Isotope fractionation of micropollutants during large-volume extraction: heads-up from a critical method evaluation for atrazine, desethylatrazine and 2,6-dichlorobenzamide at low ng/L concentrations in groundwater.

Micropollutants are frequently detected in groundwater. Thus, the question arises whether they are eliminated by natural attenuation so that pesticide degradation would be observed with increasing residence time in groundwater. Conventional analytical approaches rely on parent compound/metabolite ratios. These are difficult to interpret if metabolites are sorbed or further transformed. Compound-specific stable isotope analysis (CSIA) presents an alternative for identifying degradation based on the analysis of natural isotope abundances in pesticides and their changes during degradation. However, CSIA by gas chromatography-isotope ratio mass spectrometry is challenged by the low concentrations (ng/L) of micropollutants in groundwater. Consequently, large amounts of water need to be sampled requiring enrichment and clean-up steps from interfering matrix effects that must not introduce artefacts in measured isotope values. The aim of this study was to evaluate the accuracy of isotope ratio measurements of the frequently detected micropollutants atrazine, desethylatrazine and 2,6-dichlorobenzamide after enrichment from large water volumes (up to 100 L) by solid-phase extraction with consecutive clean-up by HPLC. Associated artefacts of isotope discrimination were found to depend on numerous factors including organic matter content and extraction volume. This emphasizes the necessity to perform a careful method evaluation of sample preparation and sample pre-treatment prior reliable CSIA.

Mass Transfer Limitation during Slow Anaerobic Biodegradation of 2-Methylnaphthalene.

While they are theoretically conceptualized to restrict biodegradation of organic contaminants, bioavailability limitations are challenging to observe directly. Here we explore the onset of mass transfer limitations during slow biodegradation of the polycyclic aromatic hydrocarbon 2-methylnaphthalene (2-MN) by the anaerobic, sulfate-reducing strain NaphS2. Carbon and hydrogen compound specific isotope fractionation was pronounced at high aqueous 2-MN concentrations (60 µM) (εcarbon = -2.1 ± 0.1‰/εhydrogen = -40 ± 7‰) in the absence of an oil phase but became significantly smaller (εcarbon = -0.9 ± 0.3‰/εhydrogen = -6 ± 3‰) or nondetectable when low aqueous concentrations (4 µM versus 0.5 µM) were in equilibrium with 80 or 10 mM 2-MN in hexadecane, respectively. This masking of isotope fractionation directly evidenced mass transfer limitations at (sub)micromolar substrate concentrations. Remarkably, oil-water mass transfer coefficients were 60-90 times greater in biotic experiments than in the absence of bacteria (korg-aq2-MN = 0.01 ± 0.003 cm h-1). The ability of isotope fractionation to identify mass transfer limitations may help study how microorganisms adapt and navigate at the brink of bioavailability at low concentrations. For field surveys our results imply that, at trace concentrations, the absence of isotope fractionation does not necessarily indicate the absence of biodegradation.

Biodegradation and photooxidation of phenolic compounds in soil-A compound-specific stable isotope approach.

Phenolic compounds occur in a variety of plants and can be used as model compounds for investigating the fate of organic wastewater, lignin, or soil organic matter in the environment. The aim of this study was to better understand and differentiate mechanisms associated with photo- and biodegradation of tyrosol, vanillin, vanillic acid, and coumaric acid in soil. In a 29 d incubation experiment, soil spiked with these phenolic compounds was either subjected to UV irradiation under sterile conditions or to the native soil microbial community in the dark. Changes in the isotopic composition (δ13C) of phenolic compounds were determined by gas chromatography-isotope ratio mass spectrometry and complemented by concentration measurements. Phospholipid-derived fatty acid and ergosterol biomarkers together with soil water repellency measurements provided information on soil microbial and physical properties. Biodegradation followed pseudo-first-order dissipation kinetics, enriched remaining phenolic compounds in 13C, and was associated with increased fungal rather than bacterial biomarkers. Growing mycelia rendered the soil slightly water repellent. High sample variation limited the reliable estimation of apparent kinetic isotope effects (AKIEs) to tyrosol. The AKIE of tyrosol biodegradation was 1.007 ±â€¯0.002. Photooxidation kinetics were of pseudo-zero- or first-order with an AKIE of 1.02 ±â€¯0.01 for tyrosol, suggesting a hydroxyl-radical mediated degradation process. Further research needs to address δ13C variation among sample replicates potentially originating from heterogeneous reaction spaces in soil. Here, nuclear magnetic resonance or nanoscopic imaging could help to better understand the distribution of organic compounds and their transformation in the soil matrix.

Mechanistic Dichotomy in Bacterial Trichloroethene Dechlorination Revealed by Carbon and Chlorine Isotope Effects.

