Isotope Tracer Experiments Research Articles

Aqueous-phase selective aerobic oxidation of 5-hydroxymethylfurfural on Ru/C in the presence of base

The aerobic oxidation of 5-hydroxymethylfurfural (HMF) was performed on an activated carbon-supported ruthenium (Ru/C) catalyst in water. The presence of Mg-Al hydrotalcite (HT, Mg/Al molar ratio = 3/1) as a base afforded higher selective oxidation of HMF to 5-formyl-2-furancarboxylic acid (FFCA) and 2,5-furandicarboxylic acid (FDCA) than with the bases MgO, Ca(OH) 2 and NaOH owing to its appropriate strength of basicity. X-ray photoelectron spectroscopy characterization confirmed that metallic Ru 0 species were the active sites for HMF oxidation. Isotopic tracer experiments conducted with 18 O 2 and 16 O 2 indicated that H 2 O rather than O 2 provided the oxygen atom for the oxidation of HMF to FFCA and FDCA via hydration of the formyl group. These results and kinetic studies of the oxidation of HMF and 2,5-diformylfuran (DFF) led to the proposition that the aerobic oxidation of HMF to FFCA follows a Langmuir-Hinshelwood mechanism. The oxidation involved dissociative adsorption of HMF and O 2 to form adsorbed alcoholate and atomic oxygen species followed by kinetically relevant abstraction of β-H from the alcoholate species via the atomic oxygen species to adsorbed DFF species on the Ru surface, which then underwent hydration and oxidation to FFCA under basic conditions. Aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 5-formyl-2-furancarboxylic acid on Ru/C with base in water follows a Langmuir-Hinshelwood mechanism, involving kinetically relevant β-H abstraction from alcoholate species, which are formed from dissociative adsorption of HMF and O 2 on the Ru surface by atomic oxygen.

Dual mode of action of metformin on mitochondrial metabolism

Metformin is commonly used in the treatment of diabetes, however recently there is increasing interest in “repurposing” the drug for cancer prevention or treatment. Metformin is believed to act by inhibiting mitochondrial complex I, leading to activation of AMPK. Interestingly, metformin treatment is not associated with the serious health consequences that are seen with classic inhibitors of complex I. To address this apparent paradox, we investigated the impact of metformin on cellular bioenergetics using in-depth respirometry analyses coupled with stable isotope tracer experiments in both cells and isolated mitochondria. We show that cells treated with metformin display reduced respiration. Metformin reduces respiration by specifically inhibiting respiration coupled to ATP production, while increasing uncoupled respiration. Thus, cells treated with metformin devote a large fraction of their respiration for uncoupled reactions and become inefficient. The impact of metformin on cellular respiration can be attributed to direct action on mitochondria as metformin inhibits complex I-dependent respiration and citric acid cycle activity in isolated mitochondria. Overall, our study reveals global bioenergetic consequences of metformin exposure, and establishes mechanistic similarities between metformin and recently described liver-specific uncoupling agents that, like metformin, improve the metabolic consequences of obesity.

A Model for Extracting Fundamental Kinetic Rates of SOFC Cathode Materials from Oxygen Isotope Exchange Experiments

