Stable Carbon Isotope Fractionation Research Articles

Variability in microbial carbon isotope fractionation of tetra- and trichloroethene upon reductive dechlorination

The variability of stable carbon isotope fractionation upon reductive dechlorination of tetra- and trichloroethene by several microbial strains was investigated to examine the uncertainties related to the in situ application of compound specific isotope analysis (CSIA) of chlorinated ethenes. Carbon isotope fractionation was investigated with a set of microorganisms representative for the currently known diversity of dehalorespirers: Dehalococcoides ethenogenes strain 195, Desulfitobacterium sp. strain Viet1, Desulfuromonas michiganensis and Geobacter lovleyi sp. strain SZ and compared to the previous reports using Sulfurospirillum spp. and Desulfitobacterium sp. strain PCE-S. Carbon isotope fractionation of tetrachloroethene (PCE) and trichlorethene (TCE) was highly variable ranging from the absence of significant fractionation to carbon isotope fractionation (εC) of 16.7 and 3.5–18.9 for PCE and TCE, respectively. Fractionation of both compounds by D. ethenogenes strain 195 (PCE: εC = 6.0; TCE: εC = 13.7) was similar to the literature data for mixed cultures containing Dehalococcoides spp. D. michiganensis (PCE: no significant fractionation; TCE: εC = 3.5) and G. lovleyi sp. strain SZ (PCE no significant fractionation; TCE: εC = 8.5) generated the lowest fractionation of all studied strains. Desulfitobacterium sp. strain Viet1 (PCE: εC = 16.7) gave the highest enrichment factor for PCE.

Carbon and hydrogen isotopic fractionation of low molecular weight organic compounds during ultraviolet degradation

Ultraviolet (UV) radiation is a ubiquitous energy source for chemical reactions in nature. In particular, UV radiation provides an important pathway for degradation of organic compounds. Although isotopic change in organic compounds exposed to UV radiation may be an indicator for evaluating the extent of degradation, the isotope behavior has not been yet clarified. In this study, stable carbon and hydrogen isotope fractionations were determined for volatile organic compounds including methanol, acetonitrile, benzene and propionic acid during UV degradation in order to reveal the isotopic behavior with respect to different functional groups. All the studied compounds became enriched in 13C and D with increase in UV exposure time. The isotopic fractionation factor ( α) ranged from 0.9968 to 0.9875 for carbon, and from 0.9890 to 0.9406 for hydrogen. The order of carbon isotopic fractionation was different from that of hydrogen isotopic fractionation, probably due to intra- and inter-molecular reactions with different functional groups. UV degradation is a process potentially able to induce isotope enrichment in organic compounds in natural environments.

Quantification of Microbial Methane Oxidation in an Alpine Peat Bog

Methane (CH4) is an important greenhouse gas that is produced in different subsurface environments. Its main biological sink, microbial CH4 oxidation, can be quantified in situ in the vadose zone using gas push‐pull tests (GPPTs). This field method is based on the comparison of breakthrough curves of the reactant CH4 and a nonreactive tracer. Under diffusion‐dominated transport conditions, previously employed noble gases are unsuitable as tracers to calculate rate constants for CH4 oxidation due to differing diffusion coefficients. Here, we show that by performing two consecutive GPPTs and coinjecting acetylene (C2H2) as an inhibitor of CH4 oxidation in the second test, the reactant CH4 can be used as a substitute tracer. Applying this procedure, apparent first‐order rate constants for CH4 oxidation ranging from 0.38 to 0.82 h−1 were obtained in the vadose zone of three hummocks in an alpine peat bog near Lucerne, Switzerland. Corresponding estimates of in situ rates ranged from 4 to 299 ng CH4 g dry weight−1 h−1 In all but one GPPT, strong stable carbon isotope fractionation due to diffusion masked isotope fractionation due to microbial oxidation. Therefore, stable carbon isotope fractionation is suitable only to a limited extent as an indicator of microbial CH4 oxidation during a GPPT with diffusion‐dominated gas transport. In contrast, the presented procedure for the quantification of microbial CH4 oxidation using GPPTs can be applied without restrictions even in systems with high porosity. Furthermore, the presented method may be useful for quantifying other processes for which suitable inhibitors but no suitable tracers are available.

