Study of Plasma Induced Chemistry by DC Discharges in CO<Subscript>2</Subscript>/N<Subscript>2</Subscript>/H<Subscript>2</Subscript>O Mixtures Above a Water Surface

The chemistry induced by atmospheric pressure DC discharges above a water surface in CO(2)/N(2)/H(2)O mixtures was investigated. The gaseous mixtures studied represent a model prebiotic atmosphere of the Earth. The most remarkable changes in the chemical composition of the treated gas were the decomposition of CO(2) and the production of CO. The concentration of CO increased logarithmically with the increasing input energy density and an increasing initial concentration of CO(2) in the gas. The highest achieved concentration of CO was 4.0 +/- 0.6 vol. %. The production of CO was crucial for the synthesis of organic species, since reactions of CO with some reactive species generated in the plasma, e. g. H* or N* radicals, were probably the starting point in this synthesis. The presence of organic species (including the tentative identification of some amino acids) was demonstrated by the analysis of solid and liquid samples by high-performance liquid chromatography, infrared absorption spectroscopy and proton-transfer-reaction mass spectrometry. Formation of organic species in a completely inorganic CO(2)/N(2)/H(2)O atmosphere is a significant finding for the theory of the origins of life.     

Emissions of CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O from undisturbed,drained and mined peatlands in Estonia

The aim of this study is to estimate emissions of greenhouse gases CO₂, CH₄ and N₂O, and the effects of drainage and peat extraction on these processes, in Estonian transitional fens and ombrotrophic bogs. Closed-chamber-based sampling lasted from January to December 2009 in nine peatlands in Estonia, covering areas with different land-use practices: natural (four study sites), drained (six sites), abandoned peat mining (five sites) and active peat mining areas (five sites). Median values of soil CO₂ efflux were 1,509, 1,921, 2,845 and 1,741 kg CO₂-C ha^?1 year^?1 from natural, drained, abandoned and active mining areas, respectively. Emission of CH₄-C (median values) was 85.2, 23.7, 0.07 and 0.12 kg ha^?1 year^?1, and N₂O-N ?0.05, ?0.01, 0.18 and 0.19 kg ha^?1 year^?1, respectively. There were significantly higher emissions of CO₂ and N₂O from abandoned and active peat mining areas, whereas CH₄ emissions were significantly higher in natural and drained areas. Significant Spearman rank correlation was found between soil temperature and CO₂ flux at all sites, and CH₄ flux with high water level at natural and drained areas. Significant increase in CH₄ flux was detected for groundwater levels above 30 cm.     

Exploiting mixtures of H<Subscript>2</Subscript>, CO<Subscript>2</Subscript>, and O<Subscript>2</Subscript> for improved production of methacrylate precursor 2-hydroxyisobutyric acid by engineered <Emphasis Type="Italic">Cupriavidus necator</Emphasis> strains

Current manufacturing of most bulk chemicals through petrochemical routes considerably contributes to common concerns over the depletion of fossil carbon sources and greenhouse gas emissions. Sustainable future production of commodities thus requires the shift to renewable feedstocks in combination with established or newly developed synthesis routes. In this study, the potential of Cupriavidus necator H16 for autotrophic synthesis of the building block chemical 2-hydroxyisobutyric acid (2-HIBA) is evaluated. A novel biosynthetic pathway was implemented by heterologous expression of the 2-hydroxyisobutyryl-coenzyme A (2-HIB-CoA) mutase from Aquincola tertiaricarbonis L108, relying on a main intermediate of strain H16’s C4 overflow metabolism, 3-hydroxybutyryl-CoA. The intention was to direct the latter to 2-HIBA instead or in addition to poly-3-hydroxybutyrate (PHB). Autotrophic growth and 2-HIBA (respectively, PHB) synthesis of wild-type and PHB-negative mutant strains were investigated producing maximum 2-HIBA titers of 3.2 g L^?1 and maximum specific 2-HIBA synthesis rates (q _2-HIBA) of about 16 and 175 μmol g^?1 h^?1, respectively. The obtained specific productivity was the highest reported to date for mutase-dependent 2-HIBA synthesis from heterotrophic and autotrophic substrates. Furthermore, expression of a G protein chaperone (MeaH) in addition to the 2-HIB-CoA mutase subunits yielded improved productivity. Analyzing the inhibition of growth and product synthesis due to substrate availability and product accumulation revealed a strong influence of 2-HIBA, when cells were cultivated at high titers. Nevertheless, the presented results imply that at the time the autotrophic synthesis route is superior to thus far established heterotrophic routes for production of 2-HIBA with C. necator.     

