Stable Isotope Fractionation Research Articles

Deciphering the Variability of Stable Isotope (C, Cl) Fractionation of Tetrachloroethene Biotransformation by Desulfitobacterium strains Carrying Different Reductive Dehalogenases Enzymes.

Kinetic isotope effects have been used successfully to prove and characterize organic contaminant transformation on various scales including field and laboratory studies. For tetrachloroethene (PCE) biotransformation, however, causes for the substantial variability of reported isotope enrichment factors (ε) are still not deciphered (εC = -0.4 to -19.0‰). Factors such as different reaction mechanisms and masking of isotope fractionation by either limited intracellular mass transfer or rate-limitations within the enzymatic multistep reaction are under discussion. This study evaluated the contribution of these factors to the magnitude of carbon and chlorine isotope fractionation of Desulfitobacterium strains harboring three different PCE-transforming enzymes (PCE-RdhA). Despite variable single element isotope fractionation (εC = -5.0 to -19.7‰; εCl = -1.9 to -6.3‰), similar slopes of dual element isotope plots (ΛC/Cl values of 2.4 ± 0.1 to 3.6 ± 0.1) suggest a common reaction mechanism for different PCE-RdhAs. Cell envelope properties of the Desulfitobacterium strains allowed to exclude masking effects due to PCE mass transfer limitation. Our results thus revealed that different rate-limiting steps (e.g., substrate channel diffusion) in the enzymatic multistep reactions of individual PCE-RdhAs rather than different reaction mechanisms determine the extent of PCE isotope fractionation in the Desulfitobacterium genus.

The assessment and remediation of mercury contaminated sites: A review of current approaches

Remediation of mercury (Hg) contaminated sites has long relied on traditional approaches, such as removal and containment/capping. Here we review contemporary practices in the assessment and remediation of industrial-scale Hg contaminated sites and discuss recent advances. Significant improvements have been made in site assessment, including the use of XRF to rapidly identify the spatial extent of contamination, Hg stable isotope fractionation to identify sources and transformation processes, and solid-phase characterization (XAFS) to evaluate Hg forms. The understanding of Hg bioavailability for methylation has been improved by methods such as sequential chemical extractions and porewater measurements, including the use of diffuse gradient in thin-film (DGT) samplers. These approaches have shown varying success in identifying bioavailable Hg fractions and further study and field applications are needed. The downstream accumulation of methylmercury (MeHg) in biota is a concern at many contaminated sites. Identifying the variables limiting/controlling MeHg production—such as bioavailable inorganic Hg, organic carbon, and/or terminal electron acceptors (e.g. sulfate, iron) is critical. Mercury can be released from contaminated sites to the air and water, both of which are influenced by meteorological and hydrological conditions. Mercury mobilized from contaminated sites is predominantly bound to particles, highly correlated with total sediment solids (TSS), and elevated during stormflow. Remediation techniques to address Hg contamination can include the removal or containment of Hg contaminated materials, the application of amendments to reduce mobility and bioavailability, landscape/waterbody manipulations to reduce MeHg production, and food web manipulations through stocking or extirpation to reduce MeHg accumulated in desired species. These approaches often rely on knowledge of the Hg forms/speciation at the site, and utilize physical, chemical, thermal and biological methods to achieve remediation goals. Overall, the complexity of Hg cycling allows many different opportunities to reduce/mitigate impacts, which creates flexibility in determining suitable and logistically feasible remedies.

Extreme Zr stable isotope fractionation during magmatic fractional crystallization.

Zirconium is a commonly used elemental tracer of silicate differentiation, yet its stable isotope systematics remain poorly known. Accessory phases rich in Zr4+ such as zircon and baddeleyite may preserve a unique record of Zr isotope behavior in magmatic environments, acting both as potential drivers of isotopic fractionation and recorders of melt compositional evolution. To test this potential, we measured the stable Zr isotope composition of 70 single zircon and baddeleyite crystals from a well-characterized gabbroic igneous cumulate. We show that (i) closed-system magmatic crystallization can fractionate Zr stable isotopes at the >0.5% level, and (ii) zircon and baddeleyite are isotopically heavy relative to the melt from which they crystallize, thus driving chemically differentiated liquids toward isotopically light compositions. Because these effects are contrary to first-order expectations based on mineral-melt bonding environment differences, Zr stable isotope fractionation during zircon crystallization may not solely be a result of closed-system thermodynamic equilibrium.