Losses from cathode polarization in solid oxide fuel cells (SOFC) limit their performance at intermediate temperatures. A fundamental understanding of the oxygen reduction mechanisms on SOFC cathodes is essential for the development of lower temperature SOFCs with high performance. In order to separate the various contributions to cathode polarization, different techniques and experiments have been carried out to gain an understanding of the kinetics occurring at the gas-solid interface. Isotope exchange is a powerful tool for analyzing the oxygen exchange process (1, 2, 3, 4, 5), however, most isotope tracer experiments are limited in their scope. Experiments typically take one of two approaches, a focus on the kinetics of oxygen adsorption on the surface (SSIKTA), or the diffusion of oxygen through the lattice (SIMS). For SOFC cathodes, we are concerned with both the dissociation of oxygen on the surface and the ability of oxygen atoms to incorporate into and diffuse through the lattice. In this study, we attempt to determine the ORR mechanism and extract fundamental kinetics rates for La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and (La0.8Sr0.2)0.95MnO3-x (LSM) using isothermal isotope exchange (IIE) (6,7). Studies of isotope exchange can greatly contribute to the elucidation of the interactions between molecular oxygen and the cathode surface. Isotope exchange can help characterize the dissociation of oxygen molecules at the oxide surface as well as the conduction of oxygen ions within the oxide. IIE experiments will be conducted at various temperatures and oxygen partial pressures.To extract fundamental kinetic rates from in-situ isotope exchange experiments on LSM and LSCF, a model has been developed based on a two-step reaction mechanism across the heterogeneous gas-solid interface. The coupled reactions can be shown in two elementary steps: dissociative adsorption and incorporation. The relation between the fundamental kinetics rates and surface exchange coefficient (kex) (8, 9) will also be linked. Acknowledgments This work was supported by the Department of Energy under contract DEFE0009084. The authors would like to thank Dr. Dongxia Liu for reviewing our model.ReferencesM.W. den Otter, B.A. Boukamp, H.J.M. Bouwmeester, Solid State Ionics 139 (2001) 89–94Henny J. M. Bouwmeester,* Chunlin Song, Jianjun Zhu, Jianxin Yi, Martin van Sint Annaland and Bernard A. Boukamp, Phys. Chem. Chem. Phys., 2009, 11, 9640–9643S. Lacombe, H. Zanthoff, and C. Mirodatos, Jounal of Catalysis, 155, 106-116(1995)M.W. den Otter, B.A. Boukamp, H.J.M. Bouwmeester, Solid State Ionics 139 (2001) 89–94Schohn L. Shannon and James G. Goodwin, Jr.*, Chem. Rev. 1995, 677-695C.C. Kan, E.D. Wachsman, Solid State Ionics 181 (2010) 338–347C.C. Kan, H. H. Kan, F. M. Van Assche IV, E. N. Armstrong, and E. D. Wachsman, Journal of The Electrochemical Society, 155 (10) B985-B993 (2008E. N. Armstrong, K. L. Duncan, D. J. Oh, J. F. Weaver, and E. D. Wachsman, Journal of The Electrochemical Society, 158 (5) B492-B499 (2011)S. Wang, P.A.W. van der Heide, C. Chavez, A.J. Jacobson, S.B. Adler, Solid State Ionics 156 (2003) 201–208

The effect of FISH and CARD-FISH on the isotopic composition of 13C- and 15N-labeled Pseudomonas putida cells measured by nanoSIMS

The use of nanoSIMS for the exploration of microbial activities in natural habitats often implies that stable isotope tracer experiments are combined with in situ hybridization techniques (i.e. fluorescence in situ hybridization (FISH) or catalyzed reporter deposition (CARD)-FISH). In this study, Pseudomonas putida grown on 13C- and 15N-labeled carbon and nitrogen, collected in exponential growth and stationary phases, was hybridized and analyzed by nanoSIMS. It was shown that 13C and 15N fractions decreased after FISH and CARD-FISH in comparison to chemically untreated cells. However, the fractions were influenced differently by various treatments. After paraformaldehyde fixation of exponentially growing cells, a reduction of the 13C and 15N fractions was measured from 94±1.2% and 89.5±3.8% to 90.2±0.8% and 64±4.6%, respectively, indicating that nitrogen isotopic composition was most influenced. A further decrease of the 13C and 15N fractions to 80.7±6.5 and 59.5±4.1%, respectively, was measured after FISH, while CARD-FISH decreased the fractions to 57.4±3.0% and 47.1±4.1%, respectively. The analysis of cells collected in different growth phases revealed that the effect of various treatments seemed to be dependent on the cell's physiological state. In addition, a mathematical model that can be used in further studies was developed in order to calculate the amount of carbon introduced into the cells by chemical treatments. These results can be valuable for environmental FISH-nanoSIMS studies where the isotopic composition of single cells will be used to quantitatively assess the importance of specific populations to certain biochemical processes and determine budget estimations.