Spatial Variation of Soil CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O Fluxes Across Topographical Positions in Tropical Forests of the Guiana Shield

The spatial variation of soil greenhouse gas fluxes (GHG; carbon dioxide—CO₂, methane—CH₄ and nitrous oxide—N₂O) remains poorly understood in highly complex ecosystems such as tropical forests. We used 240 individual flux measurements of these three GHGs from different soil types, at three topographical positions and in two extreme hydric conditions in the tropical forests of the Guiana Shield (French Guiana, South America) to (1) test the effect of topographical positions on GHG fluxes and (2) identify the soil characteristics driving flux variation in these nutrient-poor tropical soils. Surprisingly, none of the three GHG flux rates differed with topographical position. CO₂ effluxes covaried with soil pH, soil water content (SWC), available nitrogen and total phosphorus. The CH₄ fluxes were best explained by variation in SWC, with soils acting as a sink under drier conditions and as a source under wetter conditions. Unexpectedly, our study areas were generally sinks for N₂O and N₂O fluxes were partly explained by total phosphorus and available nitrogen concentrations. This first study describing the spatial variation of soil fluxes of the three main GHGs measured simultaneously in forests of the Guiana Shield lays the foundation for specific studies of the processes underlying the observed patterns.     

DFT study of CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O co-adsorption on carbon models of coal surface

The moisture content of coal affects the adsorption capacity of CO₂ on the coal surface. Since the hydrogen bonds are formed between H₂O and oxygen functional group, the H₂O cluster more easily adsorbs on the coal micropore than CO₂ molecule. The coal micropores are occupied by H₂O molecules that cannot provide extra space for CO₂ adsorption, which may leads to the reduction of CO₂ adsorption capacity. However, without considering factors of micropore and oxygen functional groups, the co-adsorption mechanisms of CO₂ and adsorbed H₂O molecule are not clear. Density functional theory (DFT) calculations were performed to elucidate the effect of adsorbed H₂O to CO₂ adsorption. This study reports some typical coal-H₂O···CO₂ complexes, along with a detailed analysis of the geometry, energy, electrostatic potential (ESP), atoms in molecules (AIM), reduced density gradient (RDG), and energy decomposition analysis (EDA). The results show that H₂O molecule can more stably adsorb on the aromatic ring surface than CO₂ molecule, and the absolute values of local ESP maximum and minimum of H₂O cluster are greater than CO₂. AIM analysis shows a detailed interaction path and strength between atoms in CO₂ and H₂O, and RDG analysis shows that the interactions among CO₂, H₂O, and coal model belong to weak van der Waals force. EDA indicates that electrostatic and long-range dispersion terms play a primary role in the co-adsorption of CO₂ and H₂O. According to the DFT calculated results without considering micropore structure and functional group, it is shown that the adsorbed H₂O can promote CO₂ adsorption on the coal surface. These results demonstrate that the micropore factor plays a dominant role in affecting CO₂ adsorption capacity, the attractive interaction of adsorbed H₂O to CO₂ makes little contribution.     

The polythiophene molecular segment as a sensor model for H<Subscript>2</Subscript>O,HCN, NH<Subscript>3</Subscript>, SO<Subscript>3</Subscript>, and H<Subscript>2</Subscript>S: a density functional theory study

Studying the interaction of some atmospheric gases (H₂O, HCN, NH₃, SO₃ and H₂S) with 3PT oligomers is important in the development of polymeric sensors for gas detection. In the present study, we studied the relaxed geometries, interaction energies, charge analysis, HOMO–LUMO orbital analysis, and UV–vis spectra of all interacted systems using first-principles density functional theory (DFT). All these analyses indicated the potential of polythiophene as an inexpensive polymeric sensor for the analytes mentioned. Interaction energy values of ?19.90, ?19.66, ?14.01, ?8.70, and ?4.76 kJ mol^?1 were achieved for adsorption of SO₃, H₂O, NH₃, HCN, and H₂S on 3PT, respectively. Consequently, clarification of their physical parameters became the major focus of this study.     

