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PubMed2006

Using continuous stable isotope measurements to partition net ecosystem CO2 exchange.

Ecosystem-scale estimation of photosynthesis and respiration using micrometeorological techniques remains an important, yet difficult, challenge. In this study, we combined micrometeorological and stable isotope methods to partition net ecosystem CO2 exchange (FN) into photosynthesis (F(A)) and respiration (F(R)) in a corn-soybean rotation ecosystem during the summer 2003 corn phase. Mixing ratios of (12)CO2 and (13)CO2 were measured continuously using tunable diode laser (TDL) absorption spectroscopy. The dynamics of the isotope ratio of ecosystem respiration (R), net ecosystem CO2 exchange (deltaN) and photosynthetic discrimination at the canopy scale (delta canopy) were examined. During the period of full canopy closure, F(N) was partitioned into photosynthesis and respiration using both the isotopic approach and the conventional night-time-derived regression methodology. Results showed that deltaR had significant seasonal variation (-32 to -11% per hundred) corresponding closely with canopy phenology. Daytime deltaN typically varied from -12 to -4% per hundred, while delta canopy remained relatively constant in the vicinity of 3% per hundred. Compared with the regression approach, the isotopic flux partitioning showed more short-term variations and was considerably more symmetric about F(N). In this experiment, the isotopic partitioning resulted in larger uncertainties, most of which were caused by the uncertainties in deltaN. and the daytime estimate of deltaR. By sufficiently reducing these uncertainties, the tunable diode laser (TDL)-micrometeorological technique should yield a better understanding of the processes controlling photosynthesis, respiration and ecosystem-scale discrimination.

Carbon Dioxide↗
PubMed1990

Deuterium D(V/K) isotope effects on ethanol oxidation in hepatocytes: importance of the reverse ADH-reaction.

The kinetic deuterium isotope effect, D(V/K), on ethanol oxidation was measured on hepatocytes from rat and pig by the radiometric competitive method using 14C-labelled ethanol containing deuterium in the (1-R)-position. The corrected D(V/K) values of 2.68 and 2.80 for rat and pig hepatocytes respectively were significantly different, suggesting differences in the amount of non-ADH ethanol oxidizing activity. The apparent isotope effects declined rapidly with time when acetaldehyde was present in the medium as a result of the reduction to ethanol of the [14C]-acetaldehyde formed from the double labelled ethanol by alcohol dehydrogenase (ADH). Fructose and cyanamide caused the acetaldehyde concentration during ethanol oxidation to increase by entirely different mechanisms, and the isotope effect to decrease with time, as did also the addition of acetaldehyde. The apparent first order rate constant for the reverse ADH reaction, assuming the reactants to be acetaldehyde and the ADH-NADH complex, was determined by two methods giving comparable results. In the presence of semicarbazide, which removes acetaldehyde, the isotope effect was nearly constant. This was the case also when the acetaldehyde concentration was very low (less than 1 microM) for other reasons, as in hepatocytes from starved animals. A mathematical formula describing the expected decrease of the apparent isotope effect with time was derived. The different response of pig and rat hepatocytes to addition of fructose (the 'fructose effect') is suggested to be caused by differences in activity of aldehyde dehydrogenases in the two species.

Acetaldehyde↗
PubMed2002

Natural iron isotope variations in human blood.

Isotopic analysis of human blood and liver and muscle tissue indicates that each individual bears a long-term iron (Fe) isotope signature in the blood. Blood and tissue differ slightly in isotopic composition and are depleted by up to 2.6 per mil in 56Fe relative to 54Fe when compared to dietary Fe. The 56Fe/54Fe isotope ratio in the blood of males is, on average, lower by 0.3 per mil than that of females. These results suggest that Fe isotope effects in the blood reflect differences in intestinal Fe absorption between individuals and genotypes.

Adult↗
PubMed1987

Natural abundances of carbon isotopes in acetate from a coastal marine sediment.

