ISSN:1025-6016    

DOI:10.1080/10256019808036362

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Anoxic sediment surfaces coloured black by iron monosulfides (“black spots”) evolve in tidal sandflats of the Wadden Sea (southern North Sea) as a result of the degradation of buried organic matter. To follow the short- and long-term effects of organic matter burial on pore water and sediment isotopic biogeochemistry, formation of artificial black spots was initiated on the Groninger Plate (site RP63) in the backbarrier tidal flats of Spiekeroog island. Changes in concentrations (DOC, TA, TOC, sulfate, sulfide, TRS, Fe) and isotopic compositions (sulfate, sulfide, TRS, pyrite, TOC) were followed for up to 12 months and compared to reference areas. 13°C ratios of TOC clearly mirror the early diagenetic degradation of organic matter. At least temporarily closed system sulfate reduction is inferred for the artificial black spot from the variation of sulfate concentrations and stable sulfur isotope partitioning, In the interstitial waters of the black spot, 34S/32S values of coexisting dissolved sulfate and sulfide yield fractionation degrees between −5 and −25%. On the reference area, 34S/32S are fractionated by −32 to −42% as calculated from the isotope composition of solid phase reduced sulfur and pore water sulfate. Sulfur isotope fractionation seems to increase with decreasing sulfate reduction rate. Limiting factor seems to be the availability of DOC. Between the pyrite pool and the dissolved sulfide in the black spot, no significant isotope exchange is observed within 12 months.

ISSN:1025-6016    

DOI:10.1080/10256019808036363

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The δ13C value of bacterial carbon is an important parameter in microbial ecology for studying the carbon flow within a microbial community and for the identification of ecological important strains involved in the mineralization of certain carbon pools in the environment. In our study, biomarkers were isolated from bacteria from a microbial consortium derived from two chemostats and δ13C values were measured. Similar isotope ratios between biomarkers such as fatty acids and outer membrane protein, biomass and substrate were observed. The δ13C analyses of outer membrane protein F ofPseudomonasand biomarker fatty acids were combined to follow bacterial assimilation of13C labelled 4-chlorocatechol. This new approach was also used in the environment where soil samples were cultivated with different13C traced substrates.

ISSN:1025-6016    

DOI:10.1080/10256019808036364

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Acetyl-CoA provides the C-2-units for the formation of many secondary compounds. The primary pattern of this intermediate is originating from glucose, further modifications are introduced by isotope effects on the pyruvate-dehydrogenase-reaction and on ester- and aldol-reactions in the course of the biosynthesis of secondary products. The overlap of these various effects is demonstrated through the pattern of some natural products.

ISSN:1025-6016    

DOI:10.1080/10256019808036365

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

L-Ascorbic acid in fruit juices is enriched in13C by 4.8 δ-units relative to its precursor glucose, and this enrichment is mainly located in position C-1. On the other hand, biotechnologically produced L-ascorbic acid preserves the pattern as expected from glucose. In grapes L-tartaric acid is synthesized from glucose via L-ascorbic acid and it is one of its main products. It preserves C-atoms 1 to 4 from glucose and the measured δ13C-value is identical to that calculated from this precursor.

ISSN:1025-6016    

DOI:10.1080/10256019808036366

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Recently the set up of a stable carbon isotope chronology of juniper tree rings of the eastern Tibet timberline has been iniliated. First results for the last 800 years will be presented.

ISSN:1025-6016    

DOI:10.1080/10256019808036367

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

ISSN:1025-6016    

DOI:10.1080/10256019808036368

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

335 [15N2]urea examinations to detect gastricHelicobacter pyloriinfection were performed using a uniform study protocol broadened to obtain a large quantity of various experimental data. The data were compiled and assessed in order to establish the most suitable parameter for a standardized test protocol. Second-hour urine following oral administration of the diagnostic agent was found to produce the clearest distinction betweenH. pylori-infected and non-infected persons. The threshold value ofc= 0.06 (the quotient of15Nexcessabundance in urinary ammonia and15Nexcessabundance in the urinary urea) is not age-dependent Though less suitable on its own, the15Nexcessabundance of urinary ammonia is useful as a second parameter for interpreting the results in the case ofcvalues slightly below 0.06. The standardized test protocol is specified.

ISSN:1025-6016    

DOI:10.1080/10256019808036369

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Catabolism of the natural branched-chain L-amino acids in overnight fasted healthy volunteers was comparatively studied by performing oral loading tests with 1-13C-labelled L-leucine, L-valine, and L-isoleucine (38 μmol x kg (body weight)−1), respectively. On the basis of the13CO2exhalation and the13C-isotope enrichment in the plasma branched-chain compounds, whole body branched-chain L-amino acid oxidation rates were estimated applying a seven compartment model and non-linear regression analysis. Mean computed in vivo oxidation rates were in the order L-leucine ≫ L-valine > L-isoleucine and amounted to 0.32 ± 0.06, 0.22 ± 0.04, and 0.17 ± 0.05 μmol x (kg body wt.)−1x min−1(n= 5), respectively. The data are discussed with respect to current estimates of human branched-chain L-amino acid requirements.

ISSN:1025-6016    

DOI:10.1080/10256019808036370

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Numerical simulation of18O exchange between CO2, H2O and HCO3−explains the ratio mass 46/mass 44 (=C18O16O/C16O2) measured by mass spectrometry to approximately 1% relative mean difference. In the presence of intact red blood cells the reaction takes place extra- and intracellularly at different reaction rates. Because CO2hydration/dehydration is accelerated intracellularly by carbonic anhydrase, a difference in intra- and extracellular concentration of labelled reactants occurs. The ensuing transfer of reactants across the cell membrane depends on their membrane permeabilities which are approximately PCO2≅ 2 cm/s, PH2O≅ 0.002 cm/s and PHCO3≅ 0.00015 cm/s (values from the literature). We found that the numerical simulation is affected most by PHCO3. The influence of the other permeabilities is at least ten times less. Therefore within the range of normal literature values, PHCO3is the only permeability that has a rate limiting effect on18O exchange. This is why, in turn, PHCO3can be derived from an experimentally determined time course of mass 46/mass 44 by a fitting procedure. Another crucial parameter that can be estimated from the experimental results is the intraerythrocytic carbonic anhydrase activityAi.

ISSN:1025-6016    

DOI:10.1080/10256019808036371

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor