Stable carbon isotope fractionation of chloroethenes by dehalorespiring isolates

Dynamics of chlorinated aliphatic hydrocarbons in the Chalk aquifer of northern France

The Chalk aquifer used for drinking-water production in the southwest of the Lille European Metropolis is threatened by the presence of chlorinated aliphatic hydrocarbons (CHCs), their concentrations in groundwater regularly exceeding the regulatory limits for drinking water in France. This hinders its use for drinking-water production. Understanding the dynamics and spatial distribution of CHC in the aquifer is a key factor for resource sustainability. For that purpose, an intensive monitoring was undertaken in several well fields and at different depths over eight years. To assess a possible migration and/or degradation of the compounds, the water column in several wells was sampled at various depths with passive samplers. Furthermore, CHC degradation mechanisms were investigated with compound-specific carbon-isotope analysis. The CHC concentrations and their distributions in the area depend on past and current industrial activity, causing plumes emphasized by pumping in the wells, such plumes being multi-source with no identified origin in most wells. In the south area of Les Ansereuilles, reductive dechlorination of tetrachloroethylene from a former industrial laundry highly impacted the surrounding area with its main degradation product cis-1,2-dichloroethylene. The same area is also affected by tetrachlroroethylene from several industrial laundries, textile factories and dyeing industries with also an anaerobic degradation. In the northern part of Les Ansereuilles, tetrachloroethylene, trichloroethane, trichloroethylene and 1,1-dichloroethylene were found as primary products, whereas cis-1,2-dichloroethylene appears to be an anaerobic degradation product of TCE. The other well fields (Houplin-Ancoisne, Seclin and Emmerin) are less impacted by CHC pollution, and it was shown that no CHC degradation occurred in the wells. However, the stratification of CHCs in the well-water columns, their constant concentration values over time caused by the large amount of available CHCs, and the minor degradation occurring in wells are of concern for water operators in the future.

Stable carbon isotope fractionation of chlorinated ethenes by a microbial consortium containing multiple dechlorinating genes

The study aimed to determine the possible contribution of specific growth conditions and community structures to variable carbon enrichment factors (Ɛ-_carbon) values for the degradation of chlorinated ethenes (CEs) by a bacterial consortium with multiple dechlorinating genes. Ɛ-_carbon values for trichloroethylene, cis-1,2-dichloroethylene, and vinyl chloride were -7.24% ± 0.59%, -14.6% ± 1.71%, and -21.1% ± 1.14%, respectively, during their degradation by a microbial consortium containing multiple dechlorinating genes including tceA and vcrA. The Ɛ-_carbon values of all CEs were not greatly affected by changes in growth conditions and community structures, which directly or indirectly affected reductive dechlorination of CEs by this consortium. Stability analysis provided evidence that the presence of multiple dechlorinating genes within a microbial consortium had little effect on carbon isotope fractionation, as long as the genes have definite, non-overlapping functions.

Identification of Degradation Pathways of Chlorohydrocarbons in Saturated Low-Permeability Sediments Using Compound-Specific Isotope Analysis

This study aims to investigate whether compound-specific carbon isotope analysis (CSIA) can be used to differentiate the degradation pathways of chlorohydrocarbons in saturated low-permeability sediments. For that purpose, a site was selected, where a complex mixture of chlorohydrocarbons contaminated an aquifer-aquitard system. Almost 50 years after contaminant releases, high-resolution concentration, CSIA, and microbial profiles were determined. The CSIA profiles showed that in the aquitard cis-dichloroethene (cDCE), first considered as a degradation product of trichloroethene (TCE), is produced by the dichloroelimination of 1,1,2,2-tetrachloroethane (TeCA). In contrast, TeCA degrades to TCE via dehydrohalogenation in the aquifer, indicating that the aquifer-aquitard interface separates two different degradation pathways for TeCA. Moreover, the CSIA profiles showed that chloroform (CF) is degraded to dichloromethane (DCM) via hydrogenolysis in the aquitard and, to a minor degree, produced by the degradation of carbon tetrachloride (CT). Several microorganisms capable of degrading chlorohydrocarbons were detected in the aquitard, suggesting that aquitard degradation is microbially mediated. Furthermore, numerical simulations reproduced the aquitard concentration and CSIA profiles well, which allowed the determination of degradation rates for each transformation pathway. This improves the prediction of contaminant fate in the aquitard and potential magnitude of impacts on the adjacent aquifer due to back-diffusion.

Evaluation of carbon isotope fractionation during anaerobic reductive dehalogenation of chlorinated and brominated benzenes

Compound specific stable isotope analysis (CSIA) has been established as a useful tool to evaluate in situ biodegradation. Here, CSIA was used to determine microbial dehalogenation of chloro- and bromobenzenes in microcosms derived from Hackensack River sediments. Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) was used to measure carbon isotope fractionation during reductive dehalogenation of hexachlorobenzene (HCB), pentachlorobenzene (PeCB), 1,2,3,5-tetrachlorobenzene (TeCB), 1,2,3,5-tetrabromobenzene (TeBB), and 1,3,5-tribromobenzene (TriBB). Strong evidence of isotope fractionation coupled to dehalogenation was not observed in the substrate, possibly due to the low solubilities of the highly halogenated benzene substrates and a dilution of the isotope signal. Nonetheless, we could measure a depletion of the δ¹³C value in the dichlorobenzene product during dechlorination of HCB, the sequential depletion and enrichment of δ¹³C value for trichlorobenzene in TeCB dechlorinating cultures, and the enrichment of δ¹³C during debromination of TriBB. This indicates that a measurable isotope fractionation occurred during reductive dehalogenation of highly halogenated chloro- and bromobenzenes in aquatic sediments. Thus, although more quantitative measurements will be needed, the data suggests that CSIA may have application for monitoring in situ microbial reductive dehalogenation of highly halogenated benzenes.

Variable dual carbon-bromine stable isotope fractionation during enzyme-catalyzed reductive dehalogenation of brominated ethenes

The potential of compound-specific stable isotope analysis (CSIA) to characterize biotransformation of brominated organic compounds (BOCs) was assessed and compared to chlorinated analogues. Sulfurospirillum multivorans and Desulfitobacterium hafniense PCE-S catalyzed the dehalogenation of tribromoethene (TBE) to either vinyl bromide (VB) or ethene, respectively. Significantly lower isotope fractionation was observed for TBE dehalogenation by S. multivorans (ε_C = -1.3 ± 0.2‰) compared to D. hafniense (ε_C = -7.7 ± 1.5‰). However, higher fractionation was observed for dibromoethene (DBE) dehalogenation by S. multivorans (ε_C = -16.8 ± 1.8‰ and -21.2 ± 1.6‰ for trans- and cis-1,2- (DBE) respectively), compared to D. hafniense PCE-S (ε_C = -9.5 ± 1.2‰ and -14.5 ± 0.7‰ for trans-1,2-DBE and cis-1,2-DBE, respectively). Significant, but similar, bromine fractionation was observed for for S. multivorans (ε_Br = -0.53 ± 0.15‰, -1.03 ± 0.26‰, and -1.18 ± 0.13‰ for trans-1,2-DBE, cis-1,2-DBE and TBE, respectively) and D. hafniense PCE-S (ε_Br = -0.97 ± 0.28‰, -1.16 ± 0.36‰, and -1.34 ± 0.32‰ for cis-1,2-DBE, TBE and trans-1,2-DBE, respectively). Variable CBr dual-element slopes were estimated at Λ (ε_C/ε_Br) = 1.03 ± 0.2, 17.9 ± 5.8, and 29.9 ± 11.0 for S. multivorans debrominating TBE, cis-1,2-DBE and trans-1,2-DBE, respectively, and at 7.14 ± 1.6, 8.27 ± 3.7, and 8.92 ± 2.4 for D. hafniense PCE-S debrominating trans-1,2-DBE, TBE and cis-1,2-DBE, respectively. A high variability in isotope fractionation, which was substrate property related, was observed for S. multivorans but not D. hafniense, similar as observed for chlorinated ethenes, and may be due to rate-limiting steps preceding the bond-cleavage or differences in the reaction mechanism. Overall, significant isotope fractionation was observed and, therefore, CSIA can be applied to monitor the fate of brominated ethenes in the environment. Isotope effects differences, however, are not systematically comparable to chlorinated ethenes.