Factors controlling the carbon isotope fractionation of tetra- and trichloroethene during reductive dechlorination by Sulfurospirillum ssp. and Desulfitobacterium sp. strain PCE-S

Carbon stable isotope fractionation of tetrachloroethene (PCE) and trichloroethene (TCE) was investigated during reductive dechlorination. Growing cells of Sulfurospirillum multivorans, Sulfurospirillum halorespirans, or Desulfitobacterium sp. strain PCE-S, the respective crude extracts and the abiotic reaction with cyanocobalamin (vitamin B(12)) were used. Fractionation of TCE (alphaC=1.0132-1.0187) by S. multivorans was more than one order of magnitude higher than values previously observed for tetrachloroethene (PCE) (alphaC=1.00042-1.0017). Similar differences in fractionation were observed during reductive dehalogenation by the close relative S. halorespirans with alphaC=1.0046-1.032 and alphaC=1.0187-1.0229 for PCE and TCE respectively. TCE carbon isotope fractionation (alphaC=1.0150) by the purified PCE-reductive dehalogenase from S. multivorans was more than one order of magnitude higher than fractionation of PCE (alphaC=1.0017). Carbon isotope fractionation of TCE by Desulfitobacterium sp. strain PCE-S (alphaC=1.0109-1.0122) as well as during the abiotic reaction with cyanocobalamin (alphaC=1.0154) was in a similar range to previously reported values for fractionation by mixed microbial cultures. In contrast with previous results with PCE, no effects due to rate limitations, uptake or transport of the substrate to the reactive site could be observed during TCE dechlorination. Our results show that prior to a mechanistic interpretation of stable isotope fractionation factors it has to be carefully verified how other factors such as uptake or transport affect the isotope fractionation during degradation experiments with microbial cultures.

The effect of iron availability on the regulation of inorganic carbon acquisition in the coccolithophore Emiliania huxleyi and the significance of cellular compartmentation for stable carbon isotope fractionation

Iron is limiting phytoplankton productivity in large parts of today’s oceans, the so-called HNLC (high nutrient low chlorophyll) areas. It is a key component in photosynthesis during which inorganic carbon fixation in most phytoplankton species is sustained by so-called carbon concentrating mechanisms (CCMs). Here we investigate CCM regulation in the coccolithophore Emiliania huxleyi in response to varying degrees of iron limitation by means of membrane-inlet mass spectrometry. Compared to iron replete conditions rates of both active CO 2 and HCO 3 - uptake were markedly reduced under iron limitation leading to significantly diminished growth rates. Moreover, there was a concomitant decrease in CCM efficiency, reflected in an increased CO 2 loss from the cell in relation to carbon fixation. Under such conditions higher values for carbon isotope fractionation ( ϵ p) would be expected. However, direct measurements of ϵ p showed that carbon isotope fractionation was insensitive to changes in growth rates and CCM activity. This can be explained by concomitant changes in internal DIC fluxes in and out of the chloroplast as demonstrated with a simple cell model comprising two compartments. Thus, carbon isotope fractionation reflects the ability of phytoplankton to actively control their inorganic carbon acquisition depending on environmental conditions. The insensitivity of carbon isotope fractionation to changes in the availability of iron could be of interest for paleoreconstructions in the HNLC areas of today’s oceans.

Evidence for substantial intramolecular heterogeneity in the stable carbon isotopic composition of phytol in photoautotrophic organisms

The ubiquitous isoprenoid, phytol, was isolated from a range of algae, terrestrial plants and a bacterium, and its two terminal carbon atoms were quantitatively removed by chemical oxidation. The product, 6,10,14-trimethylpentadecan-2-one, was depleted in 13C by 1–4‰ relative to the parent phytol. This difference is significant, and indicates that the pathway for biosynthesis of phytol induces substantial intramolecular stable carbon isotopic fractionation. The nature and magnitude of the fractionation suggest strongly that it is associated both with the biosynthesis of isopentenyl pyrophosphate via the 2-C-methylerythritol-4-phosphate pathway and with the formation of carotenoids and phytol from geranyl-geraniolphosphate. As a result of these large, intramolecular isotopic differences, diagenetic products formed by loss of C, such as pristane, may be naturally depleted in 13C by several permil relative to phytane.