Pathway of nitrous oxide consumption in isolated Pseudomonas stutzeri strains under anoxic and oxic conditions

The microbial consumption of nitrous oxide (N2O) has gained great interest since it was revealed that this process could mitigate the greenhouse effect of N2O. The consumption of N2O results from its reduction to dinitrogen gas (N2) as part of the denitrification process. However, there is ongoing debate regarding an alternative pathway, namely reduction of N2O to NH4(+), or assimilatory N2O consumption. To date, this pathway is poorly investigated and lacks unambiguous evidence. Enrichment of denitrifying activated sludge using a mineral nitrogen-free medium rendered a mixed culture capable of anoxic and oxic N2O consumption. Dilution plating, isolation and deoxyribonucleic acid fingerprinting identified a collection of Pseudomonas stutzeri strains as dominant N2O consumers in both anaerobic and aerobic enrichments. A detailed isotope tracing experiment with a Pseudomonas stutzeri isolate showed that consumption of N2O via assimilatory reduction to NH4(+) was absent. Conversely, respiratory N2O reduction was directly coupled to N2 fixation.

Quantitation of Cellular Metabolic Fluxes of Methionine

Methionine is an essential proteogenic amino acid. In addition, it is a methyl donor for DNA and protein methylation and a propylamine donor for polyamine biosynthesis. Both the methyl and propylamine donation pathways involve metabolic cycles, and methods are needed to quantitate these cycles. Here, we describe an analytical approach for quantifying methionine metabolic fluxes that accounts for the mixing of intracellular and extracellular methionine pools. We observe that such mixing prevents isotope tracing experiments from reaching the steady state due to the large size of the media pools and hence precludes the use of standard stationary metabolic flux analysis. Our approach is based on feeding cells with (13)C methionine and measuring the isotope-labeling kinetics of both intracellular and extracellular methionine by liquid chromatography-mass spectrometry (LC-MS). We apply this method to quantify methionine metabolism in a human fibrosarcoma cell line and study how methionine salvage pathway enzyme methylthioadenosine phosphorylase (MTAP), frequently deleted in cancer, affects methionine metabolism. We find that both transmethylation and propylamine transfer fluxes amount to roughly 15% of the net methionine uptake, with no major changes due to MTAP deletion. Our method further enables the quantification of flux through the pro-tumorigenic enzyme ornithine decarboxylase, and this flux increases 2-fold following MTAP deletion. The analytical approach used to quantify methionine metabolic fluxes is applicable for other metabolic systems affected by mixing of intracellular and extracellular metabolite pools.

Use of stable isotope probing to assess the fate of emerging contaminants degraded by white-rot fungus

The widespread of emerging contaminants in the environment and their potential impact on humans is a matter of concern. White-rot fungi are cosmopolitan organisms able to remove a wide range of pharmaceuticals and personal care products (PPCP) through cometabolism (i.e. laccases and peroxidases) or detoxification mechanisms (i.e. cytochrome P450 system). However, the use of PPCP as carbon source for these organisms is largely unexplored. Here, we used carbon stable isotope tracer experiments to assess the fate of anti-inflammatory diclofenac (DCF) and UV filter benzophenone-3 (BP3) during degradation by Trametes versicolor. The comparison between carbon isotopic composition of emitted carbon dioxide from 13C-labelled DCF ([acetophenyl ring-13C6]-DCF) and 13C-BP3 ([phenyl-13C6]-BP3) versus their 12C-homologue compounds showed mineralization of about 45% and 10% of the 13C contained in their respective molecules after 9days of incubation. The carbon isotopic composition of the bulk biomass and the application of amino acid-stable isotope probing (SIP) allowed distinguishing between incorporation of 13C from BP3 into amino acids, which implies the use of this emerging contaminant as carbon source, and major intracellular accumulation of 13C from DCF without implying the transformation of its labelled phenyl ring into anabolic products. A mass balance of 13C in different compartments over time provided a comprehensive picture of the fate of DCF and BP3 across their different transformation processes. This is the first report assessing biodegradation of PPCP by SIP techniques and the use of emerging contaminants as carbon source for amino acid biosynthesis.

