovery time during one full arm cycle).

SETTING

Laboratory of Sport Sciences at the University of France-Comte in France.

METHODS

Two exercise bouts of 6-min duration were performed at a constant workload: (1) non-fatigable exercise (moderate workload) and (2) fatigable exercise (heavy workload). Measurement of VO2, surface electromyographic activity (EMG) from shoulder muscles, and temporal parameters of wheelchair ergometer propulsion were collected from eight able-bodied men (26+/-4 years).

RESULTS

A progressive increase in VO2 associated with EMG alterations (P<0.05), and a decrease of the cycle and recovery time (P<0.05) during the heavy exercise. Whereas the push time remained constant, an increased muscle activation time (P<0.05) was found during heavy exercise.

CONCLUSION

Observations during wheelchair ergometry indicate the development of fatigue and inefficient muscle coordination, which may contribute to deleterious stress distributions at the shoulder joint, increasing susceptibility to injury.

Biomarker Evidence for a Major Preservation Pathway of Sedimentary Organic Carbon

Hydrogenation processes leading from biomolecules to fossil biomarkers in anoxic sediments are crucial for the preservation of organic matter. However, these processes are still poorly understood. The present identification of several reduced carotenoids in recent sediments attests that these processes operate at the earliest stages of diagenesis without structural or stereochemical specificity, implying a nonbiological reduction pathway. Sulfur species (e.g., H2S) are the hydrogen donors involved in such reduction, as demonstrated with laboratory experiments. These reactions allow the preservation of abundant organic carbon in the rock record.

Nitrate-consuming processes in a petroleum-contaminated aquifer quantified using push-pull tests combined with 15N isotope and acetylene-inhibition methods

Nitrate consumption in aquifers may result from several biogenic and abiotic processes such as denitrification, assimilatory NO3- reduction, dissimilatory NO3- reduction to ammonium (DNRA), or abiotic NO3- (or NO2-) reduction. The objectives of this study were to investigate the fate of NO3- in a petroleum-contaminated aquifer, and to assess the feasibility of using single-well push-pull tests (PPTs) in combination with 15N isotope and C2H2 inhibition methods for the quantification of processes contributing to NO3- consumption. Three consecutive PPTs were performed in a monitoring well of a heating oil-contaminated aquifer in Erlen, Switzerland. For each test, we injected 500 l of test solution containing 0.5 mM Br- as conservative tracer and either 0.5 mM unlabeled NO3- or approximately 0.3 mM 15N-labeled NO3- as reactant. Test solutions were sparged during preparation and injection with either N2, Ar or 10% C2H2 in Ar. After an initial incubation period of 1.5-3.2 h, we extracted the test solution/groundwater mixtures from the same location and measured concentrations of relevant species including Br-, NO3-, NO2-, N2O, N2, and NH4+. In addition, we determined the 15N contents of N2, N2O, NH4+, and suspended biomass from 15N/14N isotope-ratio measurements. Average total test duration was 50.5 h. First-order rate coefficients (k) were computed from measured NO3- consumption, N2-15N production and N2O-15N production. From measured NO3- consumption we obtained nearly identical estimates of k for all PPTs with small 95% confidence intervals, indicating good reproducibility and accuracy for the tests. Estimates of k from N2-15N production and N2O-15N production indicated that denitrification accounted for only 46-49% of observed NO3- consumption. Production of N2-15N in the presence of C2H2 was observed during one of the tests, which may be an indicator for abiotic NO3- reduction. Moreover, 15N isotope analyses confirmed occurrence of assimilatory NO3- reduction (0.58 at.% 15N in suspended biomass) and to a smaller extent DNRA (up to 4 at.% 15N in NH4+). Our results indicated that the combination of PPTs, 15N-isotope and C2H2 inhibition methods provided improved information on denitrification as well as alternative fates of NO3- in this aquifer.

In situ assessment of microbial sulfate reduction in a petroleum-contaminated aquifer using push-pull tests and stable sulfur isotope analyses

Anaerobic microbial activities such as sulfate reduction are important for the degradation of petroleum hydrocarbons (PHC) in contaminated aquifers. The objective of this study was to evaluate the feasibility of single-well push-pull tests in combination with stable sulfur isotope analyses for the in situ quantification of microbial sulfate reduction. A series of push-pull tests was performed in an existing monitoring well of a PHC-contaminated aquifer in Studen (Switzerland). Sulfate transport behavior was evaluated in a first test. In three subsequent tests, we injected anoxic test solutions (up to 1000 l), which contained 0.5 mM bromide (Br-) as conservative tracer and 1 mM sulfate (SO4(2-)) as reactant. After an initial incubation period of 42.5 to 67.9 h, up to 1100 l of test solution/groundwater mixture was extracted in each test from the same location. During the extraction phases, we measured concentrations of relevant species including Br-, SO4(2-) and sulfide (S(-II)), as well as stable sulfur isotope ratios (delta 34S) of extracted, unconsumed SO4(2-) and extracted S(-II). Results indicated sulfate reduction activity in the vicinity of the test well. Computed first-order rate coefficients for sulfate reduction ranged from 0.043 +/- 0.013 to 0.130 +/- 0.015 day-1. Isotope enrichment factors (epsilon) computed from sulfur isotope fractionation of extracted, unconsumed SO4(2-) ranged from 20.2 +/- 5.5@1000 to 22.8 +/- 3.4@1000. Together with observed fractionation in extracted S(-II), isotope enrichment factors provided strong evidence for microbially mediated sulfate reduction. Thus, push-pull tests combined with stable sulfur isotope analyses proved useful for the in situ quantification of microbial sulfate reduction in a PHC-contaminated aquifer.