Measurements of the natural abundances of carbon isotopes were made in acetate samples isolated from the anoxic marine sediment of Cape Lookout Bight, North Carolina. The typical value of the total acetate carbon isotope ratio (delta 13C) was -16.1 +/- 0.2 per mil. The methyl and carboxyl groups were determined to be -26.4 +/- 0.3 and -6.0 +/- 0.3 per mil, respectively, for one sample. The isotopic composition of the acetate is thought to have resulted from isotopic discriminations that occurred during the cycling of that molecule. Measurements of this type, which have not been made previously in the natural environment, may provide information about the dominant microbial pathways in anoxic sediments as well as the processes that influence the carbon isotopic composition of biogenic methane from many sources.

Acetates↗
PubMed1994

Carbon isotope effects associated with aceticlastic methanogenesis.

The carbon isotope effects associated with synthesis of methane from acetate have been determined for Methanosarcina barkeri 227 and for methanogenic archaea in sediments of Wintergreen Lake, Michigan. At 37 degrees C, the 13C isotope effect for the reaction acetate (methyl carbon) --> methane, as measured in replicate experiments with M. barkeri, was - 21.3% +/- 0.3%. The isotope effect at the carboxyl portion of acetate was essentially equal, indicating participation of both positions in the rate-determining step, as expected for reactions catalyzed by carbon monoxide dehydrogenase. A similar isotope effect, - 19.2% +/- 0.3% was found for this reaction in the natural community (temperature = 20 degrees C). Given these observations, it has been possible to model the flow of carbon to methane within lake sediment communities and to account for carbon isotope compositions of evolving methane. Extension of the model allows interpretation of seasonal fluctuations in 13C contents of methane in other systems.

Acetates↗
PubMed2001

Diversity of sulfur isotope fractionations by sulfate-reducing prokaryotes.

Batch culture experiments were performed with 32 different sulfate-reducing prokaryotes to explore the diversity in sulfur isotope fractionation during dissimilatory sulfate reduction by pure cultures. The selected strains reflect the phylogenetic and physiologic diversity of presently known sulfate reducers and cover a broad range of natural marine and freshwater habitats. Experimental conditions were designed to achieve optimum growth conditions with respect to electron donors, salinity, temperature, and pH. Under these optimized conditions, experimental fractionation factors ranged from 2.0 to 42.0 per thousand. Salinity, incubation temperature, pH, and phylogeny had no systematic effect on the sulfur isotope fractionation. There was no correlation between isotope fractionation and sulfate reduction rate. The type of dissimilatory bisulfite reductase also had no effect on fractionation. Sulfate reducers that oxidized the carbon source completely to CO2 showed greater fractionations than sulfate reducers that released acetate as the final product of carbon oxidation. Different metabolic pathways and variable regulation of sulfate transport across the cell membrane all potentially affect isotope fractionation. Previous models that explained fractionation only in terms of sulfate reduction rates appear to be oversimplified. The species-specific physiology of each sulfate reducer thus needs to be taken into account to understand the regulation of sulfur isotope fractionation during dissimilatory sulfate reduction.

Culture Media↗
PubMed2001

Simultaneous measurement of denitrification and nitrogen fixation using isotope pairing with membrane inlet mass spectrometry analysis.

A method for estimating denitrification and nitrogen fixation simultaneously in coastal sediments was developed. An isotope-pairing technique was applied to dissolved gas measurements with a membrane inlet mass spectrometer (MIMS). The relative fluxes of three N(2) gas species ((28)N(2), (29)N(2), and (30)N(2)) were monitored during incubation experiments after the addition of (15)NO(3)(-). Formulas were developed to estimate the production (denitrification) and consumption (N(2) fixation) of N(2) gas from the fluxes of the different isotopic forms of N(2). Proportions of the three isotopic forms produced from (15)NO(3)(-) and (14)NO(3)(-) agreed with expectations in a sediment slurry incubation experiment designed to optimize conditions for denitrification. Nitrogen fixation rates from an algal mat measured with intact sediment cores ranged from 32 to 390 microg-atoms of N m(-2) h(-1). They were enhanced by light and organic matter enrichment. In this environment of high nitrogen fixation, low N(2) production rates due to denitrification could be separated from high N(2) consumption rates due to nitrogen fixation. Denitrification and nitrogen fixation rates were estimated in April 2000 on sediments from a Texas sea grass bed (Laguna Madre). Denitrification rates (average, 20 microg-atoms of N m(-2) h(-1)) were lower than nitrogen fixation rates (average, 60 microg-atoms of N m(-2) h(-1)). The developed method benefits from simple and accurate dissolved-gas measurement by the MIMS system. By adding the N(2) isotope capability, it was possible to do isotope-pairing experiments with the MIMS system.

