Variability In Isotope Fractionation Research Articles

Individual growth as a non‐dietary determinant of the isotopic niche metrics

Abstract Quantitative analytical approaches for isotopic niche analysis in the trophic diversity studies are proliferating rapidly; however, the assumptions behind the isotopic niche applications are rarely tested. One of the main assumptions is independence of the niche metrics and physiological status of the animals. The aim of this experimental study was to test the relationship between growth and Layman's metrics of isotopic niche in consumers eating the same food but in different quantities and growing at different rates. Based on research indicating that individual variability in isotopic fractionation increases under suboptimal conditions, I hypothesized that a group of consumers originating from the same population and exposed to food limitation would have greater estimates of the niche breadth and diversity as a result of higher inter‐individual variability in growth rate and stable isotope signatures than in non‐limiting feeding conditions. This hypothesis was tested using Baltic mysids Neomysis integer fed isotopically uniform diet under laboratory conditions and analysing individual growth and stable isotope signature in concert. As hypothesized, mysid growth rate was a significant negative predictor of most niche metrics. These effects were related to both increased inter‐individual variability and higher trophic fractionation in malnourished animals, whereas in actively growing mysids, lower fractionation was observed together with lower values for niche size, isotopic range and trophic diversity. These findings challenge how we interpret the intrapopulation isotopic variance and evaluate isotopic evidence of individual specialization and call for integrated approaches for isotopic niche and growth assessment.

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 (εC = −1.3 ± 0.2‰) compared to D. hafniense (εC = −7.7 ± 1.5‰). However, higher fractionation was observed for dibromoethene (DBE) dehalogenation by S. multivorans (εC = −16.8 ± 1.8‰ and −21.2 ± 1.6‰ for trans- and cis-1,2- (DBE) respectively), compared to D. hafniense PCE-S (εC = −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 (εBr = −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 (εBr = −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 (εC/εBr) = 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.

Effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of Geysir, Iceland

Detectible δ30Si variations in present-day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents. Interpretations of δ30Si signatures remain problematic in view of potential post-depositional changes, of which magnitudes and underlying mechanisms are largely unknown. A critical issue in the interpretation of isotope data from cherts concerns the extent to which early-diagenetic processes modify original δ30Si signatures. Here, we report δ30Si variations in opal-A, opal-A/CT and opal-CT from fossil sinter deposits in an active discharge apron in the Geysir geothermal field, Iceland. Opal-A samples show an average δ30Si of −0.7±0.2‰, while opal-CT samples are isotopically lighter with an average δ30Si of −2.0±0.4‰, implying a sizable shift of approximately 1.3‰ between the different phases. This shift can be explained by repetitive dissolution/re-precipitation processes, diffusive transport or temperature differences during phase transitions. On average, the fossil opal-A tends to be less negative in δ30Si than amorphous silica that recently precipitated from the hydrothermal water. The difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface, or to a post-depositional release of surface 28Si at the onset of diagenetic formation. Our results corroborate the perception that original silicon isotope signatures of silica, acquired during chemical precipitation from a saturated solution, may not be preserved in the geological record, and that post-depositional changes must be taken into account when interpreting data from ancient chert deposits.

Effects of inorganic anions on carbon isotope fractionation during Fenton-like degradation of trichloroethene

Understanding the magnitude and variability in isotope fractionation with respect to specific processes is crucial to the application of stable isotopic analysis as a tool to infer and quantify transformation processes. The variability of carbon isotope fractionation during Fenton-like degradation of trichloroethene (TCE) in the presence of different inorganic ions (nitrate, sulfate, and chloride), was investigated to evaluate the potential effects of inorganic anions on carbon isotope enrichment factor (ε value). A comparison of ε values obtained in deionized water, nitrate solution, and sulfate solution demonstrated that the ε values were identical and not affected by the presence of nitrate and sulfate. In the presence of chloride, however, the ε values (ranging from −6.3±0.8 to 10±1.3‰) were variable and depended on the chloride concentration, indicating that chloride could significantly affect carbon isotope fractionation during Fenton-like degradation of TCE. Thus, caution should be exercised in selecting appropriate ε values for the field application of stable isotope analysis, as various chloride concentrations may be present due to naturally present or introduced with pH adjustment and iron salts during Fenton-like remediation. Furthermore, the effects of chloride on carbon isotope fractionation may be able to provide new insights about reaction mechanisms of Fenton-like processes.