Contrasting dual (C, Cl) isotope fractionation offers potential to distinguish reductive chloroethene transformation from breakdown by permanganate

cis-1,2-Dichloroethene (cis-DCE) and trichloroethene (TCE) are persistent, toxic and mobile pollutants in groundwater systems. They are both conducive to reductive dehalogenation and to oxidation by permanganate. In this study, the potential of dual element (C, Cl) compound specific isotope analyses (CSIA) for distinguishing between chemical oxidation and anaerobic reductive dechlorination of cis-DCE and TCE was investigated. Well-controlled cis-DCE degradation batch tests gave similar carbon isotope enrichment factors ε_C (‰), but starkly contrasting dual element isotope slopes Δδ¹³C/Δδ³⁷Cl for permanganate oxidation (ε_C=-26‰±6‰, Δδ¹³C/Δδ³⁷Cl≈-125±47) compared to reductive dechlorination (ε_C=-18‰±4‰, Δδ¹³C/Δδ³⁷Cl≈4.5±3.4). The difference can be tracked down to distinctly different chlorine isotope fractionation: an inverse isotope effect during chemical oxidation (ε_Cl=+0.2‰±0.1‰) compared to a large normal isotope effect in reductive dechlorination (ε_Cl=-3.3‰±0.9‰) (p≪0.05). A similar trend was observed for TCE. The dual isotope approach was evaluated in the field before and up to 443days after a pilot scale permanganate injection in the subsurface. Our study indicates, for the first time, the potential of the dual element isotope approach for distinguishing cis-DCE (and TCE) concentration drops caused by dilution, oxidation by permanganate and reductive dechlorination both at laboratory and field scale.

Metagenomic and Metatranscriptomic Analyses Reveal the Structure and Dynamics of a Dechlorinating Community Containing Dehalococcoides mccartyi and Corrinoid-Providing Microorganisms under Cobalamin-Limited Conditions

The aim of this study is to obtain a systems-level understanding of the interactions between Dehalococcoides and corrinoid-supplying microorganisms by analyzing community structures and functional compositions, activities, and dynamics in trichloroethene (TCE)-dechlorinating enrichments. Metagenomes and metatranscriptomes of the dechlorinating enrichments with and without exogenous cobalamin were compared. Seven putative draft genomes were binned from the metagenomes. At an early stage (2 days), more transcripts of genes in the Veillonellaceae bin-genome were detected in the metatranscriptome of the enrichment without exogenous cobalamin than in the one with the addition of cobalamin. Among these genes, sporulation-related genes exhibited the highest differential expression when cobalamin was not added, suggesting a possible release route of corrinoids from corrinoid producers. Other differentially expressed genes include those involved in energy conservation and nutrient transport (including cobalt transport). The most highly expressed corrinoid de novo biosynthesis pathway was also assigned to the Veillonellaceae bin-genome. Targeted quantitative PCR (qPCR) analyses confirmed higher transcript abundances of those corrinoid biosynthesis genes in the enrichment without exogenous cobalamin than in the enrichment with cobalamin. Furthermore, the corrinoid salvaging and modification pathway of Dehalococcoides was upregulated in response to the cobalamin stress. This study provides important insights into the microbial interactions and roles played by members of dechlorinating communities under cobalamin-limited conditions.IMPORTANCE The key chloroethene-dechlorinating bacterium Dehalococcoides mccartyi is a cobalamin auxotroph, thus acquiring corrinoids from other community members. Therefore, it is important to investigate the microbe-microbe interactions between Dehalococcoides and the corrinoid-providing microorganisms in a community. This study provides systems-level information, i.e., taxonomic and functional compositions and dynamics of the supportive microorganisms in dechlorinating communities under different cobalamin conditions. The findings shed light on the important roles of Veillonellaceae species in the communities compared to other coexisting community members in producing and providing corrinoids for Dehalococcoides species under cobalamin-limited conditions.

Experimental Determination of Isotope Enrichment Factors - Bias from Mass Removal by Repetitive Sampling

Application of compound-specific stable isotope approaches often involves comparisons of isotope enrichment factors (ε). Experimental determination of ε-values is based on the Rayleigh equation, which relates the change in measured isotope ratios to the decreasing substrate fractions and is valid for closed systems. Even in well-controlled batch experiments, however, this requirement is not necessarily fulfilled, since repetitive sampling can remove a significant fraction of the analyte. For volatile compounds the need for appropriate corrections is most evident, and various methods have been proposed to account for mass removal and for volatilization into the headspace. In this study we use both synthetic and experimental data to demonstrate that the determination of ε-values according to current correction methods is prone to considerable systematic errors even in well-designed experimental setups. Application of inappropriate methods may lead to incorrect and inconsistent ε-values entailing misinterpretations regarding the processes underlying isotope fractionation. In fact, our results suggest that artifacts arising from inappropriate data evaluation might contribute to the variability of published ε-values. In response, we present novel, adequate methods to eliminate systematic errors in data evaluation. A model-based sensitivity analysis serves to reveal the most crucial experimental parameters and can be used for future experimental design to obtain correct ε-values allowing mechanistic interpretations.

Variability in carbon isotope fractionation of trichloroethene during degradation by persulfate activated with zero-valent iron: Effects of inorganic anions

Stable carbon isotope analysis has the potential to be used for assessing the performance of in situ remediation of organic contaminants. Successful application of this isotope technique requires understanding the magnitude and variability in carbon isotope fractionation associated with the reactions under consideration. This study investigated the influence of inorganic anions (sulfate, bicarbonate, and chloride) on carbon isotope fractionation of trichloroethene (TCE) during its degradation by persulfate activated with zero-valent iron. The results demonstrated that the significant carbon isotope fractionation (enrichment factors ε ranging from -3.4±0.3 to -4.3±0.3‰) was independent on the zero-iron dosage, sulfate concentration, and bicarbonate concentration. However, the ε values (ranging from -7.0±0.4 to -13.6±1.2‰) were dependent on the chloride concentration, indicating that chloride could significantly affect carbon isotope fractionation during TCE degradation by persulfate activated with zero-valent iron. The dependence of ε values on chloride concentration, indicated that TCE degradation mechanisms may be different from the degradation mechanism caused by sulfate radical (SO4(-)). Ignoring the effect of chloride on ε value may cause numerous uncertainties in quantitative assessment of the performance of the in situ chemical oxidation (ISCO).