Geologic Sediments↗
PubMed2002

Carbon isotope fractionation during aerobic biodegradation of trichloroethene by Burkholderia cepacia G4: a tool to map degradation mechanisms.

The strain Burkholderia cepacia G4 aerobically mineralized trichloroethene (TCE) to CO(2) over a time period of approximately 20 h. Three biodegradation experiments were conducted with different bacterial optical densities at 540 nm (OD(540)s) in order to test whether isotope fractionation was consistent. The resulting TCE degradation was 93, 83.8, and 57.2% (i.e., 7.0, 16.2, and 42.8% TCE remaining) at OD(540)s of 2.0, 1.1, and 0.6, respectively. ODs also correlated linearly with zero-order degradation rates (1.99, 1.11, and 0.64 micromol h(-1)). While initial nonequilibrium mass losses of TCE produced only minor carbon isotope shifts (expressed in per mille delta(13)C(VPDB)), they were 57.2, 39.6, and 17.0 per thousand between the initial and final TCE levels for the three experiments, in decreasing order of their OD(540)s. Despite these strong isotope shifts, we found a largely uniform isotope fractionation. The latter is expressed with a Rayleigh enrichment factor, epsilon, and was -18.2 when all experiments were grouped to a common point of 42.8% TCE remaining. Although, decreases of epsilon to -20.7 were observed near complete degradation, our enrichment factors were significantly more negative than those reported for anaerobic dehalogenation of TCE. This indicates typical isotope fractionation for specific enzymatic mechanisms that can help to differentiate between degradation pathways.

Aerobiosis↗
PubMed2003

Carbon and hydrogen isotopic fractionation during anaerobic biodegradation of benzene.

Compound-specific isotope analysis has the potential to distinguish physical from biological attenuation processes in the subsurface. In this study, carbon and hydrogen isotopic fractionation effects during biodegradation of benzene under anaerobic conditions with different terminal-electron-accepting processes are reported for the first time. Different enrichment factors (epsilon ) for carbon (range of -1.9 to -3.6 per thousand ) and hydrogen (range of -29 to -79 per thousand ) fractionation were observed during biodegradation of benzene under nitrate-reducing, sulfate-reducing, and methanogenic conditions. These differences are not related to differences in initial biomass or in rates of biodegradation. Carbon isotopic enrichment factors for anaerobic benzene biodegradation in this study are comparable to those previously published for aerobic benzene biodegradation. In contrast, hydrogen enrichment factors determined for anaerobic benzene biodegradation are significantly larger than those previously published for benzene biodegradation under aerobic conditions. A fundamental difference in the previously proposed initial step of aerobic versus proposed anaerobic biodegradation pathways may account for these differences in hydrogen isotopic fractionation. Potentially, C-H bond breakage in the initial step of the anaerobic benzene biodegradation pathway may account for the large fractionation observed compared to that in aerobic benzene biodegradation. Despite some differences in reported enrichment factors between cultures with different terminal-electron-accepting processes, carbon and hydrogen isotope analysis has the potential to provide direct evidence of anaerobic biodegradation of benzene in the field.

Anaerobiosis↗
PubMed1994

Isotopic biogeochemistry of the Oxford Clay Formation (Jurassic), UK.