Nitrogen isotope fractionation in a continuous culture system containing phytoplankton and blue mussels

An experiment was conducted to examine the fractionation of nitrogen stable isotopes in a continuous culture system containing field collected estuarine phytoplankton and blue mussels, Mytilus edulis. Nitrate and phosphate were added to culture vessels at concentrations above ambient levels and nitrogen isotope ratios (δ15N) were measured in particulate matter (PM) and blue mussels over the course of the 15-day experiment. The added nutrients resulted in large productivity and chlorophyll increases in the system. Study results indicate that rapid and significant nitrogen isotope fractionation can occur during incorporation by phytoplankton grown under conditions of excess dissolved inorganic nitrogen, as shown by δ15N values depleted by as much as 9‰ in PM from the higher nutrient treatments. These lower δ15N values were also reflected in mussels exposed to culture vessels effluents. Therefore, nitrogen concentration needs to be considered when using δ15N values in biota as indicators of anthropogenic nitrogen inputs.

Lower hydrogen flux leads to larger carbon isotopic fractionation of methane and biomarkers during hydrogenotrophic methanogenesis

The isotopic composition of lipid biomarkers and biomass in sedimentary environments are widely used to infer microbial metabolisms and constrain carbon cycling processes. It has been observed that metabolic energy availability in the form of H2 impacts the stable carbon isotopes of CH4 during hydrogenotrophic methanogenesis, but it is unknown whether this relationship extends to lipids and amino acids. Since lipids and amino acids are long-lived, they can be used to reconstruct past conditions over geologic timescales. Therefore, the controls on their isotopic signatures are important to constrain to better interpret past environments.In this study, the isotopic distributions of carbon metabolized and reduced by the hyperthermophile Methanocaldococcus jannaschii were quantified following growth at 82 °C in a chemostat with high (80–83 µM) and low (15–27 µM) H2 concentrations. As has been shown previously, the stable carbon isotope fractionation factors for CH4 were >15‰ larger in low H2 experiments than in high H2 experiments. Lipid biomarkers and amino acids were similarly impacted with approximately 10‰ larger fractionation factors under low H2 conditions. The increase in fractionation factors can be related to the lower availability of thermodynamic energy, suggesting that even larger fractionation factors would be observed in methanogens living close to their threshold energy needs, as they do in most environments. The resulting isotopic signatures of long-lived lipid biomarkers synthesized by hydrogenotrophic methanogens may become as ‘superlight’ as those synthesized by archaea carrying out the anaerobic oxidation of methane. These results help to describe the underlying mechanisms that determine the isotopic composition of long-lived biomarkers and provide constraints for interpreting these signatures in the environment.

Evaluating biogeochemical indicators of methanogenic conditions and thermodynamic constraints in peat