Resiliency of Stable Isotope Fractionation (δ(13)C and δ(37)Cl) of Trichloroethene to Bacterial Growth Physiology and Expression of Key Enzymes

Quantification of in situ (bio)degradation using compound-specific isotope analysis requires a known and constant isotope enrichment factor (ε). Because reported isotope enrichment factors for microbial dehalogenation of chlorinated ethenes vary considerably we studied the potential effects of metabolic adaptation to TCE respiration on isotope fractionation (δ(13)C and δ(37)Cl) using a model organism (Desulfitobacterium hafniesne Y51), which only has one reductive dehalogenase (PceA). Cells grown on TCE for the first time showed exponential growth until 10(9) cells/mL. During exponential growth, the cell-normalized amount of PceA enzyme increased steadily in the presence of TCE (up to 21 pceA transcripts per cell) but not with alternative substrates (<1 pceA transcript per cell). Cultures initially transferred or subcultivated on TCE showed very similar isotope fractionation, both for carbon (εcarbon: -8.6‰ ± 0.3‰ or -8.8‰ ± 0.2‰) and chlorine (εchlorine: -2.7‰ ± 0.3‰) with little variation (0.7‰) for the different experimental conditions. Thus, TCE isotope fractionation by D. hafniense strain Y51 was affected by neither growth phase, pceA transcription, or translation, nor by PceA content per cell, suggesting that transport limitations did not affect isotope fractionation. Previously reported variable ε values for other organohalide-respiring bacteria might thus be attributed to different expression levels of their multiple reductive dehalogenases.

Substrate hydrophobicity and cell composition influence the extent of rate limitation and masking of isotope fractionation during microbial reductive dehalogenation of chlorinated ethenes

This study investigated the effect of intracellular microscale mass transfer on microbial carbon isotope fractionation of tetrachloroethene (PCE) and trichloroethene (TCE). Significantly stronger isotope fractionation was observed for crude extracts vs intact cells of Sulfurospirillum multivorans, Geobacter lovleyi, Desulfuromonas michiganensis, Desulfitobacterium hafniense strain PCE-S, and Dehalobacter restrictus. Furthermore, carbon stable isotope fractionation was stronger for microorganisms with a Gram-positive cell envelope compared to those with a Gram-negative cell envelope. Significant differences were observed between model organisms in cellular sorption capacity for PCE (S. multivorans-K(d-PCE) = 0.42-0.51 L g(-1); D. hafniense-K(d-PCE) = 0.13 L g(-1)), as well as in envelope hydrophobicity (S. multivorans 33.0° to 72.2°; D. hafniense 59.1° to 60.8°) when previously cultivated with fumarate or PCE as electron acceptor, but not for TCE. Cell envelope properties and the tetrachloroethene reductive dehalogenase (PceA-RDase) localization did not result in significant effects on observed isotope fractionation of TCE. For PCE, however, systematic masking of isotope effects as a result of microscale mass transfer limitation at microbial membranes was observed, with carbon isotope enrichment factors of -2.2‰, -1.5 to -1.6‰, and -1.0‰ (CI95% < ± 0.2‰) for no membrane, hydrophilic outer membrane, and outer + cytoplasmic membrane, respectively. Conclusively, rate-limiting mass transfer barriers were (a) the outer membrane or cell wall and (b) the cytoplasmic membrane in case of a cytoplasmic location of the RDase enzyme. Overall, our results indicate that masking of isotope fractionation is determined by (1) hydrophobicity of the degraded compound, (2) properties of the cell envelope, and (3) the localization of the reacting enzyme.

Combined C and Cl isotope effects indicate differences between corrinoids and enzyme (Sulfurospirillum multivorans PceA) in reductive dehalogenation of tetrachloroethene, but not trichloroethene

The role of the corrinoid cofactor in reductive dehalogenation catalysis by tetrachloroethene reductive dehalogenase (PceA) of Sulfurospirillum multivorans was investigated using isotope analysis of carbon and chlorine. Crude extracts containing PceA--harboring either a native norpseudo-B12 or the alternative nor-B12 cofactor--were applied for dehalogenation of tetrachloroethene (PCE) or trichloroethene (TCE), and compared to abiotic dehalogenation with the respective purified corrinoids (norpseudovitamin B12 and norvitamin B12), as well as several commercially available cobalamins and cobinamide. Dehalogenation of TCE resulted in a similar extent of C and Cl isotope fractionation, and in similar dual-element isotope slopes (εC/εCl) of 5.0-5.3 for PceA enzyme and 3.7-4.5 for the corrinoids. Both observations support an identical reaction mechanism. For PCE, in contrast, observed C and Cl isotope fractionation was smaller in enzymatic dehalogenation, and dual-element isotope slopes (2.2-2.8) were distinctly different compared to dehalogenation mediated by corrinoids (4.6-7.0). Remarkably, εC/εCl of PCE depended in addition on the corrinoid type: εC/εCl values of 4.6 and 5.0 for vitamin B12 and norvitamin B12 were significantly different compared to values of 6.9 and 7.0 for norpseudovitamin B12 and dicyanocobinamide. Our results therefore suggest mechanistic and/or kinetic differences in catalytic PCE dehalogenation by enzymes and different corrinoids, whereas such differences were not observed for TCE.

Variability in the carbon isotope fractionation of trichloroethene on its reductive dechlorination by vitamin B12

Stable carbon isotope fractionation through the reductive dechlorination of trichloroethylene by vitamin B12 was determined to assess the possibility of using stable carbon isotope analysis to determine the efficacy of remediation of trichloroethylene using vitamin B12. We elucidated the effects of environmental conditions, including the pH, reaction temperature, and vitamin B12 concentration, on the carbon isotope enrichment factor (ε). The ε values were relatively insensitive to the reaction temperature and vitamin B12 concentration, ranging from -15.7‰ to -16.2‰, with a mean of -15.9 ± 0.2‰, at different temperatures and vitamin B12 concentrations. Such a reproducible ε value could be particularly useful for estimating the extent of degradation in reactions in which a mass balance is difficult to achieve. However, changing the initial solution pH from 6.5 to 9.0 caused a notable change in the ε values, from -14.0‰ to -18.0‰. Reactions were investigated by calculating the apparent kinetic isotope effects for carbon, which, at 1.029-1.037, were smaller than the kinetic isotope effect values previously found for C-Cl bond cleavage. This indicates that a reaction other than the elimination of chloride may be a competitive degradation pathway. The dominant degradation pathway may be different for different initial solution pH values, and this will clearly influence carbon isotope fractionation. Therefore, if the ε value varies with reaction conditions, such as the solution pH, the calculations should take into account the actual environmental conditions that affect the rate limiting pathways.

Carbon and chlorine isotope fractionation during Fenton-like degradation of trichloroethene

Dual isotope approach has been proposed as a viable tool for characterizing and assessing in situ contaminant transformation, however, little data is currently available on its applicability to chlorinated ethenes. This study determined carbon and chlorine isotope fractionation during Fenton-like degradation of trichloroethene (TCE). Carbon and chlorine isotope enrichment factors were εC=-2.9 ± 0.3‰ and εCl=-0.9 ± 0.1‰, respectively. An observed small secondary chlorine isotope effect (AKIECl=1.001) was consistent with an initial transformation by adding hydroxyl radicals (OH) to CC bonds without cleavage of CCl bonds. The relative change in carbon and chlorine isotope ratios (Δ=Δδ(13)C/Δδ(37)Cl) was calculated to be 3.1 ± 0.2, approximately equal to the ratio of chlorine and carbon isotope enrichment factors (εC/εCl=3.2). The similarity of the Δ (or εC/εCl) values between Fenton-like degradation and microbial reductive dechlorination of TCE was observed, indicating that application of solely dual isotope approach may be limited in distinguishing the two transformation pathways.