Distinguishing Abiotic and Biotic Transformation of Tetrachloroethylene and Trichloroethylene by Stable Carbon Isotope Fractionation

Significant carbon isotope fractionation was observed during FeS-mediated reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE). Bulk enrichment factors (E(bulk)) for PCE were -30.2 +/- 4.3 per thousand (pH 7), -29.54 +/- 0.83 per thousand (pH 8), and -24.6 +/- 1.1 per thousand (pH 9). For TCE, E(bulk) values were -33.4 +/- 1.5 per thousand (pH 8) and -27.9 +/- 1.3 per thousand (pH 9). A smaller magnitude of carbon isotope fractionation resulted from microbial reductive dechlorination by two isolated pure cultures (Desulfuromonas michiganensis strain BB1 (BB1) and Sulfurospirillum multivorans (Sm)) and a bacterial consortium (BioDechlor INOCULUM (BDI)). The E(bulk) values for biological PCE microbial dechlorination were -1.39 +/- 0.21 per thousand (BB1), -1.33 +/- 0.13 per thousand (Sm), and -7.12 +/- 0.72 per thousand (BDI), while those for TCE were -4.07 +/- 0.48 per thousand (BB1), -12.8 +/- 1.6 per thousand (Sm), and -15.27 +/- 0.79 per thousand (BDI). Reactions were investigated by calculation of the apparent kinetic isotope effect for carbon (AKIEc), and the results suggest that differences in isotope fractionation for abiotic and microbial dechlorination resulted from the differences in rate-limiting steps during the dechlorination reaction. Measurement of more negative E(bulk) values at sites contaminated with PCE and TCE may suggest the occurrence of abiotic reductive dechlorination by FeS.

Isotopic and chemical investigations of Fe (III) reduction by Shewanella putrefaciens strain 200R

ABSTRACTExperiments were conducted using the Fe+3‐reducing bacterium Shewanella putrefaciens strain 200R to determine the stable carbon isotope fractionation during dissimilatory Fe (III) reduction and associated lactate oxidation at circum‐neutral pH. Previous studies used equilibrium fractionation factors (~14.3‰) between bacterial biomass and synthesized fatty acids to identify the predominant carbon fixation pathways for some of the most frequently isolated microbes including Shewanella under anaerobic conditions. We investigated the carbon isotope disproportionation among organic carbon substrate (lactate), biomass and respired carbon dioxide at the lag to stationary phase of the growth curve. Ferric citrate and sodium lactate were used as electron acceptor and donor, respectively. Sodium bicarbonate or potassium phosphate was used as buffering agent. Iron (II), iron (III), dissolved inorganic carbon (DIC) and carbon isotope ratios were measured for both bicarbonate‐ and phosphate‐buffered systems. Carbon isotope ratio measurements were made on the respired CO2 (as DIC) and microbial biomass for both buffering conditions. The fraction of lactate consumed was estimated using DIC as a proxy and was verified by direct measurement using HPLC. Our result showed that bicarbonate‐buffered system has an enhancing effect in the reduction process compared to the phosphate system. Both systems resulted in carbon isotope fractionations between the lactate substrate and DIC that could be modelled as a Rayleigh process. The biomass produced under both buffer conditions was depleted on average by ~2‰ relative to the substrate and enriched by ~5‰ relative to the DIC. This translates to an overall isotopic fractionation of 10–12‰ between the biomass and respired CO2 in both buffering systems.