Subdivision of Nitrogen Use Efficiency of Rice Based on 15 N Tracer

The nitrogen(N) uptake, and N use efficiencies(NUE) at different rice growth stages(i.e. during basal, tillering and panicle fertilizations) were studied using 15 N isotope tracing. A two-year field experiment with two N rates and two distribution ratios was conducted using two different high-yielding rice cultivars Wuyunjing 23(japonica) and Y-Liangyou 2(hybrid indica). A sub-plot of 15 N isotope tracing experiment with three duplications under the same treatment was also set up in the field. The results revealed that basal nitrogen absorbed by rice was only 1.5%–11.5% before tillering fertilization(eight days after transplanting), 6.6%–24.9% from tillering fertilization to panicle fertilization, and little after panicle fertilization. The overall recovery efficiency of basal N(NUEB) was low and ranged from 9.1% to 22.8%, not significantly affected by different cultivars and N treatments. Tillering fertilizer N was mainly absorbed from tillering fertilization to panicle fertilization, and no longer had effect after panicle fertilization. NUE of tillering N fertilizer(NUET) was 17%–34%, which is almost the same as that of basal fertilizer. NUET of Y-Liangyou 2 was higher than that of Wuyunjing 23. Compared with basal and tillering N fertilizers, NUE of panicle N fertilizer(NUEP) was the highest with a value of 54.0%–82.1%, and Wuyunjing 23 had lower NUE than Y-Liangyou 2. The whole NUE in the entire growth period of all N fertilizers decreased with the increase of N application rate, and varied from 32% to 64%. Among the total N uptake of rice, the contribution was 4.13%–10.59%(average 6.92%) for basal N fertilizer, 3.98%–11.75%(average 7.58%) for tillering fertilizer, 13.32%–37.56%(average 26.02%) for panicle fertilizer, and 45.71%–70.83%(average 59.91%) for the soil. The experiment also revealed that the more the basal and tillering fertilizers applied, the lower the total NUE. It is suggested that rice N management, the N absorption and utilization from fertilizer applied at different stages should be considered to improve the NUE of rice, and ensure the hig yield while avoid the N loss.

Sink or link? The bacterial role in benthic carbon cycling in the Arabian Sea's oxygen minimum zone

Abstract. The bacterial loop, the consumption of dissolved organic matter (DOM) by bacteria and subsequent transfer of bacterial carbon to higher trophic levels, plays a prominent role in pelagic food webs. However, its role in sedimentary ecosystems is not well documented. Here we present the results of isotope tracer experiments performed under in situ oxygen conditions in sediments from inside and outside the Arabian Sea's oxygen minimum zone (OMZ) to study the importance of the microbial loop in this setting. Particulate organic matter, added as phytodetritus, was processed by bacteria, protozoa and metazoans, while dissolved organic matter was processed only by bacteria and there was very little, if any, transfer to higher trophic levels within the 7 day experimental period. This lack of significant transfer of bacterial-derived carbon to metazoan consumers indicates that the bacterial loop is rather inefficient, in sediments both inside and outside the OMZ. Moreover, metazoans directly consumed labile particulate organic matter resources and thus competed with bacteria for phytodetritus.

Nitrogen Biogeochemistry in the Caribbean Sponge, Xestospongia muta: A Source or Sink of Dissolved Inorganic Nitrogen?