Tetrachloroethene (PCE) and trichloroethene (TCE) are significant groundwater contaminants. Microbial reductive dehalogenation at contaminated sites can produce nontoxic ethene but often stops at toxic cis-1,2-dichloroethene ( cis-DCE) or vinyl chloride (VC). The magnitude of carbon relative to chlorine isotope effects (as expressed by ΛC/Cl, the slope of δ13C versus δ37Cl regressions) was recently recognized to reveal different reduction mechanisms with vitamin B12 as a model reactant for reductive dehalogenase activity. Large ΛC/Cl values for cis-DCE reflected cob(I)alamin addition followed by protonation, whereas smaller ΛC/Cl values for PCE evidenced cob(I)alamin addition followed by Cl- elimination. This study addressed dehalogenation in actual microorganisms and observed identical large ΛC/Cl values for cis-DCE (ΛC/Cl = 10.0 to 17.8) that contrasted with identical smaller ΛC/Cl for TCE and PCE (ΛC/Cl = 2.3 to 3.8). For TCE, the trend of small ΛC/Cl could even be reversed when mixed cultures were precultivated on VC or DCEs and subsequently confronted with TCE (ΛC/Cl = 9.0 to 18.2). This observation provides explicit evidence that substrate adaptation must have selected for reductive dehalogenases with different mechanistic motifs. The patterns of ΛC/Cl are consistent with practically all studies published to date, while the difference in reaction mechanisms offers a potential answer to the long-standing question of why bioremediation frequently stalls at cis-DCE.

Monitoring Microbial Mineralization Using Reverse Stable Isotope Labeling Analysis by Mid-Infrared Laser Spectroscopy.

Assessing the biodegradation of organic compounds is a frequent question in environmental science. Here, we present a sensitive, inexpensive, and simple approach to monitor microbial mineralization using reverse stable isotope labeling analysis (RIL) of dissolved inorganic carbon (DIC). The medium for the biodegradation assay contains regular organic compounds and 13C-labeled DIC with 13C atom fractions (x(13C)DIC) higher than natural abundance (typically 2-50%). The produced CO2 (x(13C) ≈ 1.11%) gradually dilutes the initial x(13C)DIC allowing to quantify microbial mineralization using mass-balance calculations. For 13C-enriched CO2 samples, a newly developed isotope ratio mid-infrared spectrometer was introduced with a precision of x(13C) < 0.006%. As an example for extremely difficult and slowly degradable compounds, CO2 production was close to the theoretical stoichiometry for anaerobic naphthalene degradation by a sulfate-reducing enrichment culture. Furthermore, we could measure the aerobic degradation of dissolved organic carbon (DOC) adsorbed to granular activated carbon in a drinking water production plant, which cannot be labeled with 13C. Thus, the RIL approach can be applied to sensitively monitor biodegradation of various organic compounds under anoxic or oxic conditions.

Cytochrome P450-catalyzed dealkylation of atrazine by Rhodococcus sp. strain NI86/21 involves hydrogen atom transfer rather than single electron transfer.

Cytochrome P450 enzymes are responsible for a multitude of natural transformation reactions. For oxidative N-dealkylation, single electron (SET) and hydrogen atom abstraction (HAT) have been debated as underlying mechanisms. Combined evidence from (i) product distribution and (ii) isotope effects indicate that HAT, rather than SET, initiates N-dealkylation of atrazine to desethyl- and desisopropylatrazine by the microorganism Rhodococcus sp. strain NI86/21. (i) Product analysis revealed a non-selective oxidation at both the αC and ßC-atom of the alkyl chain, which is expected for a radical reaction, but not SET. (ii) Normal (13)C and (15)N as well as pronounced (2)H isotope effects (εcarbon: -4.0‰ ± 0.2‰; εnitrogen: -1.4‰ ± 0.3‰, KIEH: 3.6 ± 0.8) agree qualitatively with calculated values for HAT, whereas inverse (13)C and (15)N isotope effects are predicted for SET. Analogous results are observed with the Fe(iv)[double bond, length as m-dash]O model system [5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(iii)-chloride + NaIO4], but not with permanganate. These results emphasize the relevance of the HAT mechanism for N-dealkylation by P450.

13C/12C and 15N/14N isotope analysis to characterize degradation of atrazine: evidence from parent and daughter compound values.

Atrazine (Atz) and its metabolite desethylatrazine (DEA) frequently occur in the environment. Conclusive interpretation of their transformation is often difficult. This study explored evidence from (13)C/(12)C and (15)N/(14)N isotope trends in parent and daughter compounds when Atz was dealkylated by (i) permanganate and (ii) the bacterium Rhodococcus sp. NI86/21. In both transformations, (13)C/(12)C ratios of atrazine increased strongly (ε(carbon/permanganate) = -4.6 ± 0.6‰ and ε(carbon/Rhodoccoccus) = -3.8 ± 0.2‰), whereas nitrogen isotope fractionation was small. (13)C/(12)C ratios of DEA showed the following trends. (i) When DEA was formed as the only product (Atz + permanganate), (13)C/(12)C remained constant, close to the initial value of Atz, because the carbon atoms involved in the reaction step are not present in DEA. (ii) When DEA was formed together with desisopropylatrazine (biodegradation of Atz), (13)C/(12)C increased but only within 2‰. (iii) When DEA was further biodegraded, (13)C/(12)C increased by up to 9‰ giving strong testimony of the metabolite's breakdown. Two lines of evidence emerge. (a) Enrichment of (13)C/(12)C in DEA, compared to initial Atz, may contain evidence of further DEA degradation. (b) Dual element ((15)N/(14)N versus (13)C/(12)C) isotope plots for dealkylation of atrazine agree with indirect photodegradation but differ from direct photolysis and biotic hydrolysis. Trends in multielement isotope data of atrazine may, therefore, decipher different degradation pathways.