Burial of organic matter in deep peat deposits has already been experimentally demostrated to slow down or even inhibit anaerobic decomposition due to lack of diffusive transport and end-product accumulation. However, so far little is known about potential biogeochemical or thermodynamic indicators for the observed inhibition of further decomposition. For example, theoretical energy yields for methanogenesis, hydrogen partial pressures, stable isotope fractionation factors between CO2 and CH4, and electrochemical properties of dissolved organic matter have been proposed as thermodyamic indicators for such inhibition. To test the applicability and explanatory power of these indicators to identify conditions inhibiting organic matter decomposition, we incubated homogenized ombrotrophic peat for 300 days at 20 °C under diffusive flux conditions as control, and compared the observed effects to a treatment with vertical advective transport by water circulation and to a treatment in which both the unsaturated and water-saturated zone of the peat profile were kept anoxic. Results of energy yields of acetoclastic and hydrogenotrophic methanogenesis were compared to hydrogen partial pressures, to 13C isotope fractionation factors and to redox properties of dissolved organic matter as obtained from mediated electrochemical oxidation and reduction. While CO2 and CH4 production slowed substantially in the deep peat profile, a concomitant decrease of Gibs free energy yields available to hydrogenotrophic and acetoclastic methanogensis and hydrogen and acetate concentrations over time supported a thermodynamic constraint on methanogenesis. Although, energy yields for the hydrogenotrophic pathway were close to or below the theoretical energy minimum levels already after 15 days. Transiently elevated H2 concentrations, not related to actual methanogenesis rates were observed for about 150–225 days. Thereafter, hydrogen concentrations diminished to levels below thresholds to thermodynamically support ongoing methanogenesis. Thus even on incubation timescales of 150–225 days, steady-state hydrogen concentrations as would be expected from thermodynamic considerations did not adjust on the bulk scale of observation. Gibbs free energy estimates for methanogenesis based on hydrogen partial pressures were consequently biased and did not reach the minimum required threshold despite ovious net CH4 production. Ratios between electron accepting (EAC) and donating (EDC) capacity of dissolved organic matter, however, turned out to provide suitable indicators of predominant redox conditions along gradients, stabilizing at low values upon onset of methanogenesis. Thus, our study demonstrated that the thermodynamically driven slow down of decomposition in deep peat deposits, preventing the peat to decompose further, cannot be easily identified based on a single indicator. However, constant and high concentrations of decomposition end-products, indicating zero net turnover and low energy yields, and constantly low EAC/EDC ratios, indicating no further availability of terminal electron acceptors, seem to be characteristic of the onset of conditions inhibiting further significant decomposition of peat.

Influence of physical and chemical hydrology on bioremediation of a U-contaminated aquifer informed by reactive transport modeling incorporating 238U/235U ratios

Microbially-catalyzed reductive immobilization of aqueous uranium (U) as a solid phase has been proposed as a U remediation technique. Both laboratory and field experiments have demonstrated that this reduction reaction alters the 238U/235U ratio, producing a 238U-enriched U(IV) solid. In contrast, other major U reactive transport processes fractionate these isotopes much less. This suggests the potential to quantify the extent of bioreduction occurring in groundwater containing U using the 238U/235U ratio, which would substantially improve upon current practices largely relying on U concentration measurements alone. Current reactive transport models for uranium dynamics include only concentration measurements, which are strongly influenced by highly coupled reactive transformation and aqueous transport processes. The complex physical and chemical behavior of U potentially compromises such quantitative analysis of U storage and release. Here we report the first numerical reactive transport model which explicitly incorporates variations in the 238U/235U ratio of U and demonstrates improved interpretation of the principal chemical reactions and groundwater transport processes affecting the subsurface mobility and distribution of this widespread contaminant.Recent U bioreduction studies performed in a contaminated aquifer in Rifle, Colorado, USA applied Rayleigh distillation models to interpret U stable isotope fractionation observed as a result of acetate amendment. These simplified models were unable to resolve the spatiotemporal pattern of U isotope fractionation recorded in the aqueous solutes. Here, we employ the multi-component, isotope-enabled CrunchTope reactive transport software to interpret these measured U isotope ratios, and demonstrate accurate reproduction of observed trends in both geochemistry and 238U/235U ratios for two consecutive years of field experiments. Our results indicate that accurately modeling both the U concentration and isotope ratio distributions greatly constrains the parameter space of the model. We find that the transport properties of U in the Rifle aquifer are governed by the presence of low-permeability regions, which the isotopes are uniquely sensitive to. When U reduction is spatially constrained by these low-permeability regions, the shift in the 238U/235U ratio becomes more muted. Accurate modeling of observed U isotope ratios thus provides a powerful means to better understand bioremediation, and the current study serves to advance the application of this novel method.