Comparison of 1,2-dichloroethane, dichloroethene and vinyl chloride carbon stable isotope fractionation during dechlorination by two Dehalococcoides strains

Carbon stable isotope fractionation during 1,2-dichloroethane (1,2-DCA), dichloroethene (DCE) and vinyl chloride (VC) dechlorination was analysed for two Dehalococcoides strains, Dehalococcoides mccartyi strain 195 (formerly Dehalococcoides ethenogenes strain 195) and D. mccartyi strain BTF08, and used to characterize the reaction. The isotope enrichment factors (εC) determined for 1,2-DCA were -30.8 ± 1.3‰ and -29.0 ± 3.0‰ for D. mccartyi strain BTF08 and D. mccartyi strain 195, respectively. Enrichment factors (εC) determined for chlorinated ethenes with strain BTF08 were -28.8 ± 1.5‰ (VC), -30.5 ± 1.5‰ (cis-DCE) and -12.4 ± 1.1‰ (1,1-DCE). Product, ethene, related enrichment factors (εC1,2-DCA-ethene) calculated for 1,2-DCA (-34.1 and -32.3‰ for strain BTF08 and strain 195, respectively) were similar to substrate based enrichment factors (εC1,2-DCA), supporting the hypothesis that ethene is the direct product of 1,2-DCA dichloroelimination but that VC was a side product as result of branching in the reaction.

Effects of varying growth conditions on stable carbon isotope fractionation of trichloroethene (TCE) by tceA-containing Dehalococcoides mccartyi strains

To quantify in situ bioremediation using compound specific isotope analysis (CSIA), isotope fractionation data obtained from the field is interpreted according to laboratory-derived enrichment factors. Although previous studies that have quantified dynamic isotopic shifts during the reductive dechlorination of trichloroethene (TCE) indicate that fractionation factors can be highly variable from culture-to-culture and site-to-site, the effects of growth condition on the isotope fractionation during reductive dechlorination have not been previously examined. Here, carbon isotope fractionation by Dehalococcoides mccartyi 195 (Dhc195) maintained under a variety of growth conditions was examined. Enrichment factors quantified when Dhc195 was subjected to four suboptimal growth conditions, including decreased temperature (-13.3 ± 0.9‰), trace vitamin B12 availability (-12.7 ± 1.0‰), limited fixed nitrogen (-14.4 ± 0.8‰), and elevated vinyl chloride exposure (-12.5 ± 0.4‰), indicate that the fractionation is similar across a range of tested conditions. The TCE enrichment factors for two syntrophic cocultures, Dhc195 with Desulfovibrio vulgaris Hildenborough (-13.0 ± 2.0‰) and Dhc195 with Syntrophomonas wolfei (-10.4 ± 1.2‰ and -13.3 ± 1.0‰), were also similar to a control experiment. In order to test the stability of enrichment factors in microbial communities, the isotope fractionation was quantified for Dhc-containing groundwater communities before and after two-year enrichment periods under different growth conditions. Although these enrichment factors (-8.9 ± 0.4‰, -6.8 ± 0.8‰, -8.7 ± 1.3‰, -9.4 ± 0.7‰, and -7.2 ± 0.3‰) were predominantly outside the range of values quantified for the isolate and cocultures, all tested enrichment conditions within the communities produced nearly similar fractionations. Enrichment factors were not significantly affected by changes in any of the tested growth conditions for the pure cultures, cocultures or the mixed communities, indicating that despite a variety of temperature, nutrient, and cofactor-limiting conditions, stable carbon isotope fractionations remain consistent for given Dehalococcoides cultures.

3D-CSIA: carbon, chlorine, and hydrogen isotope fractionation in transformation of TCE to ethene by a Dehalococcoides culture

Carbon (C), chlorine (Cl), and hydrogen (H) isotope effects were determined during dechlorination of TCE to ethene by a mixed Dehalococcoides (Dhc) culture. The C isotope effects for the dechlorination steps were consistent with data published in the past for reductive dechlorination (RD) by Dhc. The Cl effects (combined with an inverse H effect in TCE) suggested that dechlorination proceeded through nucleophilic reactions with cobalamin rather than by an electron transfer mechanism. Depletions of (37)Cl in daughter compounds, resulting from fractionation at positions away from the dechlorination center (secondary isotope effects), further support the nucleophilic dechlorination mechanism. Determination of C and Cl isotope ratios of the reactants and products in the reductive dechlorination chain offers a potential tool for differentiation of Dhc activity from alternative transformation mechanisms (e.g., aerobic degradation and reductive dechlorination proceeding via outer sphere mechanisms), in studies of in situ attenuation of chlorinated ethenes. Hydrogenation of the reaction products (DCE, VC, and ethene) showed a major preference for the (1)H isotope. Detection of depleted dechlorination products could provide a line of evidence in discrimination between alternative sources of TCE (e.g., evolution from DNAPL sources or from conversion of PCE).

Reductive dechlorination of TCE by chemical model systems in comparison to dehalogenating bacteria: insights from dual element isotope analysis (13C/12C, 37Cl/35Cl)

Chloroethenes like trichloroethene (TCE) are prevalent environmental contaminants, which may be degraded through reductive dechlorination. Chemical models such as cobalamine (vitamin B12) and its simplified analogue cobaloxime have served to mimic microbial reductive dechlorination. To test whether in vitro and in vivo mechanisms agree, we combined carbon and chlorine isotope measurements of TCE. Degradation-associated enrichment factors ε(carbon) and ε(chlorine) (i.e., molecular-average isotope effects) were -12.2‰ ± 0.5‰ and -3.6‰ ± 0.1‰ with Geobacter lovleyi strain SZ; -9.1‰ ± 0.6‰ and -2.7‰ ± 0.6‰ with Desulfitobacterium hafniense Y51; -16.1‰ ± 0.9‰ and -4.0‰ ± 0.2‰ with the enzymatic cofactor cobalamin; -21.3‰ ± 0.5‰ and -3.5‰ ± 0.1‰ with cobaloxime. Dual element isotope slopes m = Δδ(13)C/ Δδ(37)Cl ≈ ε(carbon)/ε(chlorine) of TCE showed strong agreement between biotransformations (3.4 to 3.8) and cobalamin (3.9), but differed markedly for cobaloxime (6.1). These results (i) suggest a similar biodegradation mechanism despite different microbial strains, (ii) indicate that transformation with isolated cobalamin resembles in vivo transformation and (iii) suggest a different mechanism with cobaloxime. This model reactant should therefore be used with caution. Our results demonstrate the power of two-dimensional isotope analyses to characterize and distinguish between reaction mechanisms in whole cell experiments and in vitro model systems.

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

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.