Dual (C, H) isotope fractionation in anaerobic low molecular weight (poly)aromatic hydrocarbon (PAH) degradation: potential for field studies and mechanistic implications.

Anaerobic polycyclic aromatic hydrocarbon (PAH) degradation is a key process for natural attenuation of oil spills and contaminated aquifers. Assessments by stable isotope fractionation, however, have largely been limited to monoaromatic hydrocarbons. Here, we report on measured hydrogen isotope fractionation during strictly anaerobic degradation of the PAH naphthalene. Remarkable large hydrogen isotopic enrichment factors contrasted with much smaller values for carbon: ε(H) = -100‰ ± 15‰, ε(C) = -5.0‰ ± 1.0‰ (enrichment culture N47); ε(H) = -73‰ ± 11‰, ε(C) = -0.7‰ ± 0.3‰ (pure culture NaphS2). This reveals a considerable potential of hydrogen isotope analysis to assess anaerobic degradation of PAHs. Furthermore, we investigated the conclusiveness of dual isotope fractionation to characterize anaerobic aromatics degradation. C and H isotope fractionation during benzene degradation (ε(C) = -2.5‰ ± 0.2‰; ε(H) = -55‰ ± 4‰ (sulfate-reducing strain BPL); ε(C) = -3.0‰ ± 0.5‰; ε(H) = -56‰ ± 8‰ (iron-reducing strain BF)) resulted in dual isotope slopes (Λ = 20 ± 2; 17 ± 1) similar to those reported for nitrate-reducers. This breaks apart the current picture that anaerobic benzene degradation by facultative anaerobes (denitrifiers) can be distinguished from that of strict anaerobes (sulfate-reducers, fermenters) based on the stable isotope enrichment factors.

C, N, and H isotope fractionation of the herbicide isoproturon reflects different microbial transformation pathways.

The fate of pesticides in the subsurface is of great interest to the public, industry, and regulatory authorities. Compound-specific isotope analysis (CSIA) is a promising tool complementary to existing methods for elucidating pesticide degradation reactions. Here, we address three different initial biotransformation reactions of the phenylurea herbicide isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea) in pure culture experiments with bacterial and fungal strains. When analyzing isotopic changes in different parts of the isoproturon molecule, hydroxylation of the isopropyl group by fungi was found to be associated with C and H isotope fractionation. In contrast, hydrolysis by Arthrobacter globiformis D47 caused strong C and N isotope fractionation, albeit in a different manner than abiotic hydrolysis so that isotope measurements can distinguish between both modes of transformation. No significant isotope fractionation was observed during N-demethylation by Sphingomonas sp. SRS2. The observed isotope fractionation patterns were in agreement with the type of reactions and elements involved. Moreover, their substantially different nature suggests that isotope changes in natural samples may be uniquely attributed to either pathway, allowing even to distinguish the abiotic versus biotic nature of hydrolysis. Our investigations show how characteristic isotope patterns may significantly add to the present understanding of the environmental fate of pesticides.

C and N isotope fractionation suggests similar mechanisms of microbial atrazine transformation despite involvement of different enzymes (AtzA and TrzN).

Transformation of atrazine to hydroxyatrazine in the environment may be underestimated by current assessment schemes since immobilization and further transformation of the metabolite can render parent-to-daughter compound ratios unreliable. This study reports significant C and N isotope fractionation of atrazine in transformation to hydroxyatrazine by Chelatobacter heintzii, Pseudomonas sp. ADP, and Arthrobacter aurescens TC1 highlighting an alternative approach to detecting this natural transformation pathway. Indistinguishable dual isotope slopes big up tri, open (= delta(15)N/delta(13)C approximately epsilon(N)/epsilon(C)) for Chelatobacter heintzii (-0.65 +/- 0.08) and Arthrobacter aurescens TC1 (-0.61 +/- 0.02) suggest the same biochemical transformation mechanism despite different hydrolyzing enzymes (AtzA versus TrzN). With Pseudomonas sp. ADP (also AtzA) significantly smaller fractionation indicates masking effects by steps prior to enzyme catalysis, while a distinguishable big up tri, open = -0.32 +/- 0.06 suggests that some of these steps showed slight isotope fractionation. Abiotic reference experiments reproduced the pattern of biotic transformation at pH 3 (enrichment of (13)C, depletion of (15)N in atrazine), but showed enrichment of both (13)C and (15)N at pH 12. This indicates that the organisms activated atrazine by a similar Lewis acid complexation (e.g., with H(+)) prior to nucleophilic aromatic substitution, giving the first detailed mechanistic insight into this important enzymatic reaction.

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).