Sn isotope fractionation during volatilization of Sn(IV) chloride: Laboratory experiments and quantum mechanical calculations

Volatilization is an important pathway of element transport in nature, and this process may be associated with stable isotope fractionation, which could be used to understand the elemental volatilization mechanisms. In this study, we report that evaporation of Sn(IV) chloride solution under experimental conditions (96 °C, 1 atmospheric pressure) results in significant loss of aqueous Sn(IV) and Sn stable isotope fractionation. The δ122/116Sn of the residue solution can increase by up to 0.50‰ (or a 0.33‰ increase in δ122/118Sn) after repeated evaporation, indicating preferential partitioning of isotopically light Sn species into the vapor phase during evaporation. The observed Sn loss and associated Sn isotope fractionation during the evaporation experiments can be described using a Rayleigh fractionation function, with a best-fitting isotope fractionation factor of −0.36‰ in Δ122/116Snvapor-aq (or −0.24‰ in Δ122/118Snvapor-aq). We also performed quantum mechanical calculations to assess the stability of different potential Sn(IV) species in aqueous and vapor phases, and derived the equilibrium Sn isotope fractionation factors between these Sn(IV) species. The calculation results suggest that the dominant gaseous species of Sn(IV), SnCl4, is isotopically heavier than the aqueous Sn(IV) species by 1.24‰–0.35‰ in δ122/116Sn (or 0.82‰–0.19‰ in δ122/118Sn) under equilibrium at 96 °C, which is opposite to the experimental results. Such contrast in the direction of Sn isotope fractionation implies that kinetic isotope fractionation, rather than thermodynamic equilibrium isotope fractionation, took place for SnCl4 in the evaporation experiments at 96 °C. The observed experimental data can be explained by a kinetic isotope fractionation model involving backward reaction of SnCl4 vaporization at the solution-vapor boundary. This study, in combination with a recent report of positive Δ122/116Snvapor-aq (or Δ122/118Snvapor-aq) factor during evaporation of SnCl4 at 150 °C (Wang et al., 2019a), suggests that Sn(IV) volatilization mechanisms may be different with and without fluid boiling. Combined laboratory experiments and quantum mechanical calculations on the isotopic effects of Sn(IV) chloride solution evaporation provide important constraints for understanding the rapidly accumulating Sn isotopes data from studies on Sn ore-forming processes, bronze metallurgy and archeology, and volatile elements in planetary processes.

Molecular simulations of kinetic stable calcium isotope fractionation at the calcite-aqueous interface

The measurement of calcium stable isotope fractionation during crystal growth can provide insights into the dominant pathways and mechanisms of crystal growth, but these interpretations require a detailed understanding of mass dependent reaction rates and free energy states at the molecular scale. Here we report the results of free energy calculations used to determine the rate-limiting step controlling kinetic stable isotope fractionation during calcium ion attachment to active growth sites – kink sites – at the calcite surface. The free energy profile for attachment is combined with ion coordination states to reconstruct the multi-step molecular pathway for the ion attachment process. Dissociative desolvation of a water molecule coordinating the calcium ion approaching the kink is identified as the rate-limiting step, with a free energy barrier of ∼18kJ/mol, which is equal to the energetic cost of water molecule leaving Ca2+ coordination shell in the aqueous phase. Molecular dynamics simulations were used to investigate the dependence of this desolvation step on calcium ion mass. Both brute force MD and an analysis of mass dependent water exchange coefficients (kwex) give identical kinetic stable isotope fractionation factor values αf44/40=0.992, which corresponds to Δ44/40Ca of ∼ -8‰, originating from a mass-dependent transition frequency for the Ca-bound solvation water at the kink. This value is significantly higher than the fractionation factor associated with water exchange dynamics around Ca2+ in bulk aqueous solution determined here (αf44/40=0.999). Our results confirm that the growing calcite surface should be depleted in 44Ca with respect to the aqueous phase due to kinetically preferred desolvation of the lighter isotope. We conclude that the maximum kinetic stable isotope fractionation associated with calcium attachment to surfaces is not constant but can vary due to changes in the interfacial environment that affect local solvent dynamics during desolvation.