Stable Carbon Isotope Fractionation of Chloroethenes by Dehalorespiring Isolates

Stable carbon isotope fractionation during the reductive dechlorination of chloroethenes by two bacterial strains that dechlorinate to ethene, Dehalococcoides ethenogenes 195 and Dehalococcoides sp. strain BAV1 as well as Sulfurospirillum multivorans and Dehalobacter restrictus strain PER-K23, isolates that do not dechlorinate past DCE, are reported. Fractionation by a Dehalococcoides-containing enrichment culture is also measured for comparison to the isolates. All data adequately fit the Rayleigh model and results are presented as enrichment factors. For strain 195, the measured enrichment factors were -9.6 +/- 0.4, -21.1 +/- 1.8, and -5.8 +/- 0.5 when degrading TCE, cDCE, and 1,1-DCE, respectively. Strain BAV1 exhibited enrichment factors of -16.9 +/- 1.4, -8.4 +/- 0.3, -21.4 +/- 0.9, and -24.0 +/- 2.0 for cDCE, 1,1-DCE, tDCE, and VC, respectively. The surprisingly large differences in enrichment factors caused by individual reductases (RDases) reducing different chloroethenes is likely the result of chemical structure differences among the chloroethenes. For TCE reduction, S. multivorans and D. restrictus strain PER-K23 exhibited enrichment factors of -16.4 +/- 1.5 and -3.3 +/- 0.3, respectively. While all of the organisms studied here utilize RDases that require corrinoid cofactors, the biotic TCE enrichment factors varied widely from those reported for the abiotic cobalamin-catalyzed reaction, indicating that additional factors affect the extent of fractionation in these biological systems. The enrichment factors measured for the Dehalococcoides-containing enrichment culture did not match well with those from any of the isolates, demonstrating the inherent difficulties in predicting fractionation factors of undefined communities. Although compound-specific isotope fractionation is a powerful tool for evaluating the progress of in situ bioremediation in the field, given the wide range of enrichment factors associated with functionally similar and phylogenetically diverse organisms, caution must be exercised when applying enrichment factors for the interpretation of dechlorination data.

Diet–Feather Stable Isotope (δ15N and δ13C) Fractionation in Common Murres and Other Seabirds

Abstract We measured the fractionation of stable nitrogen (δ15N) and carbon (δ13C) isotopes in the breast and primary feathers of 11 Common Murres (Uria aalge) maintained on a diet of capelin (Mallotus villosus). Diet–feather δ15N fractionation from delipidated capelin muscle to murre feathers was 3.6‰ ± 0.2‰ in breast feathers and 3.7‰ ± 0.2‰ in primary feathers. Fractionation of δ13C was 2.5‰ ± 0.2‰ in breast feathers and 1.9‰ ± 0.3‰ in primary feathers. Prey–feather fractionation (for delipidated, muscle-only prey samples) for nine other species of seabirds ranged from 3.0‰ to 4.6‰ for δ15N and 0.1‰ to 2.5‰ for δ13C. Studies that did not remove lipids from prey samples showed higher δ15N and δ13C fractionation, and those that used whole prey items rather than muscle tissue alone showed higher δ15N fractionation. We suggest that: (1) prey samples be delipidated to facilitate interpretation of δ13C fractionation, (2) high interstudy and interspecific variation in δ13C makes species-specific studies essential, and (3) use of muscle tissue rather than whole bodies of fish will minimize unexplained variation in δ15N fractionation.

Investigation of the stable isotope fractionation in speleothems with laboratory experiments

Many speleothems show evidence for calcite precipitation under disequilibrium conditions. To improve the understanding of these kinetic processes, several laboratory experiments were performed to study the fractionation of stable oxygen and carbon isotopes during the precipitation of calcite. Carbonate was precipitated under controlled conditions from both a body of standing water (beaker experiments) and a solution flowing along a channel (channel experiments) at a relative humidity of 100%. Slow degassing of CO 2, simulated by the beaker experiments, results in δ 18O values in equilibrium with the solution. In contrast, the δ 13C values show a significant enrichment, inversely proportional to the height of the solution in the beakers. Fast degassing of CO 2, simulated by the channel experiments, showed an enrichment of both δ 13C and δ 18O and a slope of Δδ 13C/Δδ 18O of 1.4±0.6. These results represent experimental evidence for the Hendy effect, which is manifested in (i) a progressive increase in δ 18O and δ 13C away from the growth axis and (ii) a positive correlation between δ 18O and δ 13C along a single growth layer of a stalagmite.