BackgroundSponges have long been known to be ecologically important members of the benthic fauna on coral reefs. Recently, it has been shown that sponges are also important contributors to the nitrogen biogeochemistry of coral reefs. The studies that have been done show that most sponges are net sources of dissolved inorganic nitrogen (DIN; NH4 + and NO3 −) and that nitrification, mediated by their symbiotic prokaryotes, is the primary process involved in supplying DIN to adjacent reefs.Methodology/Principal FindingsA natural experiment was conducted with the Caribbean sponge Xestospongia muta from three different locations (Florida Keys, USA; Lee Stocking Island, Bahamas and Little Cayman, Cayman Islands). The DIN fluxes of sponges were studied using nutrient analysis, stable isotope ratios, and isotope tracer experiments. Results showed that the fluxes of DIN were variable between locations and that X. muta can be either a source or sink of DIN. Stable isotope values of sponge and symbiotic bacterial fractions indicate that the prokaryotic community is capable of taking up both NH4 + and NO3 − while the differences in δ 15N between the sponge and bacterial fractions from the NH4 + tracer experiment suggest that there is translocation of labeled N from the symbiotic bacteria to the host.Conclusions/SignificanceNitrogen cycling in X. muta appears to be more complex than previous studies have shown and our results suggest that anaerobic processes such as denitrification or anammox occur in these sponges in addition to aerobic nitrification. Furthermore, the metabolism of this sponge and its prokaryotic symbionts may have a significant impact on the nitrogen biogeochemistry on Caribbean coral reefs by releasing large amounts of DIN, including higher NH4 + concentrations that previously reported.

Negative Priming Effect on Organic Matter Mineralisation in NE Atlantic Slope Sediments

The priming effect (PE) is a complex phenomenon which describes a modification (acceleration or retardation) in the mineralisation rate of refractory organic matter (OM) following inputs of labile material. PEs are well-studied in terrestrial ecosystems owing to their potential importance in the evolution of soil carbon stocks but have been largely ignored in aquatic systems despite the fact that the prerequisite for their occurrence, i.e. the co-existence of labile and refractory OM, is also true for sediments. We conducted stable isotope tracer experiments in continental margin sediments from the NE Atlantic (550–950 m) to study PE occurrence and intensity in relation to labile OM input. Sediment slurries were treated with increasing quantities of the 13C-labelled diatom Thalassiosira rotula and PE was quantified after 7, 14 and 21 days. There was a stepwise effect of diatom quantity on its mineralisation although mineralisation efficiency dropped with increasing substrate amounts. The addition of diatomaceous OM yielded a negative PE (i.e. retardation of existing sediment OM mineralisation) at the end of the experiment regardless of diatom quantity. Negative PE is often the result of preferential utilisation of the newly deposited labile material by the microbial community (“preferential substrate utilization”, PSU) which is usually observed at excessive substrate additions. The fact that PSU and the associated negative PE occurred even at low substrate levels in this study could be attributed to limited amounts of OM subject to priming in our study area (∼0.2% organic carbon [OC]) which seems to be an exception among continental slopes (typically >0.5%OC). We postulate that PEs will normally be positive in continental slope sediments and that their intensity will be a direct function of sediment OC content. More experiments with varying supply of substrate targeting C-poor vs. C-rich sediments are needed to confirm these hypotheses.

Simultaneous determination of natural‐abundance δ15N values and quantities of individual amino acids in proteins from milk of lactating women and from infant hair using gas chromatography/isotope ratio mass spectrometry