A total of 165 samples was obtained from the Oxford Clay Formation at seven different sites. Nearly all were from the Peterborough Member (Lower Oxford Clay), but seven were from the Stewartby and Weymouth Members (Middle and Upper Oxford Clay respectively). Five samples from the underlying Kellaways Formation were also examined. Stratigraphic relationships were estimated on the basis of ammonite subzones and results from all locations can be placed along a single stratigraphic scale. The following were determined for all samples: abundance and isotopic composition of organic carbon, abundances of carbonate carbon and total sulphur, and the Rock-Eval pyrolysis parameters hydrogen index, oxygen index and Tmax. For a subset of eight samples selected to be representative of geochemical and apparent palaeoenvironmental variations, soluble organic compounds were extracted and the isotopic composition of pristane, phytane, and long-chain n-alkanes determined by isotope-ratio-monitoring gas chromatograph mass spectrometry. Concentrations of organic carbon in samples from the Peterborough Member ranged from 0.5 to 16.6 % and delta values of total organic carbon (TOC) ranged from -27.7 to -23.1% v. PDB. Shales dominated by epifaunal bivalve assemblages have high concentrations of TOC and values of H index approaching 800, indicating preservation of hydrogen-rich organic material. Conversely, shell beds and calcareous and silty clay beds have lower abundances of TOC and values of H index dropping below 100, indicating extensive oxidation of the organic matter. Isotopic composition of pristane and phytane in the Peterborough and Stewartby Members average -31.7%, those in the Weymouth Member average -29.8. Values of delta for long-chain n-alkanes average -28%. Together these results indicate delta values for primary inputs as follows: terrestrial vascular plants, -23.5%; Peterborough Member algae, -28.2; Stewartby Member algae, -29.1%; Weymouth Member algae, -26.6% Comparison of primary delta values to those of TOC indicates that in some cases secondary processes enriched TOC relative to primary inputs by as much as 4%. Paleontological evidence in these same beds indicates development of extensive food-webs and supports attribution of this isotopic enrichment to heterotrophic reworking.

Alkanes↗
PubMed1992

Model equations for condensation biosynthesis using stable isotopes and radioisotopes.

Important syntheses in living systems occur by condensation reactions of the type nA----1B (where n is the number of A molecules needed to synthesize 1 molecule of B). Quantitative relationships for estimating the rate of synthesis of B from radioactive and stable isotope tracers are compared. With radioisotope tracers, only a single quantity is detected, the amount of radioactivity in B. In contrast, isotopes of varying mass produce multiple mass isotopomers B that are detected using mass spectrometry. The analysis demonstrates that the rate of synthesis of B is identifiable from stable isotope data but not from radioisotope data. This results because the isotopomer distribution of B at any time after tracer addition is a function of only the multinomial distribution representing the synthesis of B from n molecules of A and two parameters representing the fractional fluxes of isotopically enriched molecules to the sampled compartment of B. The model considers the possibility that the sampled compartment of B may not reach isotopic steady state during the experiment. A graphical method for obtaining initial estimates of the two parameters is presented.

Acetates↗
PubMed2006

Subcutaneous infusion and capillary "finger stick" sampling of stable isotope tracer in metabolic studies.

Metabolic studies utilizing stable isotope tracer in humans have typically used intravenous tracer infusions and venous blood sampling. These studies explore subcutaneous infusion of isotope and "finger stick" capillary blood sampling to measure glucose turnover. Five subjects received simultaneous 8-h infusions of glucose labeled with isotope: [1-(13)C]glucose subcutaneously and [6,6-(2)H(2)]glucose intravenously. At regular intervals, venous and finger stick blood specimens were obtained. Finger stick blood was applied to filter paper. Substrate and isotopic steady state was reached after 7.0 h with both routes of infusion. The isotopic enrichments of finger stick and venous specimens did not differ significantly for the subcutaneously infused [1-(13)C]glucose (p = 0.33 and p = 0.23, respectively) but the finger stick [6,6-(2)H(2)]glucose enrichment was slightly higher (p < 0.03) than that of the venous sample. Using [6,6-(2)H(2)]glucose infusion and venous plasma sampling as the reference method, the [1-(13)C]glucose gave estimates of glucose R(a) that were 13% (plasma) and 17% (finger stick) lower (p < 0.001 and p < 0.02, respectively). This difference could be attributed to recycling of (13)C label. In conclusion, subcutaneous infusion and finger stick specimen collection onto filter paper represent a potential method of conducting in vivo studies of substrate metabolism outside of a hospital-based research unit.

Adult↗
PubMed2001

Isotope ratio by HRGC-MS of citrus Junos tanaka (yuzu) essential oils: m/z 137/136 of terpene hydrocarbons.