Effect of temperature and ration size on carbon and nitrogen stable isotope trophic fractionation

Summary Stable isotope data are widely used to track the origins and transformations of materials in food webs. Reliable interpretation of these data requires knowledge of the factors influencing isotopic fractionation between diet and consumer. For practical reasons, isotopic fractionation is often assumed to be constant but, in reality, a range of factors may affect fractionation. To investigate effects of temperature and feeding rate on fractionation of carbon and nitrogen stable isotopes in a marine predator, we reared European sea bass Dicentrarchus labrax on identical diets at 11 and 16 °C on three ration levels for 600 days. Nitrogen trophic fractionation (Δδ15N) was affected by temperature. Bass Δδ15N was 4·41‰ at 11 °C and 3·78‰ at 16 °C. Carbon fractionation (Δδ13C) was also affected by temperature. Bass Δδ13C was 1·18‰ at 11 °C and 1·64‰ at 16 °C. The higher lipid content in the tissues of bass reared at cooler temperatures accounted for the temperature effect on Δδ13C. When Δδ13C was determined using mathematically defatted values, there was a direct effect of ration size and Δδ13C was 2·51, 2·39 and 2·31‰ for high, medium and low rations, respectively. Reported Δδ15N for all treatments exceeded the mean of 3·4‰ widely used in ecological studies of fish populations and communities. This would confound the interpretation of δ15N as an indicator of trophic level when comparing populations that are exposed to different temperatures. The Δδ13C of 0–1‰ commonly applied in food web studies did not hold under any of the temperature or feeding regimes considered and a value of 2‰ would be more appropriate.

DIET–FEATHER STABLE ISOTOPE (δ15N AND δ13C) FRACTIONATION IN COMMON MURRES AND OTHER SEABIRDS

Abstract We measured the fractionation of stable nitrogen (δ15N) and carbon (δ13C) isotopes in the breast and primary feathers of 11 Common Murres (Uria aalge) maintained on a diet of capelin (Mallotus villosus). Diet–feather δ15N fractionation from delipidated capelin muscle to murre feathers was 3.6‰ ± 0.2‰ in breast feathers and 3.7‰ ± 0.2‰ in primary feathers. Fractionation of δ13C was 2.5‰ ± 0.2‰ in breast feathers and 1.9‰ ± 0.3‰ in primary feathers. Prey–feather fractionation (for delipidated, muscle-only prey samples) for nine other species of seabirds ranged from 3.0‰ to 4.6‰ for δ15N and 0.1‰ to 2.5‰ for δ13C. Studies that did not remove lipids from prey samples showed higher δ15N and δ13C fractionation, and those that used whole prey items rather than muscle tissue alone showed higher δ15N fractionation. We suggest that: (1) prey samples be delipidated to facilitate interpretation of δ13C fractionation, (2) high interstudy and interspecific variation in δ13C makes species-specific studies essen...

Stable carbon and nitrogen isotopic fractionation between diet and swine tissues

Naturally occurring stable isotope ratios can be a powerful tool in studies of animal nutrition, provided that the assumptions required for dietary reconstruction are validated by studies such as the one presented here. The objective of this study was to document the magnitude of isotopic fractionation between swine diet and their different tissues. For this, the isotopic ratios of carbon and nitrogen of the diet and selected tissues (hair, nail, liver, muscle, fat and cartilage) were determined. The delta13C and delta15N of the diet were -15.9‰ and 1.3‰, respectively, and all delta15N of swine tissues were 2.2 to 3.0‰ enriched in 15N in relation to the diet. Little variation in delta15N occurred among tissues, with exception to liver that was less enriched in 15N than the nail. Nail and hair presented no 13C enrichment relative to diet. Cartilage was ~1.0‰ enriched in 13C as compared to diet. Liver and muscle were on average 2.1‰ more depleted in 13C in relation to diet as well as fat tissues. Some of the C and N isotope ratios of swine tissues differed in organs, but the isotopic fractionation trends among tissues appears to be similar to other mammals. Therefore our data provide a good baseline to interpret stable isotope patterns in domestic mammals (such as swine) in controlled or semi-controlled experiments.