Effect of organic C on stable Fe isotope fractionation and isotope exchange kinetics between aqueous Fe(II) and ferrihydrite at neutral pH

Low-temperature geochemical cycling of Fe, C, nutrients, and toxic metals in nature are largely regulated by the stability and reactivity of fine-grained ferrihydrite. The presence of impurities such as C and Si in ferrihydrite structure are found to inhibit the rapid transformation of ferrihydrite to more stable iron oxides upon interaction with aqueous Fe(II). Therefore, understanding the factors controlling the reactivity of ferrihydrite, especially in the presence of C, is critical to evaluate the role of ferrihydrite in geochemical cycles. Equilibrium stable isotope fractionations are fundamental thermodynamic properties and therefore equilibrium fractionation in 56Fe/54Fe ratios reflects the nature of Fe bonding in ferrihydrite and its reactivity. In this study, we investigated stable Fe isotope fractionation between aqueous Fe(II) and ferrihydrite-organic matter coprecipitates to evaluate whether previously documented inhibition of mineralogical transformation by organic C is accompanied by changes in Fe isotope fractionation. Experiments conducted using Suwannee River natural organic matter (SRNOM) that was coprecipitated with ferrihydrite (molar C:Fe = 1.2) produced an equilibrium 56Fe/54Fe fractionation of −2.36 ± 0.26‰ between aqueous Fe(II) and ferrihydrite-SRNOM coprecipitates. This fractionation factor significantly differs from that previously determined between Fe(II)aq and pure ferrihydrite (−3.20‰) but is similar to that measured for Fe(II)aq-Si-ferrihydrite (−2.58 ± 0.14‰, molar Fe:Si = 1). Furthermore, the addition of C in the ferrihydrite structure at molar C:Fe ∼1.2 markedly increased the extent of Fe isotope exchange to a similar degree as observed with Si-ferrihydrite with molar Fe:Si ∼1. These observations suggest similar effects on both bonding and reactivity of ferrihydrite upon addition of equimolar C and Si. Due to the coexistence of ferrihydrite with organic matter in nature (e.g., wetlands), these results are important for understanding Fe isotope fractionation and exchange kinetics during mineral-fluid interactions in natural ferrihydrite-bearing systems.

Responses of archaeal, bacterial, and functional microbial communities to growth season and nitrogen fertilization in rice fields

Fertilization provides excess N to soil microorganisms, thus possibly affecting soil microbial abundance, activity, and community composition during rice cultivation. The abundance and diversity of archaeal, bacterial, and functional microbial communities in rice soils upon different N fertilization regimes (no fertilizer, urea, and controlled-release fertilizer) were investigated by sampling four seasonal growth stages (seedling, tillering, vegetative, maturing) under field conditions. The abundance of bacteria was significantly affected by fertilization and seasonal time, while that of the archaea was not significantly affected. Analysis of terminal restriction fragment polymorphism (T-RFLP) of 16S rRNA genes showed no effect of N fertilization on the archaeal and bacterial community composition, but changes with plant growth time. This result was confirmed by the patterns of pyrosequencing of bacterial 16S rRNA genes. The function of the methanogenic microbial community was assayed at maturing plant growth stage by determining CH4 production rates and stable isotope fractionation in the absence and presence of methyl fluoride, an inhibitor of acetoclastic methanogenesis. N fertilization had a pronounced effect on the CH4 production rate but not on the pathway of CH4 formation. Additionally, the abundance of functional microbial communities related to CH4 and N2O emissions was measured by qPCR of functional genes. Similarly to the taxonomic composition, rice growth season showed a significant effect on the abundance of the functional microbial communities represented by the mcrA, pmoA, nirK, nirS, and nosZ genes, while N addition had usually no significant effect. A similar result was also obtained by correlation analysis between CH4 and N2O emission rates and abundances of the functional microbial gene copies. In summary, rice growth time had pronounced effects on abundance, composition, and function of microbial communities in the rice soil, while the effect of N fertilization was negligible on the level of both specific functional genes and taxonomic 16S rRNA genes.

Calcium carbonate precipitation mediated by bacterial carbonic anhydrase in a karst cave: Crystal morphology and stable isotopic fractionation