Tracking transformation processes of organic micropollutants in aquatic environments using multi-element isotope fractionation analysis

The quantitative description of enzymatic or abiotic transformations of man-made organic micropollutants in rivers, lakes, and groundwaters is one of the major challenges associated with the risk assessment of water resource contamination. Compound-specific isotope analysis enables one to identify (bio)degradation pathways based on changes in the contaminants’ stable isotope ratios even if multiple reactive and non-reactive processes cause concentrations to decrease. Here, we investigated how the magnitude and variability of isotope fractionation in some priority pollutants is determined by the kinetics and mechanisms of important enzymatic and abiotic redox reactions. For nitroaromatic compounds and substituted anilines, we illustrate that competing transformation pathways can be assessed via trends of N and C isotope signatures.

Apparent seasonal cycle in isotopic discrimination of carbon in the atmosphere and biosphere due to vapor pressure deficit

We explore seasonal variability in isotopic fractionation by analyzing observational data from the atmosphere and the biosphere, as well as simulated data from a global model. Using simulated values of atmospheric CO2 and its carbon isotopic composition, we evaluated different methods for specifying background concentrations when calculating the isotopic signature of source CO2 (δs) to the atmosphere. Based on this evaluation, we determined that free troposphere measurements should be used when available as a background reference when calculating δs from boundary layer observations. We then estimate the seasonal distribution of δs from monthly climatologies derived from several atmospheric sampling sites. This approach yields significant seasonal variations in δs with more enriched values during the summer months that exceed the uncertainty of δs estimated for any given month. Intra‐annual measurements of δ13C in the cellulose of Pinus taeda growing in the southeastern U.S. also reveal seasonal isotopic variations that are consistent in phase but not necessarily amplitude with atmospherically derived estimates of δs. Coherent seasonal patterns in δs inferred from the atmosphere and observed in the biosphere were not consistent with the seasonal isotopic discrimination simulated by a commonly used biosphere model. However, δs seasonality consistent with observations from the atmosphere and biosphere was retrieved with a revised biosphere model when stomatal conductance, and thus isotopic discrimination, was allowed to vary in response to vapor pressure deficit rather than relative humidity. Therefore, in regions where vapor pressure deficit and relative humidity are positively covariant over the growth season, such as the sub‐tropics, different stomatal conductance models may yield very different estimates of CO2 and H2O exchange between the biosphere and atmosphere.

Stable carbon isotope fractionation during degradation of dichloromethane by methylotrophic bacteria

Stable carbon isotope fractionation during dichloromethane (DCM) degradation by methylotrophic bacteria was investigated under aerobic and nitrate-reducing conditions. The strains studied comprise several Hyphomicrobium strains, Methylobacterium, Methylopila, Methylophilus and Methylorhabdus spp. that are considered to degrade DCM by a glutathione (GSH)-dependent dehalogenase enzyme system in the initial step. The stable carbon isotope fractionation factors (alphaC) of the strains varied under aerobic conditions between 1.043 and 1.071 and under nitrate-reducing conditions between 1.048 and 1.065. Comparison of isotope fractionation under aerobic and nitrate-reducing conditions by individual strains revealed only minor to no differences. The variability in isotope fractionation among strains was found to be related to the polymorphism of the functional genes encoding the DCM dehalogenase.

Wavelength dependence of isotope fractionation in N<sub>2</sub>O photolysis

Abstract. In previous reports on isotopic fractionation in the ultraviolet photolysis of nitrous oxide (N2O) only enrichments of heavy isotopes in the remaining N2O fraction have been found. However, most direct photolysis experiments have been performed at wavelengths far from the absorption maximum at 182 nm. Here we present high-precision measurements of the 15N and 18O fractionation constants (ε) in photolysis at 185 nm. Small, but statistically robust depletions of heavy isotopes for the terminal atoms in the linear N2O molecule are found. This means that the absorption cross sections σ(15N 14N 16O) and σ(14N218O) are larger than σ(14N216O) at this specific wavelength. In contrast, the central N atom becomes enriched in 15N. The corresponding fractionation constants (±1 standard deviation) are 15ε1 = σ(15N 14N 16O)/σ(14N2 16O) - 1 = (3.7±0.2) %o 18ε = σ(14N218O)/σ(14N216O) - 1 = (4.5±0.2) %o and 15ε2 = σ(14N 15N 16O)/σ(14N216O) - 1 = (-18.6±0.5) %o To our knowledge, this is the first documented case of such a heavy isotope depletion in the photolysis of N2O which supports theoretical models and pioneering vacuum ultraviolet spectroscopic measurements of 15N substituted N2O species that predict fluctuations of ε around zero in this spectral region (Selwyn and Johnston, 1981). Such a variability in isotopic fractionation could have consequences for atmospheric models of N2O isotopes since actinic flux varies also strongly over narrow wavelength regions between 175 and 200 nm due to the Schumann-Runge bands of oxygen. However, the spacing between maxima and minima of the fractionation constants and of the actinic flux differ by two orders of magnitude in the wavelength domain. The wavelength dependence of fractionation constants in N2O photolysis can thus be approximated by a linear fit with negligible consequences on the actual value of the spectrally averaged fractionation constant. In order to establish this linear fit, additional measurements at wavelengths other than 185 nm were made using broadband light sources, namely D2, Hg/Xe and Sb lamps. The latter lamp was used in conjunction with various interference filters to shift the peak photolysis rate to longer wavelengths. From these experiments and existing data in the literature, a comprehensive picture of the wavelength dependence of N2O photolysis near room-temperature is created.