Stable carbon isotope fractionation during trichloroethene degradation in magnetite-catalyzed Fenton-like reaction

Mineral-catalyzed Fenton-like oxidation of chlorinated ethylenes is an attractive technique for in situ soil and groundwater remediation. Stable carbon isotope enrichment factors associated with magnetite-catalyzed Fenton-like oxidation of trichloroethylene (TCE) have been determined, to study the possibility of applying stable carbon isotope analysis as a technique to assess the efficacy of remediation implemented by Fenton-like oxidation. The carbon enrichment factors (ε values) ranged from -2.7‰ to -3.6‰ with a mean value of -3.3±0.3‰, and only small differences were observed for different initial reactive conditions. The ε values were robust and reproducible, and were relatively insensitive to a number of environmental factors such as ratios of reactants and PCE co-contamination, which can reduce the uncertainty associated with application of isotope enrichment factors for quantification of in situ remediation by Fenton-like reaction. ε values for Fenton-like oxidation of TCE were intermediate in those previously reported for aerobic biological processes (ε=-1.1 to -20.7‰). Thus, field-derived ε values that are more negative than those for Fenton-like oxidation, may indicate the occurrence of aerobic biodegradation at contaminated sites undergoing in situ remediation with Fenton-like reaction. However, stable carbon isotope analysis is unable to determine whether there is the occurrence of biodegradation processes if field-derived ε values are less negative than those for Fenton-like oxidation.

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

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.

Field applicability of Compound-Specific Isotope Analysis (CSIA) for characterization and quantification of in situ contaminant degradation in aquifers

Microbial processes govern the fate of organic contaminants in aquifers to a major extent. Therefore, the evaluation of in situ biodegradation is essential for the implementation of Natural Attenuation (NA) concepts in groundwater management. Laboratory degradation experiments and biogeochemical approaches are often biased and provide only indirect evidence of in situ degradation potential. Compound-Specific Isotope Analysis (CSIA) is at present among the most promising tools for assessment of the in situ contaminant degradation within aquifers. One- and two-dimensional (2D) CSIA provides qualitative and quantitative information on in situ contaminant transformation; it is applicable for proving in situ degradation and characterizing degradation conditions and reaction mechanisms. However, field application of CSIA is challenging due to a number of influencing factors, namely those affecting the observed isotope fractionation during biodegradation (e.g., non-isotope-fractionating rate-limiting steps, limited bioavailability), potential isotope effects caused by processes other than biodegradation (e.g., sorption, volatilization, diffusion), as well as non-isotope-fractionating physical processes such as dispersion and dilution. This mini-review aims at guiding practical users towards the sound interpretation of CSIA field data for the characterization of in situ contaminant degradation. It focuses on the relevance of various constraints and influencing factors in CSIA field applications and provides advice on when and how to account for these constraints. We first evaluate factors that can influence isotope fractionation during biodegradation, as well as potential isotope-fractionating and non-isotope-fractionating physical processes governing observed isotope fractionation in the field. Finally, the potentials of the CSIA approach for site characterization and the proper ways to account for various constraints are illustrated by means of a comprehensive CSIA field study at the benzene, toluene, ethylbenzene, and xylene (BTEX)-contaminated site Zeitz.

Pathway-dependent isotope fractionation during aerobic and anaerobic degradation of monochlorobenzene and 1,2,4-trichlorobenzene

Stable carbon isotope fractionation is a valuable tool for monitoring natural attenuation and to establish the fate of groundwater contaminants. In this study, we measured carbon isotope fractionation during aerobic and anaerobic degradation of two chlorinated benzenes: monochlorobenzene (MCB) and 1,2,4-trichlorobenzene (1,2,4-TCB). MCB isotope fractionation was measured in anaerobic methanogenic microcosms, while 1,2,4-TCB isotope experiments were carried out in both aerobic and anaerobic microcosms. Large isotope fractionation was observed in both the anaerobic microcosm experiments. Enrichment factors (ε) for anaerobic reductive dechlorination of MCB and 1,2,4-TCB were -5.0‰ ± 0.2‰ and -3.0‰ ± 0.4‰, respectively. In contrast, no significant isotope fractionation was found during aerobic microbial degradation of 1,2,4-TCB. The cleavage of a C-Cl σ bond occurs during anaerobic reductive dechlorination of MCB and 1,2,4-TCB, while no σ bond cleavage is involved during aerobic degradation via dioxygenase. The difference in isotope fractionation for aerobic versus anaerobic biodegradation of MCB and 1,2,4-TCB can be explained by the difference in the initial step of aerobic versus anaerobic biodegradation pathways.

Influence of humic acid on the trichloroethene degradation by Dehalococcoides-containing consortium

By taking an anaerobic Dehalococcoides-containing consortium (designated UC-1) as the research object, the influence of humic acid on the degradation of TCE by UC-1 was examined. The results indicated that (i) TCE was more rapidly degraded in the presence of humic acid compared with the control and the TCE removal efficiencies increased with the increase of concentrations of humic acid; and (ii) at the end of experiments, in the presence of humic acid, much more ethene was produced compared with the control, whereas less VC was accumulated in the medium. Presumably, humic acid improves the activity of organisms in dechlorinating populations resulting in more ethene accumulated in the medium, and (iii) the degradation of TCE stimulated by humic acid by UC-1 might be a biotic process or an abiotic process. Thus, humic acid could influence the degradation of TCE by UC-1 directly via enhancing electron transfer between UC-1 and TCE. This work is a preliminary step for accelerating the degradation of TCE in the groundwater environment using a kind of natural organic matter - humic acid.

Anaerobic degradation of vinyl chloride in aquifer microcosms

The anaerobic degradation potential at a chloroethene-contaminated site was investigated by operating two anoxic column aquifer microcosms enriched in iron(III). One column was fed with vinyl chloride (VC) only (column A) and one with VC and acetate (column B). In column A, after about 600 pore volume exchanges (PVEs), VC started to disappear and reached almost zero VC recovery in the effluent after 1000 PVEs. No formation of ethene was observed. In column B, effluent VC was almost always only a fraction of influent VC. Formation of ethene was observed after 800 PVEs and started to become an important degradation product after 1550 PVEs. However, ethene was never observed in stoichiometric amounts compared with disappeared VC. The average stable isotope enrichment factor for VC disappearance in column A was determined to be -4.3‰. In column B, the isotope enrichment factor shifted from -10.7 to -18.5‰ concurrent with an increase in ethene production. Batch microcosms inoculated with column material showed similar isotope enrichment factors as the column microcosms. These results indicated that two degradation processes occurred, one in column A and two in parallel in column B with increasing importance of reductive dechlorination with time. This study suggests that in addition to reductive dechlorination, other degradation processes such as anaerobic oxidation should be taken into account when evaluating natural attenuation of VC and that isotope analysis can help to differentiate between different pathways of VC removal.

Stable carbon isotope enrichment factors for cis-1,2-dichloroethene and vinyl chloride reductive dechlorination by Dehalococcoides

Compound-specific stable isotope analysis (CSIA) is a promising tool for monitoring in situ microbial activity, and enrichment factors (ε values) determined using CSIA can be employed to estimate compound transformation. Although ε values for some dechlorination reactions catalyzed by Dehalococcoides (Dhc) have been reported, reproducibility between independent experiments, variability between different Dhc strains, and congruency between pure and mixed cultures are unknown. In experiments conducted with pure cultures of Dhc sp. strain BAV1, ε values for 1,1-DCE, cis-DCE, trans-DCE, and VC were -5.1, -14.9, -20.8, and -23.2‰, respectively. The ε value for 1,1-DCE dechlorination was 48.9% higher than the value reported in a previous study, but ε values for other chlorinated ethenes were equal between independent experiments. For the dechlorination of cis-DCE and VC by Dhc strains BAV1, FL2, GT, and VS, average ε values were -18.4 and -23.2‰, respectively. cis-DCE and VC ε values determined in pure Dhc cultures with different reductive dehalogenase genes (e.g., vcrA vs bvcA) varied by less than 36.8 and 8.3%, respectively. In the BDI consortium, ε values for cis-DCE and VC dechlorination were -25.3‰ and -19.9‰, 31.6% higher and 15.3% lower, respectively, compared to the average ε value for Dhc pure cultures. As cis-DCE and VC ε values are all within the same order-of-magnitude and fractionation is always measured during Dhc dechlorination, CSIA may be a valuable approach for monitoring in situ cis-DCE and VC reductive dechlorination.