Stable oxygen (δ18O) and carbon (δ13C) isotopes of speleothems are widely used as important proxies for paleoclimatic and paleoenvironment reconstructions. However, the influence of microbial activity on carbon and oxygen isotope fractionation during speleothem precipitation remains unclear. Bacterial carbonic anhydrase (CA) can promote calcium carbonate precipitation to catalyze the mutual transformation between CO2 and HCO3-. CA-producing bacteria (Lysinibacillus sp. strain LHXY2) were separated in the Xueyu Cave, Chongqing, SW China, and used in laboratory and cave in situ models to investigate their influence on the precipitation amount, mineral components, crystal morphology and carbon and oxygen isotope fractionation of CaCO3. A CA activity gradient was applied in the laboratory model by considering various CA inhibitor acetazolamide (AZ) concentrations, which showed that the CA activity could substantially enhance precipitation, alter the mineral components and morphology, and reduce the δ13C and δ18O values of the CaCO3 formed. Most importantly, the laboratory and in situ model results revealed approximately -7‰ and -1.4‰ δ13C shifts, respectively, compared to the bacteria-free model results, which indicated that microbial-driven carbon isotopic fractionation can cause great uncertainties in paleoclimate and paleoenvironment reconstructions.

Stable chromium isotope fractionation during magmatic differentiation: Insights from Hawaiian basalts and implications for planetary redox conditions

The stable isotope compositions of chromium (Cr) are fractionated during magmatic differentiation of lunar mare basalts, which might be attributed to redox-related mineral crystallization. It has yet to be demonstrated whether magmatic differentiation fractionates Cr isotope composition of terrestrial samples. Here, we present high-precision stable Cr isotope measurements, reported as δ53Cr relative to NIST SRM 979, of well-characterized Hawaiian tholeiitic basalts from Koolau, Mauna Kea and Kilauea. The studied Makapuu-stage Koolau lavas have MgO ranging from 6.58 to 21.54 wt.%, and they have homogeneous δ53Cr ranging from −0.21‰ to −0.17‰. Similarly, studied Mauna Kea lavas have MgO ranging from 9.11 to 17.90 wt.%, and they also have homogeneous δ53Cr ranging from −0.17‰ to −0.13‰. Some Makapuu-stage Koolau and Mauna Kea lavas experienced subaerial or submarine alteration. The homogenous δ53Cr within each sample suites implies that the post-magmatic alterations have not significantly changed the Cr isotope compositions of these samples. Conversely, nine Kilauea Iki basalts have MgO ranging from 7.77 to 26.87 wt.% reflecting varying degrees of magmatic differentiation, and they show resolvable Cr isotope variations with δ53Cr ranging from −0.18‰ to 0.00‰. The δ53Cr values of the Kilauea Iki samples are positively correlated with indicators of magmatic differentiation such as Cr and MgO contents, and Mg# values. The most evolved samples have the lightest isotope compositions, whereas the olivine-spinel cumulates display complementary heavy isotope compositions. This fractionation is most likely generated by the crystallization and accumulation of spinel, which is dominated by Cr3+ and, hence, enriched in heavier Cr isotopes relative to the residual melt. At a given MgO content, Kilauea and Mauna Kea lavas, both Kea-trend volcanoes, have higher δ53Cr than Makapuu-stage Koolau lavas, a Loa-trend volcano. This difference might reflect recycling of altered oceanic crusts or redox differences of their magmatic sources, with the mantle source of Makapuu-stage lavas being more reducing.To understand the different Cr isotope fractionation behaviors of terrestrial and extraterrestrial basalts, we present a quantitative model that relates the Cr isotope compositions of basalts from the Earth, the Moon and Vesta, to the crystallization assemblage, the degree of fractional crystallization, and the Cr2+/ΣCr ratios of minerals and melts, which are related to the oxygen fugacity during differentiation. The primitive Hawaiian basaltic magma for Kilauea Iki and Mauna Kea lavas is estimated to have δ53Cr of −0.15‰, which is close to the average value of the BSE (−0.14‰ to −0.12‰). We further speculate that the initial lunar mantle is relatively homogeneous with BSE-like isotope composition (−0.16‰ to −0.09‰). The observed low δ53Cr in lunar mafic rocks is the result of redox-dominated fractional crystallization and accumulation processes of lunar mafic magmas. These magmas might be derived from variable degrees of partial melting of the primitive lunar mantle.Combined with previous results on the variations in Cr valences and contents in silicate melts and minerals related to oxygen fugacity, Cr concentration and isotope composition can serve as a useful oxybarometer for understanding the redox conditions of planetary differentiation and magmatic evolution.