In isotope tracer experiments used in nutritional studies, it is frequently desirable both to determine the (15)N/(14)N ratios of target compounds and to quantify these compounds. This report shows how this can be achieved in a single chromatographic run for protein amino acids using an isotope ratio mass spectrometer. Protein hydrolysis by acidic digestion was used to release amino acids, which were then derivatized as their N-pivaloyl-O-isopropyl esters. Suitable conditions for sample preparation were established for both hair and milk proteins. The N-pivaloyl-O-isopropyl esters of amino acids were separated by gas chromatography (GC) on a 60 m ZB-WAX column linked via a combustion interface to an isotope ratio mass spectrometer. The (15)N/(14)N ratios were obtained from the m/z 28, 29 and 30 peak intensities and the quantities from the Area All (Vs) integrated peak areas. It is shown from a five-point calibration curve that both parameters can be measured reliably within the range of 1.0 to 2.0 mg/mL for the major amino acids derived from the hydrolysis of human maternal milk or hair samples. The method was validated in terms of inter-day and inter-user repeatability for both parameters for 14 amino acids. The amino acid percentage composition showed a good correlation with literature values. The method was applied to determine the variability in a population of lactating mothers and their infants. It has been established that δ(15)N values can be simultaneously determined for a complex mixture of amino acids at variable concentrations. It is shown that the percentage composition obtained correlates well with that obtained by calculation from the protein composition or from literature values. This procedure should provide a significant saving in analysis time, especially in those cases where the GC run-time is necessarily long. It allows the satisfactory determination of the variation within a sample population.

Acceleration of Denitrification in Turbid Rivers Due to Denitrification Occurring on Suspended Sediment in Oxic Waters

High suspended sediment (SPS) concentration exists in many rivers of the world. In the present study, the effects of SPS concentration on denitrification were investigated in airtight chambers with sediment samples collected from the Yellow River which is the largest turbid river in the world. Results from the nitrogen stable ((15)N) isotopic tracer experiments showed that denitrification could occur on SPS in oxic waters and the denitrification rate increased with SPS concentration; this was probably caused by the presence of low-oxygen microsites in SPS. For the water systems with both bed-sediment and SPS, the denitrification kinetics fit well to Logistic model, and the denitrification rate constant increased linearly with SPS concentration (p < 0.01). The denitrification caused by the presence of SPS accounted for 22%, 38%, 53%, and 67% of the total denitrification in systems with 2.5, 8, 15, and 20 g L(-1) SPS, respectively. The activity of denitrifying bacteria in SPS was approximately twice that in bed-sediment, and the denitrifying bacteria population showed an increasing trend with SPS concentration in both SPS and bed-sediment, leading to the increase of denitrification rate with SPS concentration. Furthermore, the denitrification in bed-sediment was accelerated by increased diffusion of nitrate from overlying water to bed-sediment under agitation conditions, which accompanied with the presence of SPS. When with 8 g L(-1) SPS, approximately 66% of the increased denitrification compared to that without SPS was attributed to denitrification on SPS and 34% to agitation conditions. This is the first report of the occurrence of denitrification on SPS in oxic waters. The results suggest that SPS plays an important role in denitrification in turbid rivers; its effect on nitrogen cycle should be considered in future study.

Metabolic isotopomer labeling systems. Part III: Path tracing

Isotope labeling systems (ILSs) are sets of balance equations that quantitatively describe the distribution of isotopic tracers in metabolic networks. The solution of ILSs, i.e., the calculation of isotopic labeling distributions (mostly in steady state) is the fundamental computational step of 13C metabolic flux analysis (MFA). Aiming at a deeper analytical understanding of ILSs a new approach for solving ILSs is developed. It is based on the straightforward idea of tracing labeled molecules through the metabolic network. The new approach allows to calculate the label distribution in isotopic tracer experiments in an analytical way that directly reflects the underlying network structure. The theory of path tracing is formally developed by introducing regular expressions for representing all possible paths through the labeling network. These expressions are generated by classical path tracing algorithms, e.g. by the Kleene algorithm. As a central theoretical result, a framework for proving the correctness of such path tracing algorithms in their application to ILSs is developed. Finally, by mapping path expressions to algebraic expressions, the solution of an ILS is computed. As an offspring of the developed theory, the relation between path tracing and former approaches for ILS solution is worked out and several consequences for the numerical solution and analysis of ILSs and – more general – compartmental systems used in pharmaco-kinetic modeling will be sketched.