Transformation and carbon isotope fractionation of tetra- and trichloroethene to trans-dichloroethene by Dehalococcoides sp. strain CBDB1

Dehalococcoides sp. strain CBDB1 reductively dechlorinated perchloroethene (PCE) and trichloroethene (TCE) to predominantly trans-1,2-dichloroethene (trans-DCE). Cell counting by direct microscopy showed that strain CBDB1 used PCE and TCE as electron acceptors for respiratory growth obtaining a growth yield of 3.9 × 10(12) cells per mol of chloride released in both cases. PCE and TCE were dechlorinated to trans- and cis-DCE at an average constant ratio of 3.4 (±0.2):1, which is consistent with the ratios found in several trans-DCE-producing sediments and soils containing uncultured Dehalococcoides-like species. Significant carbon isotope fractionation was observed during PCE and TCE reductive dehalogenation. The enrichment factor of TCE (εC = -11.2) was within the range of previously reported values for TCE dechlorination by other Dehalococcoides species although the tceA gene responsible for ethene generation in the latter cultures was absent in strain CBDB1. On the contrary, the enrichment factor of PCE (εC = -1.6) was 3.8-times lower than that obtained for Dehalococcoides sp. strain 195 although both strains shared a high similarity in the pceA gene responsible for PCE dechlorination in strain 195. In addition, the product-related enrichment factors for TCE dehalogenation were calculated based on product isotope signature of the two accumulated products cis-DCE (εC TCE→cis-DCE = -11.0) and trans-DCE (εC TCE→trans-DCE = -15.9). These results are of particular interest since strain CBDB1 constitutes, together with the recent isolated strain MB, the unique Dehalococcoides species unable to dechlorinate PCE and TCE beyond DCE.

Stable isotope fractionation to investigate natural transformation mechanisms of organic contaminants: principles, prospects and limitations

Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) has made it possible to analyze natural stable isotope ratios (e.g., (13)C/(12)C, (15)N/(14)N, (2)H/(1)H) of individual organic contaminants in environmental samples. They may be used as fingerprints to infer contamination sources, and may demonstrate, and even quantify, the occurrence of natural contaminant transformation by the enrichment of heavy isotopes that arises from degradation-induced isotope fractionation. This review highlights an additional powerful feature of stable isotope fractionation: the study of environmental transformation mechanisms. Isotope effects reflect the energy difference of isotopologues (i.e., molecules carrying a light versus a heavy isotope in a particular molecular position) when moving from reactant to transition state. Measuring isotope fractionation, therefore, essentially allows a glimpse at transition states! It is shown how such position-specific isotope effects are "diluted out" in the compound average measured by GC-IRMS, and how a careful evaluation in mechanistic scenarios and by dual isotope plots can recover the underlying mechanistic information. The mathematical framework for multistep isotope fractionation in environmental transformations is reviewed. Case studies demonstrate how isotope fractionation changes in the presence of mass transfer, enzymatic commitment to catalysis, multiple chemical reaction steps or limited bioavailability, and how this gives information about the individual process steps. Finally, it is discussed how isotope ratios of individual products evolve in sequential or parallel transformations, and what mechanistic insight they contain. A concluding session gives an outlook on current developments, future research directions and the potential for bridging the gap between laboratory and real world systems.

Insights into enzyme kinetics of chloroethane biodegradation using compound specific stable isotopes

While compound specific isotope analysis (CSIA) has been used extensively to investigate remediation of chlorinated ethenes, to date considerably less information is available on its applicability to chlorinated ethanes. In this study, biodegradation of 1,1,1-trichloroethane (1,1,1-TCA) and 1,1-dichloroethane (1,1-DCA) was carried out by a Dehalobacter-containing mixed culture. Carbon isotope fractionation factors (ε) measured during whole cell degradation demonstrated that values for 1,1,1-TCA and 1,1-DCA (-1.8‰ and -10.5‰, respectively) were significantly smaller than values reported for abiotic reductive dechlorination of these same compounds. Similar results were found in experiments degrading these two priority pollutants by cell free extracts (CFE) where values of -0.8‰ and -7.9‰, respectively, were observed. For 1,1,1-TCA in particular, the large kinetic isotope effect expected for cleavage of a C-Cl bond was almost completely masked during biodegradation by both whole cells and CFE. Comparison to previous studies demonstrates that these patterns of isotopic fractionation are not attributable to transport effects across the cell membrane, as had been seen for other compounds such as PCE. In contrast these results reflect significant differences in the kinetics of the enzymes catalyzing chlorinated ethane degradation.

Impacts of microbial community composition on isotope fractionation during reductive dechlorination of tetrachloroethylene

Isotope fractionation has been used with increasing frequency as a tool to quantify degradation of chlorinated aliphatic pollutants in the environment. The objective of this research was to determine if the electron donor present in enrichment cultures prepared from uncontaminated sediments influenced the extent of isotope fractionation of tetrachloroethylene (PCE), either directly, or through its influence on microbial community composition. Two PCE-degrading enrichment cultures were prepared from Duck Pond (DP) sediment and were incubated with formate (DPF) or H(2) (DPH) as electron donor. DPF and DPH were significantly different in both product distribution and extent of isotope fractionation. Chemical and isotope analyses indicated that electron donors did not directly affect the product distribution or the extent of isotope fractionation for PCE reductive dechlorination. Instead, restriction fragment length polymorphism (RFLP) and sequence analysis of the 16S rRNA clone libraries of DPF and DPH identified distinct microbial communities in each enrichment culture, suggesting that differences in microbial communities were responsible for distinct product distributions and isotope fractionation between the two cultures. A dominant species identified only in DPH was closely related to known dehalogenating species (Sulfurospirillum multivorans and Sulfurospirillum halorespirans) and may be responsible for PCE degradation in DPH. Our study suggests that different dechlorinators exist at the same site and can be preferentially stimulated by different electron donors, especially over the long-term (i.e., years), typical of in-situ ground water remediation.

Current approaches for the assessment of in situ biodegradation

Considering the high costs and technical difficulties associated with conventional remediation strategies, in situ biodegradation has become a promising approach for cleaning up contaminated aquifers. To verify if in situ biodegradation of organic contaminants is taking place at a contaminated site and to determine if these processes are efficient enough to replace conventional cleanup technologies, a comprehensive characterization of site-specific biodegradation processes is essential. In recent years, several strategies including geochemical analyses, microbial and molecular methods, tracer tests, metabolite analysis, compound-specific isotope analysis, and in situ microcosms have been developed to investigate the relevance of biodegradation processes for cleaning up contaminated aquifers. In this review, we outline current approaches for the assessment of in situ biodegradation and discuss their potential and limitations. We also discuss the benefits of research strategies combining complementary methods to gain a more comprehensive understanding of the complex hydrogeological and microbial interactions governing contaminant biodegradation in the field.