Determination of Gallium Isotopic Compositions in Reference Materials

The interest in the study of gallium (Ga) stable isotope fractionation in low‐ and high‐temperature environments has increased significantly in the last few years. However, a unified reference material (RM) is still lacking for the Ga isotope research community, which hinders interlaboratory comparison between different groups. Consequently, certification of Ga isotopic reference materials for interlaboratory comparison is of high priority. In this study, Ga isotope ratio data for ten geological RMs including silicates, shales and ferromanganese nodules, and two pure Ga RMs including NIST SRM 994 and NIST SRM 3119a reported by three different groups, were determined by MC‐ICP‐MS. Sample matrices of geological RMs were separated by a two‐column separation method with the use of AG MP‐1M and AG 50‐X8 resin, separately, and quantitative recoveries of > 99% Ga were obtained for all geological RMs. Instrumental mass bias was corrected by the combined calibrator‐sample bracketing and internal normalisation model. Validation of the proposed method was performed by analysing synthetic solutions. After normalisation of all available δ71Ga data of geological RMs to a single Ga RM, results obtained in our study are in agreement with previously reported results.

Stable Isotope Fractionation by Transport and Transformation

Stable Isotope Fractionation by Transport and Transformation

Stable Isotope Trophic Fractionation (13C/12C and 15N/14N) in Mycophagous Diptera Larvae

The use of isotopic analysis for reconstructing the structure of food webs requires determination of the trophic fractionation of carbon and nitrogen stable isotopes (Δ13C and Δ15N). Fungi and mycophagous animals play a key role in soil communities, but there are very limited field data on the degree of isotope fractionation in animals that feed on fungi. We studied the bulk isotopic composition of mycetophagous Diptera larvae inhabiting fruit bodies of saprotrophic and mycorrhizal macromycetes, as well as larva feeding on parasitic rust fungi. Trophic enrichment in 13С and 15N was at the minimum (0.0 and 0.9‰, respectively) in the larvae of gall midges Mycodiplosis sp. feeding on rust fungi (Pucciniales). In the larvae of dipterans inhabiting fruiting bodies of saprotrophic and mycorrhizal macromycetes, the Δ13C and Δ15N values averaged 0.9 and 3.4‰, respectively. This corresponds to the values usually observed in grazing food chains. The accumulation of 15N was more pronounced in the larvae that fed on saprotrophic fungi, but no clear relationship was found between the degree of trophic fractionation and the taxonomic affiliation of animals or fungi. As suggested by our data and the analysis of published studies, the variations in the trophic fractionation in mycophages are strong, but they are not likely to impede the identification of the “mycorrhizal” and “saprotrophic” energy channels in the soil food webs.

Assessment of hexachlorcyclohexane biodegradation in contaminated soil by compound-specific stable isotope analysis

Compound-specific isotope analysis (CSIA) was firstly applied to explore the biodegradation of hexachlorcyclohexane (HCH) isomers in contaminated soil. Concentrations and compound-specific carbon isotope ratio profiles of HCH in different specific ex-situ pilot-scale contaminated soil mesocosms were determined. The addition of nutrients and Sphingobium spp. significantly enhanced the degradation of HCH in contaminated soils within 90 days. Isomer specific biodegradation of HCHs was observed with α- and γ-HCH being more degradable than β and δ-HCH. Stable carbon isotope fractionation of HCH was observed and the δ13C values shifted from −28.8 ± 0.3‰ to −24.8 ± 0.7‰ upon 87.3% removal, −27.9 ± 0.2‰ to −25.9 ± 0.5‰ upon 72.8% removal, −29.4 ± 0.3‰ to −19.9 ± 0.6‰ upon 95.8% removal, and −27.8 ± 0.5‰ to −23.6 ± 0.7‰ after 96.9% removal for α, β, γ, and δ-HCH, respectively. Furthermore, the enrichment factor ε for α, β, γ, and δ-HCH biodegradation in soil was obtained for the first time as −2.0‰, −1.5‰, −3.2‰, and −1.4‰, which could play a critical role in assessing in situ biodegradation of HCH isomers in field site soil. Results from ex-situ pilot-scale experiments clearly demonstrated that CSIA could be a promising tool to qualitatively and quantitatively evaluate in situ biodegradation of HCH in contaminated field site.