Can algal photosynthetic inorganic carbon isotope fractionation be predicted in lakes using existing models?

Differential fractionation of inorganic carbon stable isotopes during photosynthesis is an important cause of variability in algal carbon isotope signatures. Several physiological models have been proposed to explain algal photosynthetic fractionation factors (ep). These models generally consider CO2 concentration, growth rate, or cell morphometry and have been supported by empirical evidence from laboratory cultures. Here, we explore the applicability of these models to a broad range of lakes with mixed phytoplankton communities. Understanding this fractionation is necessary for using carbon stable isotopes for studies ranging from food webs to paleolimnology. In our largest comparative study, values of δ13C-POC ranged from − 35.1‰ to − 21.3 ‰. Using several methods to obtain an algal isotopic signature, we found high variability in fractionation among lakes. There was no relationship between ep and one of the most important predictors in existing models, pCO2. A whole-lake inorganic 13C addition was used to create distinct algal isotope signatures to aid in examining ep. Measurements and a statistical model from the isotope addition revealed that algal fractionation was often low (0 –15 ‰).

Trophic fractionation of carbon and nitrogen stable isotopes in Chironomus riparius reared on food of aquatic and terrestrial origin

Summary1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll®).2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources.3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed.4. Microbial degradation of food particles resulted in an approximate doubling of the δ15N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ13C increased only marginally, from −20.66 ± 0.11‰ to −20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.

Biogeochemistry of methane in the permanently ice-covered Lake Untersee, central Dronning Maud Land, East Antarctica

We found unprecedentedly high abundances of microbially produced CH4 in the anoxic deep waters of Lake Untersee, an oligotrophic, perennially ice-covered Antarctic freshwater lake. The maximum CH4 concentration (approaching 21.8 ± 1.4 mmol L−1) is one of the highest observed so far in a natural aquatic ecosystem. Although surficial lake sediments are the predominant source of CH4 in Lake Untersee, methanogenesis occurs also within the anoxic waters. Radiocarbon labeling experiments show that H2/CO2 reduction is the predominant methanogenic pathway (90–100%) both in the sediments and the water column, whereas acetate is only a minor CH4 precursor. This result is consistent with the stable carbon isotope fractionation between coexisting CH4 and CO2. In the water column, CH4 is partly consumed by both aerobic and anaerobic microbial oxidation as evidenced by CH4 concentration patterns, stable isotope analyses (13C, 2H), and 14C-CH4 assays. Dissimilatory sulfate reduction also occurs and peaks at 84 m water depth (1.83 µmol SO4 L−1 d−1). Intense methanogenesis in surficial lake sediments, diffusion of CH4 from sediments to the water column, additional CH4 production in the water column, gross CH4 production higher than CH4 consumption, and lack of mixing because of the permanent ice cover cause the exceptionally high CH4 concentration in the lake. Our studies demonstrate that H2/CO2 reduction may sometimes be the major pathway of methanogenesis in low-sulfate freshwater environments even at low temperatures. This pathway is obviously more important in Antarctic lakes than hitherto assumed.

Fractionation of carbon isotopes in biosynthesis of fatty acids by a piezophilic bacterium Moritella japonica strain DSK1