HPLC-RTS (high performance liquid chromatography-retention time shift)를 이용한 암모늄 이온의 안정동위원소 측정방법의 개선

Despite the usefulness of nitrogen isotope tracer experiments in nitrogen cycling studies, there are not such many measurement data mainly due to the difficulties in analytical methods. Although Gardner et al. (1996) developed a relatively simple and accurate method that can measure ammonium isotope using HPLC and used it widely in various N dynamics studies, the technique was not adopted to other laboratories. An HPLC-RTS system using updated HPLC pumps that can perform the same measurements as that of Gardner et al. (1996) was built. The result of standard sample showed linear increase of RTS with the proportions. Centroid retention times calculated with Matlab program enhanced the linearity of the response. In a sea water incubation experiment spiked with , the uptake and regeneration of ammonium could be separately estimated using the temporal change of .

Isotope ratio measurements in biological tissues using LA-ICP-MS – possibilities, limitations, and perspectives

This article describes the advantages and possibilities of using LA-ICP-MS for isotope ratio measurements, briefly comparing quadrupole, sector-field and multi-collector systems. It also discusses problems and limitations of the respective set-ups. A special focus lies on biological samples from stable isotope tracer experiments and on spatially resolved isotope ratio measurements including imaging of isotope ratios. The article explores if there are valid reasons why until now almost no one used LA-ICP-MS to image isotope ratios. It wants to encourage the ICP-MS community to realise the potential of using LA-ICP-MS for isotope ratio measurements in biological samples and to hopefully spur further developments and applications.

Advanced Characterization Techniques in High-Temperature Oxidation and Corrosion Studies

Recent improvements in characterization techniques have greatly advanced fundamental understanding of the complex processes associated with high temperature oxidation and corrosion. These techniques allow for chemical and structural analyses at high resolutions, now clearly in the micrometer range, but also submicron for some and even down to the atomic scale with such techniques as atom probe and aberration corrected electron microscopy. Added to these is the possibility to conduct advanced analyses in situ. In this present issue, it will be clearly demonstrated that substrate and oxide scale microstructures, playing a key role in a number of systems, can now be characterized with exquisite detail and often in three-dimensions, such as by the coupling of FIB, MEB/FEG and even EBSD. In addition, residual stress mapping by Raman spectroscopy, the study of semiconductor properties with photoelectrochemistry techniques, and isotopic tracer experiments in conjunction with SIMS analysis to identify principal reacting and transport processes, all provide excellent examples of what are available to better understand the mechanisms involved in oxide-scale formation at elevated temperatures.

A Fourier transform infrared trace gas and isotope analyser for atmospheric applications

Abstract. Concern in recent decades about human impacts on Earth's climate has led to the need for improved and expanded measurement capabilities of greenhouse gases in the atmosphere. In this paper we describe in detail an in situ trace gas analyser based on Fourier Transform Infrared (FTIR) spectroscopy that is capable of simultaneous and continuous measurements of carbon dioxide (CO2), methane (CH4), carbon monoxide (CO), nitrous oxide (N2O) and 13C in CO2 in air with high precision. High accuracy is established by reference to measurements of standard reference gases. Stable water isotopes can also be measured in undried airstreams. The analyser is automated and allows unattended operation with minimal operator intervention. Precision and accuracy meet and exceed the compatibility targets set by the World Meteorological Organisation – Global Atmosphere Watch for baseline measurements in the unpolluted troposphere for all species except 13C in CO2. The analyser is mobile and well suited to fixed sites, tower measurements, mobile platforms and campaign-based measurements. The isotopic specificity of the optically-based technique and analysis allows its application in isotopic tracer experiments, for example in tracing variations of 13C in CO2 and 15N in N2O. We review a number of applications illustrating use of the analyser in clean air monitoring, micrometeorological flux and tower measurements, mobile measurements on a train, and soil flux chamber measurements.