A method for the extraction of ammonium from freshwaters for nitrogen isotope analysis

The measurement of delta15N values of inorganic nitrogen species is an important analytical tool to trace nitrogen species in order to understand nitrogen cycling in aquatic systems. Nitrogen isotope analysis of freshwater ammonium has, however, been hindered by the lack of a simple and reliable technique to measure delta15N values at natural abundance levels. We present a simple and rapid method to concentrate ammonium from freshwater samples for on-line N-isotope ratio determination. Ammonium is collected by adsorption on N-free cation exchange resins. The dried N-loaded exchange resin is then directly combusted to produce N2 gas for subsequent delta15N analysis. The method was evaluated with simulated freshwater solutions containing varying amounts of standard NH4+-N (delta15N = 2.1 per thousand) and potentially interfering inorganic and organic compounds. In general, the cation exchange resin method gives accurate and reproducible delta15N values (sigma1 < 0.3 per thousand; n = 10). Because of adsorption interference, high concentrations of cations in solution may cause ammonium loss but do not result in measurable isotope fractionation. Replicate extractions of the ammonium standard added to water collected from four Swiss lakes demonstrate the good performance of this method when applied to low ionic strength natural water samples with modest concentrations of dissolved organic nitrogen.

Hydrogen and carbon isotope fractionation during aerobic biodegradation of benzene

The main aim of the study was to evaluate hydrogen and carbon isotope fractionation during biodegradation of benzene as a possible tool to trace the process in contaminated environments. Aerobic biodegradation of benzene by two bacterial isolates, Acinetobacter sp. and Burkholderia sp., was accompanied by significant hydrogen and carbon isotope fractionation with hydrogen isotope enrichment factors of -12.8 +/- 0.7 per thousand and -11.2 +/- 1.8 per thousand, respectively, and average carbon isotope enrichment factors of -1.46 +/- 0.06 per thousand and -3.53 +/- 0.26 per thousand, respectively. Inorganic carbon produced by Acinetobacter sp. was depleted in 13C by 3.6-6.2 per thousand as compared to the initial delta13C of benzene, while the produced biomass was enriched in 13C by 3.8 per thousand. The secondary aim was to determine isotope ratios of benzenes from different manufacturers with regard to the use of isotopes for source differentiation. While two of the four analyzed benzenes had similar delta13C values, each of them had a distinct delta2H-delta13C pair and delta2H values spread over a range of 66.5 per thousand. Thus, combined analyses of hydrogen and carbon isotopes may be a more promising approach to trace sources and/or biodegradation of benzene than measuring carbon isotopes only.

Limitations of using delta 18O for the source identification of nitrate in agricultural soils

The stable isotopic composition (delta 15N and delta 18O) of nitrate was analyzed in two lysimeter field experiments in order to identify the conditions under which the dual isotope approach can be applied to identify the main source of nitrate in agricultural soils. The first field experiment involved six lysimeters beneath fields that had been fertilized for 10 yr with the same type of fertilizer (NH4NO3; delta 15N = +1.2@1000, delta 18O = +18.6@1000). The isotope ratios of NO3- in the leachate (delta 15N approximately 0@1000; delta 18O approximately +2@1000) could not be interpreted in a conventional way with either fertilizer or soil organic nitrogen as main sources. These results provided clear evidence for the microbial immobilization and subsequent mineralization and nitrification to NO3- (mineralization-immobilization turnover concept). This process masked the original oxygen isotope ratio of the fertilizer source during the summer when microbial activity was high. A second experiment involving the application of Ca(NO3)2 to three lysimeters during the winter confirmed that the dual isotope approach remains valid for the source identification of nitrate under conditions of low microbial activity. The study reveals the limitation of the dual isotope approach to characterize nitrate sources under biologically active conditions and the ability to quantify microbial processes when the main sources can be controlled.

Use of isotopic and molecular techniques to link toluene degradation in denitrifying aquifer microcosms to specific microbial populations

Microcosms were inoculated with sediments from both a petroleum-hydrocarbon (PHC)-contaminated aquifer and from a nearby pristine aquifer and incubated under anoxic denitrifying conditions with [methyl-13C]toluene. These microcosms served as a laboratory model system to evaluate the combination of isotope (13C-labeling of polar-lipid-derived fatty acids) and molecular techniques (16S rRNA-targeting gene probes) to identify the toluene-metabolizing population. After total depletion of toluene, the following bacterial phospholipid fatty acids (PLFA) were 13C-enriched: 16:1omega7c, 16:1omega7t, 16:0, cy17:0, and 18:1omega7c. Pure culture experiments demonstrated that these compounds were also found in PLFA profiles of PHC-degrading Azoarcus spp. (beta-Proteobacteria) and related species. The origin of the CO2 evolved in the microcosms was determined by measurements of stable carbon isotope ratios. Toluene represented 11% of the total pool of mineralized substrates in the contaminated sediment and 54% in the pristine sediment. The microbial community in the microcosm incubations was characterized by using DAPI staining and whole-cell hybridization with specific fluorescently labeled 16S rRNA-targeted oligonucleotide probes. Results revealed that 6% of the DAPI-stained cells in the contaminated sediment and 32% in the pristine sediment were PHC-degrading Azoarcus spp. In biotic control microcosms (incubated under denitrifying conditions, no toluene added), Azoarcus spp. cells remained at less than 1% of the DAPI-stained cells. The results show that isotope analysis in combination with whole-cell hybridization is a promising approach to identify and to quantify denitrifying toluene degraders within microbial communities.