Aerobic biodegradation of cis-1,2-dichloroethene as sole carbon source: Stable carbon isotope fractionation and growth characteristics

Cis-1,2-dichloroethene (cDCE) is a compound of concern at many chloroethene-contaminated sites, since it tends to accumulate during reductive dechlorination of the higher chlorinated ethenes. Stable carbon isotope fractionation during aerobic cDCE biodegradation was observed in groundwater microcosms under varying incubation conditions (room temperature/groundwater temperature; with/without inorganic nutrients), and resulted in an average stable carbon isotope enrichment factor of -15.2+/-0.5 per thousand. A new enrichment culture, obtained from groundwater microcosms, degraded cDCE concentrations up to 100mgL(-1), was active at temperatures between 4 and 23 degrees C, had a pH optimum of approximately 7, and could withstand prolonged periods (250d) of starvation. Microbial growth during degradation of cDCE as sole carbon and energy source was demonstrated by protein formation in mineral medium not containing any known auxiliary substrate. The obtained growth yield was 12.5+/-1.9g of proteinMol(-1) of cDCE, with a doubling time of 53+/-2h at 23 degrees C. Aerobic degradation of cDCE as sole carbon and energy source appears to be a promising biological process for site remediation.

Influence of mass-transfer limitations on carbon isotope fractionation during microbial dechlorination of trichloroethene

Mass transfer of organic contaminants from nonaqueous phase liquids to the aqueous phase can significantly modulate the observable carbon isotope fractionation behavior associated with contaminant transformation. We evaluated the effects of kinetic interphase mass transfer between tetradecane and water on the observable (13)C enrichment factor, epsilon(obs), pertinent to the reductive dechlorination of trichloroethene (TCE) by Sulfurospirillum sp. in laboratory batch model systems containing organic, aqueous and gaseous phases. We propose a conceptual model, which includes the kinetics of tetradecane-water and gas-water mass transfer, microbial growth, and isotope-sensitive parameters describing dehalorespiration, for quantifying variable (13)C enrichment factors. While the C isotope fractionation of TCE reduction to cis-dichloroethene (cDCE) in the absence of phase-transfer effects can be characterized by a constant epsilon-value of -18.8 +/- 0.6 per thousand, mass-transfer limitations impede describing this process with a constant enrichment factor typically used in Rayleigh equations. Owing to the masking of kinetic isotope effects by the transfer of TCE from tetradecane to the aqueous phase, (obs)-values gradually changed from -18.4 per thousand to -5.9 per thousand. Such variations may complicate the interpretation of compound-specific isotope analysis in the assessment of chloroethene biodegradation in field applications.

Variations in expression of carbon isotope fractionation of chlorinated ethenes during biologically enhanced PCE dissolution close to a source zone

The stable carbon isotope values of tetrachloroethene (PCE) and its degradation products were monitored during studies of biologically enhanced dissolution of PCE dense nonaqueous phase liquid (DNAPL) to determine the effect of PCE dissolution on observed isotope values. The degradation of PCE was monitored in a 2-dimensional model aquifer and in a pilot test cell (PTC) at Dover Air Force Base, both with emplaced PCE DNAPL sources. Within the plume down gradient from the source, the isotopic fractionation of dissolved PCE and its degradation products were consistent with those observed in biodegradation laboratory studies. However, close to the source zone significant shifts in the isotope values of dissolved PCE were not observed in either the model aquifer or PTC due to the constant input of newly dissolved, non fractionated PCE, and the small isotopic fractionation associated with PCE reductive dechlorination by the mixed microbial culture used. Therefore the identification of reductive dechlorination in the presence of PCE DNAPL was based upon the appearance of daughter products and the isotope values of those daughter products. An isotope model was developed to simulate isotope values of PCE during the dissolution and degradation of PCE adjacent to a DNAPL source zone. With the exception of very high degradation rate constants (>1/day) stable carbon isotope values of PCE estimated by the model remained within error of the isotope value of the PCE DNAPL, consistent with measured isotope values in the model aquifer and in the PTC.

Aerobic biodegradation of chlorinated ethenes in a fractured bedrock aquifer: quantitative assessment by compound-specific isotope analysis (CSIA) and reactive transport modeling

A model-based analysis of concentration and isotope data was carried out to assess natural attenuation of chlorinated ethenes in an aerobic fractured bedrock aquifer. Tetrachloroethene (PCE) concentrations decreased downgradient of the source, but constant delta13C signatures indicated the absence of PCE degradation. In contrast, geochemical and isotopic data demonstrated degradation of trichloroethene (TCE) and cis-1,2-dichloroethene (DCE) under the prevailing oxic conditions. Numerical modeling was employed to simulate isotopic enrichment of chlorinated ethenes and to evaluate alternative degradation pathway scenarios. Existing field information on groundwater flow, solute transport, geochemistry, and delta13C signatures of the chlorinated ethenes was integrated via reactive transport simulations. The results provided strong evidence for the occurrence of aerobic TCE and DCE degradation. The chlorinated ethene concentrations together with stable carbon isotope data allowed us to reliably constrain the assessment of the extent of biodegradation at the site and plume simulations quantitatively linked aerobic biodegradation with isotope signatures in the field. Our investigation provides the first quantitative assessment of aerobic biodegradation of chlorinated ethenes in a fractured rock aquifer based on compound specific stable isotope measurements and reactive transport modeling.

Stable carbon isotope fractionation of 1,2-dichloropropane during dichloroelimination by Dehalococcoides populations

The isotope fractionation of 1,2-dichloropropane (1,2-D) during dichloroelimination to propene by Dehalococcoides populations was explored in laboratory experiments in order to provide data for the characterization of the fate of 1,2-D in heterogeneous subsurface systems. Compound specific stable carbon isotope analysis (CSIA) was used to determine the bulk enrichment factors (epsilonbulk), reactive position specific enrichment factors (epsilonreactive), and apparent kinetic isotope effect (AKIE) values for 1,2-D dichloroelimination in two distinct Dehalococcoides-containing cultures. The epsilonbulk factors calculated in the two cultures were statistically identical, -10.8 +/- 0.9 and -11.3 +/- 0.8 per thousand, even though the cultures were derived from geographically distinct locations. AKIE values for 1,2-D dichloroelimination assuming stepwise and concerted reaction mechanisms were approximately 1.033 and 1.017, respectively. These values are within the range of previously reported values for dichloroelimination reactions and were equivalent to values reported for biotic 1,2-dichloroethane and abiotic 1,1,2,2,-tetrachloroethane and pentachloroethane dichloroelimination reactions.

Proteomic and transcriptomic analyses reveal genes upregulated by cis-dichloroethene in Polaromonas sp. strain JS666

Polaromonas sp. strain JS666 is the only bacterial isolate capable of using cis-dichloroethene (cDCE) as a sole carbon and energy source. Studies of cDCE degradation in this novel organism are of interest because of potential bioremediation and biocatalysis applications. The primary cellular responses of JS666 to growth on cDCE were explored using proteomics and transcriptomics to identify the genes upregulated by cDCE. Two-dimensional gel electrophoresis revealed upregulation of genes annotated as encoding glutathione S-transferase, cyclohexanone monooxygenase, and haloacid dehalogenase. DNA microarray experiments confirmed the proteomics findings that the genes indicated above were among the most highly upregulated by cDCE. The upregulation of genes with antioxidant functions and the inhibition of cDCE degradation by elevated oxygen levels suggest that cDCE induces an oxidative stress response. Furthermore, the upregulation of a predicted ABC transporter and two sodium/solute symporters suggests that transport is important in cDCE degradation. The omics data were integrated with data from compound-specific isotope analysis (CSIA) and biochemical experiments to develop a hypothesis for cDCE degradation pathways in JS666. The CSIA results indicate that the measured isotope enrichment factors for aerobic cDCE degradation ranged from -17.4 to -22.4 per thousand. Evidence suggests that cDCE degradation via monooxygenase-catalyzed epoxidation (C C cleavage) may be only a minor degradation pathway under the conditions of these experiments and that the major degradation pathway involves carbon-chloride cleavage as the initial step, a novel mechanism. The results provide a significant step toward elucidation of cDCE degradation pathways and enhanced understanding of cDCE degradation in JS666.