We examined stable carbon isotope fractionation in biosynthesis of fatty acids of a piezophilic bacterium Moritella japonica strain DSK1. The bacterium was grown to stationary phase at pressures of 0.1, 10, 20, and 50 MPa in media prepared using sterile-filtered natural seawater supplied with glucose as the sole carbon source. Strain DSK1 synthesized typical bacterial fatty acids (C 14–19 saturated, monounsaturated, and cyclopropane fatty acids) as well as long-chain polyunsaturated fatty acids (PUFA) (20:6 ω3). Bacterial cell biomass and individual fatty acids exhibited consistent pressure-dependent carbon isotope fractionations relative to glucose. The observed Δδ FA-glucose (−1.0‰ to −11.9‰) at 0.1 MPa was comparable to or slightly higher than fractionations reported in surface bacteria. However, bulk biomass and fatty acids became more depleted in 13C with pressure. Average carbon isotope fractionation (Δδ FA-glucose) at high pressures was much higher than that for surface bacteria: −15.7‰, −15.3‰, and −18.3‰ at 10, 20, and 50 MPa, respectively. PUFA were more 13C depleted than saturated and monounsaturated fatty acids at all pressures. The observed isotope effects may be ascribed to the kinetics of enzymatic reactions that are affected by hydrostatic pressure and to biosynthetic pathways that are different for short-chain and long-chain fatty acids. A simple quantitative calculation suggests that in situ piezophilic bacterial contribution of polyunsaturated fatty acids to marine sediments is nearly two orders of magnitude higher than that of marine phytoplankton and that the carbon isotope imprint of piezophilic bacteria can override that of surface phytoplankton. Our results have important implications for marine biogeochemistry. Depleted fatty acids reported in marine sediments and the water column may be derived simply from piezophilic bacteria resynthesis of organic matter, not from bacterial utilization of a 13C-depleted carbon source (i.e., methane). The interpretation of carbon isotope signatures of marine lipids must be based on principles derived from piezophilic bacteria.

Carbon isotope fractionation during degradation of benzene, trichloroethene, and tetrachloroethene under ultraviolet light

Stable carbon isotope fractionation factors associated with the degradation of benzene, trichloroethene (TCE), and tetrachloroethene (PCE) under ultraviolet light (254 nm) have been measured experimentally using simple and inexpensive quartz reaction vessels equipped with a syringe valve and septum. Carbon isotope enrichment factors of ebenzene = -6.2 ± 0.3‰, eTCE = -6.8 ± 0.2‰, and ePCE = -4.1 ± 0.2‰ were obtained. The results demonstrate that the simple and inexpensive experimental design used in this study has potential application to determine isotope enrichment factors associated with degradation under conditions more representative of important atmospheric processes, and hence has future application to the study of the behavior of important trace gases such as non-methane hydrocarbons and other volatile organic compounds in the atmosphere.

Physiology and Phylogeny of Green Sulfur Bacteria Forming a Monospecific Phototrophic Assemblage at a Depth of 100 Meters in the Black Sea

The biomass, phylogenetic composition, and photoautotrophic metabolism of green sulfur bacteria in the Black Sea was assessed in situ and in laboratory enrichments. In the center of the western basin, bacteriochlorophyll e (BChl e) was detected between depths of 90 and 120 m and reached maxima of 54 and 68 ng liter(-1). High-pressure liquid chromatography analysis revealed a dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e. Only traces of BChl e (8 ng liter(-1)) were found at the northwestern slope of the Black Sea basin, where the chemocline was positioned at a significantly greater depth of 140 m. Stable carbon isotope fractionation values of farnesol indicated an autotrophic growth mode of the green sulfur bacteria. For the first time, light intensities in the Black Sea chemocline were determined employing an integrating quantum meter, which yielded maximum values between 0.0022 and 0.00075 micromol quanta m(-2) s(-1) at the top of the green sulfur bacterial layer around solar noon in December. These values represent by far the lowest values reported for any habitat of photosynthetic organisms. Only one 16S rRNA gene sequence type was detected in the chemocline using PCR primers specific for green sulfur bacteria. This previously unknown phylotype groups with the marine cluster of the Chlorobiaceae and was successfully enriched in a mineral medium containing sulfide, dithionite, and freshly prepared yeast extract. Under precisely controlled laboratory conditions, the enriched green sulfur bacterium proved to be capable of exploiting light intensities as low as 0.015 micromol quanta m(-2) s(-1) for photosynthetic 14CO2 fixation. Calculated in situ doubling times of the green sulfur bacterium range between 3.1 and 26 years depending on the season, and anoxygenic photosynthesis contributes only 0.002 to 0.01% to total sulfide oxidation in the chemocline. The stable population of green sulfur bacteria in the Black Sea chemocline thus represents the most extremely low-light-adapted and slowest-growing type of phototroph known to date.