Isotope fractionation approach to characterize the reactive transport processes governing the fate of hexachlorocyclohexanes at a contaminated site in India

The transformation processes of hexachlorocyclohexane isomers (HCHs) from production sites of Lindane across the landscape and along the food web were studied as an example to understand the fate of POPs in the environment. Therefore, we studied the concentration and isotope composition of HCHs in different matrices in the vicinity of a dumpsite and a chemical plant producing HCHs in India. Carbon isotope compositions (δ13C) of HCHs and the enantiomer fraction (EF) of α-HCH were used as indicators to characterize in situ degradation in soil, groundwater, and sediment as well as along the food web. The HCHs were detected in plants growing on contaminated soil. Elevated concentrations of HCHs were found in a number of crops, which indicates an important transfer pathway of HCHs entering food webs. The EF value of α-HCH and the δ13C signature of HCHs indicated that degradation processes occurred in the rhizosphere or within the plants potentially attenuating the contamination of HCHs. The isotope enrichment of HCHs in dung and milk samples showed that degradation of HCHs may take place in the digestive track of cow and buffalo as well as during their metabolism. The δ13C of HCHs was used to analyze the potential dispersion routes on the landscape scale in order to understand the reactive transport pathways starting at the source of HCHs. In this study, the potential of carbon isotope fractionation and EF for characterizing uptake of HCHs into plants and accumulation in the food web were examined. To the best of our knowledge, this is the first study using the combination of stable isotope fractionation and EF to track the reactive transport processes in a complex environment including the food web.

Key Factors of Precipitation Stable Isotope Fractionation in Central-Eastern Africa and Central Mediterranean

The processes of isotope fractionation in the hydrological cycle naturally occur during vapor formation, vapor condensation, and moisture transportation. These processes are therefore dependent on local and regional surface and atmospheric physical features such as temperature, pressure, wind speed, and land morphology, and hence on the climate. Because of the strong influence of climate on the isotope fractionation, latitudinal and altitudinal effects on the δ18O and δ2H values of precipitation at a global scale are observed. In this study, we present and compare the processes governing precipitation isotope fractionation from two contrasting climatic regions: Virunga in Central-Eastern Africa and the Central Mediterranean (Stromboli and Sicily, Italy). While Virunga is a forested rainy tropical region located between Central and Eastern Africa, the Mediterranean region is characterized by a rainy mild winter and a dry hot summer. The reported δ18O and δ2H dataset are from precipitation collected on rain gauges sampled either on a monthly or an approximately bimonthly basis and published in previous papers. Both regions show clearly defined temporal and altitudinal variations of δ18O and δ2H, depending on precipitation amounts. The Central Mediterranean shows a clear contribution of local vapor forming at the sea–air interface, and Virunga shows a contribution from both local and regional vapor. The vapor of Virunga is from two competing sources: the first is the continental recycled moisture from soil/plant evaporation that dominates during the rainy season, and the second is from the East African Great Lakes evaporation that dominates during the dry season.

Isotope-based water-use efficiency of major greening plants in a sponge city in northern China.

To tackle urban water issues, the Chinese government has promoted the construction of sponge cities in recent years. Thirty cities have been designated as experimental sites to serve as models for future sponge city construction, as more than 80% of the built-up urban areas in China must reach the standards of sponge cities by 2030. Greening plants play an important role in sponge cities, and water-use efficiency (WUE) is a vital index to determine whether plants could adapt to and grow healthily in environments with water deficits. In this study, WUE of greening plants was quantified by measuring the stable carbon isotope fractionation. Suitable plants for the green spaces in Guyuan sponge city, in northern China, were selected based on their WUE, and the main factors affecting WUE were studied in four habitats within the city. Plant species identity had the greatest effect on WUE, while habitat and plant life form had lower effect, illustrating that WUE is a relatively stable and reliable index for the classification of plant species. We can improve the WUE and ecological function of green spaces in sponge cities by using isotope technology to select suitable plant species with high WUE. To our knowledge, this study is the first to select plant species for sponge city by using this method, providing a quick and scientific method for the selection of greening plants for future sponge cities.