The relative contributions of abiotic and microbial processes to the transformation of tetrachloroethylene and trichloroethylene in anaerobic microcosms

Reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE) was studied in well-defined microcosms prepared with aquifer materials from three locations. Electron donors and terminal electron acceptors were added to both stimulate microbial activity and generate reactive minerals via microbial iron and sulfate reduction. The relative importance of abiotic and microbial PCE and TCE reductive dechlorination was then assessed by analysis of reaction products and kinetics and, in some cases, by stable carbon isotope fractionation. The predominant PCE and TCE transformation pathway in most microcosms was microbial reductive dechlorination. Rates of abiotic transformation were similar in magnitude to those for microbial reductive dechlorination in only a few cases where the activity of dechlorinating bacteria was low. Comparison of geochemical conditions with abiotic product recoveries showed thatthe greatest extent of abiotic reductive dechlorination occurred under iron- and sulfate-reducing conditions. Under these two geochemical conditions, high concentrations of Fe(II) and S(-II) solid species were present, suggesting the involvement of Fe(II) and S(-II) minerals in abiotic reductive dechlorination. Both abiotic and microbial dechlorination of PCE and TCE took place under almost all microcosm conditions; the relative rates of the two processes under field conditions will depend on factors such as the abundance of dechlorinating bacteria, soil properties, and the mass loading of reactive minerals.

Precise and accurate compound specific carbon and nitrogen isotope analysis of atrazine: critical role of combustion oven conditions

Compound-specific stable isotope analysis by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) is increasingly used to assess origin and fate of organic substances in the environment. Although analysis without isotopic discrimination is essential, it cannot be taken for granted for new target compounds. We developed and validated carbon isotope analysis of atrazine, a herbicide widely used in agriculture. Combustion was tested with reactors containing (i) CuO/NiO/Pt operating at 940 degrees C; (ii) CuO operating at 800 degrees C; (iii) Ni/NiO operating at 1150 degrees C and being reoxidized for 2 min during each gas chromatographic run. Accurate and precise carbon isotope measurements were only obtained with Ni/NiO reactors giving a mean deviation delta delta(13)C from dual inlet measurements of -0.1-0.2% per hundred and a standard deviation (SD) of +/- 0.4% per hundred. CuO at 800 degrees C gave precise, but inaccurate values (delta delta(13)C = -1.3% per hundred, SD +/- 0.4% per hundred), whereas CuO/NiO/Pt reactors at 940 degrees C gave inaccurate and imprecise data. Accurate (delta delta(15)N = 0.2% per hundred) and precise (SD +/- 0.3% per hundred) nitrogen isotope analysis was accomplished with a Ni/NiO-reactor previously used for carbon isotope analysis. The applicability of the method was demonstrated for alkaline hydrolysis of atrazine at 20 degrees C and pH 12 (nucleophilic aromatic substitution) giving epsilon(carbon) = -5.6% per hundred +/- 0.1% per hundred (SD) and epsilon(nitrogen) = -1.2% per hundred +/- 0.1% per hundred (SD).

Tracking in situ biodegradation of 1,2-dichloroethenes in a model wetland

The spatial and temporal biogeochemical development of a model wetland loaded with cis- and trans-1,2-dichloroethene contaminated groundwater was characterized over 430 days by hydrogeochemical and compound-specific isotope analyses (CSIA). The hydrogeochemistry dramatically changed over time from oxic to strongly reducing conditions as emphasized by increasing concentrations of ferrous iron, sulfide, and methane since day 225. delta(13)C values for trans- and cis-DCE substantially changed over the flow path and correlated over time with DCE removal. The carbon enrichment factor values (epsilon) retrieved from the wetland became progressively larger over the investigation period, ranging from -1.7 +/- 0.3% per hundred to -32.6 +/- 2.2% per hundred. This indicated that less fractionating DCE oxidation was progressively replaced by reductive dechlorination, associated with a more pronounced isotopic effect and further confirmed by the detection of vinyl chloride and ethene since day 250. This study demonstrates the linkage between hydrogeochemical variability and intrinsic degradation processes and highlights the potential of CSIA to trace the temporal and spatial changes of the dominant degradation mechanism of DCE in natural or engineered systems.

Identifying abiotic chlorinated ethene degradation: characteristic isotope patterns in reaction products with nanoscale zero-valent iron

Carbon isotope fractionation is of great interest in assessing chlorinated ethene transformation by nanoscale zero-valent iron at contaminated sites, particularly in distinguishing the effectiveness of an implemented abiotic degradation remediation scheme from intrinsic biotic degradation. Transformation of trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), and vinyl chloride (VC) with two types of nanoscale iron materials showed different reactivity trends, but relatively consistent carbon isotope enrichment factors (epsilon) of -19.4 per thousand +/- 1.8 per thousand (VC), -21.7 per thousand +/- 1.8 per thousand (cis-DCE), and -23.5 per thousand +/- 2.8 per thousand (TCE) with one type of iron (FeBH), and from -20.9 per thousand +/- 1.1 per thousand to -26.5 per thousand +/- 1.5 per thousand (TCE) with the other (FeH2). Products of the dichloroelimination pathway (ethene, ethane, and acetylene) were consistently 10 per thousand more isotopically depleted than those of the hydrogenolysis pathway (cis-DCE from TCE, VC from cis-DCE), displaying a characteristic pattern that may serve as an indicator of abiotic dehalogenation reactions and as a diagnostic parameter for differentiating the effects of abiotic versus biotic degradation. In contrast, the product-related enrichment factors of each respective pathway varied significantly in different experiments. Because such variation would not be expected for independent pathways with constant kinetic isotope effects, our data give preliminary evidence that the two pathways may share an irreversible first reaction step with subsequent isotopically sensitive branching.

Growth kinetics and stable carbon isotope fractionation during aerobic degradation of cis-1,2-dichloroethene and vinyl chloride

Assessing changes in the isotopic signature of contaminants is a promising new tool to monitor microbial degradation processes. In this study, chloroethene degradation was proven by depletion of chloroethenes, formation of chloride, increase in protein content and stable carbon isotope fractionation. Aerobic degradation of vinyl chloride (VC) was found to proceed metabolically, with degradation rates of 0.48 and 0.29 d(-1); and growth yields of 9.7 and 6.4 g of protein/mol of VC at room and groundwater temperature, respectively. Cis-1,2-dichloroethene (cDCE) was degraded cometabolically under aerobic conditions when VC was provided as growth substrate. Aerobic degradation was associated with significant stable carbon isotope fractionation, with enrichment factors ranging from -5.4+/-0.4 per thousand for metabolic degradation of VC to -9.8+/-1.7 per thousand for cometabolic degradation of cDCE. Thus, it was demonstrated that stable carbon isotope fractionation is suitable for assessing aerobic chloroethene degradation, which can contribute significantly to site remediation.