The isotope ratios of monoterpene hydrocarbons in Citrus junos Tanaka (yuzu) essential oils from different origins were determined by ordinary high-resolution gas chromatography-mass spectrometry (HRGC-MS). Both intensities of the molecular mass peaks (m/z 136) and of the isotope peaks (m/z 137) of monoterpene hydrocarbons were measured by single-ion monitoring with an MS analysis. The isotope ratios (m/z 137/136) of the ten monoterpene hydrocarbons commonly contained in citrus essential oils, alpha-pinene, beta-pinene, sabinene, myrcene, alpha-phellandrene, alpha-terpinene, limonene, gamma-terpinene, beta-phellandrene and terpinolene, were determined in yuzu samples of the highest commercial quality from 42 different production districts. Statistical treatment of these data by the t-test and sign test revealed significant differences of the isotope effects in each yuzu sample. It is suggested that this technique will be applicable for evaluating the quality, genuineness and origin of citrus fruits and their products. The isotope fingerprints were also demonstrated in several citrus fruits other than the yuzu samples.

Citrus↗
PubMed1994

Use of inductively coupled plasma-mass spectrometry in boron-10 stable isotope experiments with plants, rats, and humans.

The commercial availability of inductively coupled plasma-mass spectrometry technology (ICP-MS) has presented the opportunity to measure the boron concentrations and isotope ratios in a large number of samples with minimal sample preparation. A typical analytical sequence for fecal samples consists of 25 acid blanks, 1 digestion blank, 5 calibration solutions, 4 standard reference material solutions, 10 samples, and 4 natural abundance bias standards. Boron detection limits (3 x 1 sigma) for acid blanks are 0.11 ppb for 10B, and 0.40 ppb for 11B. Isotope ratios were measured in fecal samples with 20 to 50 ppb boron with < 2% relative standard deviation. Rapid washout and minimal memory effects were observed for a 50 ppb beryllium internal standard, but a 200 ppb boron biological sample had a 1.0 ppb boron memory after a 6-min washout. Boron isotope ratios in geological materials are highly variable; apparently this variability is reflected in plants of a fixed natural abundance value for boron requires that a natural abundance ratio be determined for each sample or related data set. The natural abundance variability also prevents quantitation and calculation of isotope dilution by instrument-supplied software. To measure boron transport in animal systems, 20 micrograms of 10B were fed to a fasted rat. During the 3 days after a 10B oral dose, 95% of the 10B was recovered from the urine and 4% from the feces. Urinary isotope ratios, 11B/10B, changed from a natural abundance of 4.1140 to an enriched value of 0.95077, a 77% change.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2003

Inductively coupled plasma mass spectrometric analysis of calcium isotopes in human serum: a low-sample-volume acid-equilibration method.

BACKGROUND: Analytical methods for measuring the calcium isotope distribution in enriched human serum samples that use low blood volumes, simple preparation methods, and rapid analysis are important in clinical studies of calcium kinetics. Previously, sample preparation by oxalate precipitation typically required 500 micro L of serum. This method was time-consuming, and the blood volume required was limiting in circumstances when only a small amount of serum could be obtained. METHODS: Serum was collected from humans who were administered (42)Ca, and 20 micro L of serum was mixed with 2 mL of 0.22-0.67 mol/L HNO(3) at room temperature for between 1 min and 16 h. The (42)Ca/(43)Ca ratio in the supernatant was measured by a magnetic sector inductively coupled plasma mass spectrometer (ICP-MS). Calcium isotope ratios from these equilibration solutions were compared with data from oxalate-precipitated serum samples to determine the optimum equilibrium time and the effect of acid concentration on equilibrium. RESULTS: Various amounts of aggregated particles developed in different acid-serum mixtures. These affected the time required for isotope equilibration in the mixture. The shortest equilibrium time needed for the calcium isotopes varied from 1 to 6 h for samples acidified with 0.22-0.45 mol/L HNO(3). Data obtained from these solutions were consistent with data from oxalate-precipitated calcium. The precision of (42)Ca/(43)Ca ratio measurements was better than 0.5%. CONCLUSIONS: We have developed a simple, rapid sample preparation technique for ICP-MS analysis in which 20 micro L of serum can be used for accurate measurement of the calcium isotope distribution in a sample with good precision and a rapid analysis time.