Evidence for Only Oxygenative Cleavage of Aldehydes to Alk(a/e)nes and Formate by Cyanobacterial Aldehyde Decarbonylases

Cyanobacterial aldehyde decarbonylases (ADs) catalyze the conversion of C(n) fatty aldehydes to formate (HCO(2)(-)) and the corresponding C(n-1) alk(a/e)nes. Previous studies of the Nostoc punctiforme (Np) AD produced in Escherichia coli (Ec) showed that this apparently hydrolytic reaction is actually a cryptically redox oxygenation process, in which one O-atom is incorporated from O(2) into formate and a protein-based reducing system (NADPH, ferredoxin, and ferredoxin reductase; N/F/FR) provides all four electrons needed for the complete reduction of O(2). Two subsequent publications by Marsh and co-workers [ Das, et al. ( 2011 ) Angew. Chem. Int. Ed. 50 , 7148 - 7152 ; Eser, et al. ( 2011 ) Biochemistry 50 , 10743 - 10750 ] reported that their Ec-expressed Np and Prochlorococcus marinus (Pm) AD preparations transform aldehydes to the same products more rapidly by an O(2)-independent, truly hydrolytic process, which they suggested proceeded by transient substrate reduction with obligatory participation by the reducing system (they used a chemical system, NADH and phenazine methosulfate; N/PMS). To resolve this discrepancy, we re-examined our preparations of both AD orthologues by a combination of (i) activity assays in the presence and absence of O(2) and (ii) (18)O(2) and H(2)(18)O isotope-tracer experiments with direct mass-spectrometric detection of the HCO(2)(-) product. For multiple combinations of the AD orthologue (Np and Pm), reducing system (protein-based and chemical), and substrate (n-heptanal and n-octadecanal), our preparations strictly require O(2) for activity and do not support detectable hydrolytic formate production, despite having catalytic activities similar to or greater than those reported by Marsh and co-workers. Our results, especially of the (18)O-tracer experiments, suggest that the activity observed by Marsh and co-workers could have arisen from contaminating O(2) in their assays. The definitive reaffirmation of the oxygenative nature of the reaction implies that the enzyme, initially designated as aldehyde decarbonylase when the C1-derived coproduct was thought to be carbon monoxide rather than formate, should be redesignated as aldehyde-deformylating oxygenase (ADO).

Using 15N tracers to estimate N2O and N2 emissions from nitrification and denitrification in coastal plain wetlands under contrasting land-uses

Microbial nitrification and denitrification both can emit nitrous oxide (N2O), a major greenhouse gas, and the relative contribution of each pathway depends strongly on soil moisture conditions. We conducted a stable isotope tracer experiment to determine the contribution of nitrification and denitrification to N2O and dinitrogen (N2) fluxes in coastal plain wetlands, and to determine the response of these processes to changing soil moisture. We added 15N-labeled nitrate (NO3−) or ammonium (NH4+) to intact soil cores collected from an agricultural field, a restored wetland, and a preserved forested wetland, and subjected the cores to a drying or wetting hydrologic manipulation. Across all soils and treatments, the combined fluxes of N2O and N2 ranged widely, between 0.23 and 2900 μg N m−2 h−1, and N2O accounted for as little as 0% to as much as 43% of the total gaseous nitrogen (N) fluxes. Fluxes of both gases increased with increasing soil moisture in all soils and tracer treatments, but the relative enhancement of the two gases varied by soil type and N source. The N2O yields [N2O/(N2O + N2)] derived from both nitrification and denitrification were low (1–3%) in five of eight soils in each tracer experiment. Surprisingly, nitrification-derived N2O yields were highest (13–31%) in soils with the highest organic matter and soil moisture (restored wetland under simulated rain and forested wetland under drained and simulated rain), while denitrification-derived N2O yields (12–36%) were highest under simulated rain in the two mineral soils (agricultural field and mineral soils of the restored wetland), and under drained conditions in the forested wetland. These results are consistent with field-measured N2O fluxes in our previous work in these sites. We suggest that nitrification plays an important and underappreciated role in contributing to N2O fluxes from freshwater wetlands with often-saturated, acid-organic soils, while incomplete denitrification is the likely source of N2O following rain events in agricultural soils in southeastern U.S. coastal plain wetlands.