Assessment of Bacterial Degradation of Aromatic Hydrocarbons in the Environment by Analysis of Stable Carbon Isotope Fractionation

13C/12C stable carbon isotope fractionation was used to assess biodegradation in contaminated aquifers with toluene as a model compound. Different strains of anaerobic bacteria (Thauera aromatica, Geobacter metallireducens, and the sulfate-reducing strain TRM1) showed consistent 13C/12C carbon isotope fractionation with fractionation factors between αC = 1.0017 and 1.0018. In contrast, three cultures of aerobic organisms, using different mono- and dioxygenase enzyme systems to initiate toluene degradation, showed variable isotope fractionation factors of αC = 1.0027 (Pseudomonasputida strain mt-2), αC = 1.0011 (Ralstonia picketii), andαC = 1.0004 (Pseudomonas putida strain F1). The great variability of isotope fractionation between different aerobic bacterial strains suggests that interpretation of isotope data in oxic habitats can only be qualitative. A soil column was run as a model system for contaminated aquifers with toluene as the carbon source and sulfate as the electron acceptor and samples were taken at different ports along the column. Microbial toluene degradation was calculated based on the 13C/12C isotope fractionation factors of the batch culture experiments together with the observed 13C/12C isotope shifts of the residual toluene fractions. The calculated percentage of biodegradation, B, correlated well with the decreasing toluene concentrations at the sampling ports and indicated the increasing extent of biodegradation along the column. The theoretical toluene concentrations as calculated based on the isotope values matched the measured concentrations at the different sampling ports indicating that the Rayleigh equation can be used to calculate biodegradation in quasi closed systems based on measured isotope shifts. A similar attempt was performed to assess toluene degradation in a contaminated, anoxic aquifer. A transect of groundwater wells was monitored along the main direction of the groundwater flow and revealed decreasing concentrations accompanied with an increase in the 13C/12C stable carbon isotope ratio of the residual toluene. Calculation of the extent of biodegradation based on the isotope values and laboratory derived isotope fractionation factors showed that the residual toluene was degraded to more than 99% by microbial activity. Calculation of the theoretical residual toluene concentrations based on the measured isotope values described the strongly decreasing concentrations along the plume. Other aromatic hydrocarbons like benzene and naphthalene which were analysed in the same course also showed decreasing concentrations along the groundwater flow path accompanied by increasing δ13C values indicating biodegradation.

Total energy expenditure in intravenously fed patients measured by the doubly labeled water technique

The potential clinical use of the doubly labeled water (DLW) technique to measure total energy expenditure (TEE) in intravenously fed patients was assessed by applying the technique to 13 stable patients receiving total parenteral nutrition (TPN). TEE was compared with resting energy expenditure (REE), which was measured using a ventilated hood (n = 13), and with 24-hour energy expenditure (24h EE) measured in a whole-body calorimeter (n = 6). Daily measurements of urinary enrichment in 2H and 18O showed predominantly small and covariant fluctuations in elimination rates of these two tracers. The standard error of the estimate (SE FCO2) for the CO 2 production rate (F CO2), which includes additional errors due to variability in isotopic fractionation, was calculated to be 2.65% ± 0.70% of F CO2. TPN-related changes in background isotopic enrichment in subjects who were not dosed with isotope virtually ceased 10 days after starting intravenous feeding. TEE (4.99 ± 1.10 kJ/min), TEE REE (1.21 ± 0.16 kJ/min), TEE/basal metabolic rate ([BMR] 1.31 ± 0.16 kJ/min), and TEE (isotopic)/24h EE (calorimetric; 1.07 ± 0.18 kJ/min) showed wide variability. This study provides some evidence for the validity of the DLW method in selected patients receiving TPN, since significant deviations from the assumptions of the method were not found in the subjects studied. The error associated with 2H sequestration during fatty acid synthesis is likely to be small, but could not be accurately measured. The contributions of activity and thermogenesis to TEE varied greatly, but overlapped with those reported in normal subjects.