Calcium↗
PubMedDate not supplied

[Effect of microorganisms and seasonal factors on the isotope composition of organic carbon from Black Sea suspensions].

The isotopic composition of particulate organic carbon (POC) from the Black Sea deep-water zone was studied during a Russian-Swiss expedition in May 1998. POC from the upper part of the hydrogen sulfide zone (the C-layer) was found to be considerably enriched with the 12C isotope, as compared to the POC of the oxycline and anaerobic zone. In the C-layer waters, the concurrent presence of dissolved oxygen and hydrogen sulfide and an increased rate of dark CO2 fixation were recorded, suggesting that the change in the POC isotopic composition occurs at the expense of newly formed isotopically light organic matter of the biomass of autotrophic bacteria involved in the sulfur cycle. In the anaerobic waters below the C-layer, the organic matter of the biomass of autotrophs is consumed by the community of heterotrophic microorganisms; this results in weighting of the POC isotopic composition. Analysis of the data obtained and data available in the literature allows an inference to be made about the considerable seasonable variability of the POC delta 13C value, which depends on the ratio of terrigenic and planktonogenic components in the particulate organic matter.

Carbon Isotopes↗
PubMed1999

A method for the measurement of glucose oxidation using the constant infusion of stable isotope.

We developed a method to measure the oxidation of glucose using the primed constant infusion of [U-13C] glucose in critically ill patients fed by total parenteral nutrition. The results obtained from the isotopic method were compared to those from indirect calorimetry in the critically ill patients. A patient with esophageal carcinoma was used for the preliminary study. The study was performed on the third postoperative day, assuming severely stressed state. Priming doses of NaH13CO3 at a dosage of 0.32 mg/kg and D-[U-13C] glucose at a dosage of 0.32 mg/kg were injected. D-[U-13C] glucose was then infused at an infusion rate of 0.004 mg/kg/min. It was revealed that the time required for an isotopic plateau was approximately 45 min in plasma glucose and 120 min in an expired air in highly stressed state. Isotopic measurement and indirect calorimetry were performed simultaneously pre- and postoperatively on three patients who underwent surgery for esophageal carcinoma. Increased fat oxidation was obtained by the isotopic method, whereas indirect calorimetry indicated nonprotein RQ above 1.0. Isotopic measurement offered a useful information that cannot be obtained from indirect calorimetry concerning the energy metabolism in the critical illness. Thus our method for the measurement of glucose oxidation is both simple and useful in investigating the energy metabolism in critically ill patients.

Blood Glucose↗
PubMedDate not supplied

[Carbon isotope (13C/12C) effect of photorespiration in photosynthetic organisms. Evidence for existence, probable mechanism].

Experimental evidence in favor of the new phenomenon predicted for photosynthesizing organisms, the fractionation of carbon isotopes in photorespiration is presented. A possible mechanism of this process is discussed. The fractionation of carbon in isotopes photorespiration occurs in the oxygenase phase of the functioning of ribulosebisphosphate carboxylase/oxygenase (rubisco), the key enzyme of photosynthesis, which is capable to act as carboxylase and oxygenase. Which function of the enzyme is active depends on CO2/O2 concentration ratio, which periodically changes in a cell. The key reaction in the mechanism of carbon isotope fractionation in photorespiration is glycine decarboxylation, which results in the splitting and removal from the cell of CO2 enriched with 12C and the accumulation of 13C photorespiratory carbon flow. The coupling of photorespiration and CO2 photoassimilation gives rise to two isotopically different carbon flows, which fill up separate carbohydrate pools, which are the sources of carbon in the following syntheses in the dark phase of photosynthesis. This enables one to identify, from the carbon isotope ratio of metabolites, their involvement in the photorespiratory and assimilatory carbon flows, to investigate the pathways of carbon metabolism, and to estimate more thoroughly the biosynthetic role of photorespiration.

Carbon Isotopes↗