Photosynthetic CO<Subscript>2</Subscript>/H<Subscript>2</Subscript>O gas exchange and dynamics of carbohydrates content in maize leaves under drought

We studied the temporal sequence of changes in the photosynthetic CO₂/H₂O gas exchange intensity, as well as leaf water status, contents of soluble carbohydrates, starch, proline, pigments, and MDA, in maize seedlings (Zea mays L., cv. Luchistaya) under adaptation to increasing water deficit. The duration of drought was 2, 3, 5, and 6 days. Withholding water from maize plants caused gradual increase in the intensity of water deficit: from mild (2 or 3 days) to moderate (5 days) and nearly severe (6 days) water stress. After 6 days, relative leaf water content decreased by 19.8% as compared to the control. On the second day after the onset of drought, slight reduction in the photosynthetic CO₂/H₂O gas exchange intensity of the treated plants was observed. After 6 days, photosynthesis and transpiration of leaves synchronously reduced almost threefold due to stomatal closure. The progressive soil drought had substantial impact on the carbohydrate metabolism. After 2 days of water deficit, the content of reducing sugars and sucrose increased slightly, whereas after 6 days, it increased ten and four times, respectively. After 2, 3, and 5 days of drought, the starch content declined slightly; however, under severe drought (6 days), it increased by 30% as compared to the control. Simultaneously with the increase in the content of soluble sugars, proline content increased significantly and it was the highest on the sixth day of drought. At all stages of water deficit, the proline content increased more significantly than the content of reducing carbohydrates and sucrose. Under increasing water deficit (5 and 6 days), the content of MDA was found to rise. At the initial drought stage (2 or 3 days) and under severe water deficit (6 days), no significant changes in the pigment content were observed. Thus, at the initial stages of progressive drought, in the leaves of this maize cultivar, a decline in photosynthetic activity proceeded simultaneously with accumulation of reducing sugars, sucrose, and proline. The results obtained showed that, at the first stages of adaptation of maize seedlings to drought, the changes in carbohydrate and proline metabolism have been observed, which have increased upon further plant dehydration.     

Temporal Variability of CO<Subscript>2</Subscript> and N<Subscript>2</Subscript>O Flux Spatial Patterns at a Mowed and a Grazed Grassland

Spatial patterns of ecosystem processes constitute significant sources of uncertainty in greenhouse gas flux estimations partly because the patterns are temporally dynamic. The aim of this study was to describe temporal variability in the spatial patterns of grassland CO₂ and N₂O flux under varying environmental conditions and to assess effects of the grassland management (grazing and mowing) on flux patterns. We made spatially explicit measurements of variables including soil respiration, aboveground biomass, N₂O flux, soil water content, and soil temperature during a 4-year study in the vegetation periods at grazed and mowed grasslands. Sampling was conducted in 80 × 60 m grids of 10 m resolution with 78 sampling points in both study plots. Soil respiration was monitored nine times, and N₂O flux was monitored twice during the study period. Altitude, soil organic carbon, and total soil nitrogen were used as background factors at each sampling position, while aboveground biomass, soil water content, and soil temperature were considered as covariates in the spatial analysis. Data were analyzed using variography and kriging. Altitude was autocorrelated over distances of 40–50 m in both plots and influenced spatial patterns of soil organic carbon, total soil nitrogen, and the covariates. Altitude was inversely related to soil water content and aboveground biomass and positively related to soil temperature. Autocorrelation lengths for soil respiration were similar on both plots (about 30 m), whereas autocorrelation lengths of N₂O flux differed between plots (39 m in the grazed plot vs. 18 m in the mowed plot). Grazing appeared to increase heterogeneity and linkage of the spatial patterns, whereas mowing had a homogenizing effect. Spatial patterns of soil water content, soil respiration, and aboveground biomass were temporally variable especially in the first 2 years of the experiment, whereas spatial patterns were more persistent (mostly significant correlation at p < 0.05 between location ranks) in the second 2 years, following a wet year. Increased persistence of spatial patterns after a wet year indicated the recovery potential of grasslands following drought and suggested that adequate water supply could have a homogenizing effect on CO₂ and N₂O fluxes.     

The subcommissural organ expresses D<Subscript>2</Subscript>, D<Subscript>3</Subscript>, D<Subscript>4</Subscript>, and D<Subscript>5</Subscript> dopamine receptors

Dopamine receptors have been found in certain populations of non-neuronal cells in the brain, viz., discrete areas of ciliated ependyma and the ependymal cells of the choroid plexus. We have studied the presence of both tyrosine-hydroxylase-immunoreactive nerve fibers and dopamine receptors in the subcommissural organ (SCO), an ependymal brain gland that is located in the roof of the third ventricle and that secretes, into the cerebrospinal fluid, glycoproteins that aggregate to form Reissners fiber (RF). Antibodies against D₂, D₃, D₄, and D₅ dopamine receptors were used in immunoblots of bovine striatum, fresh SCO, and organ-cultured SCO, and in immunocytochemistry of the bovine, rat, and mouse SCO. Only a few tyrosine-hydroxylase fibers appeared to reach the SCO. However, virtually all the secretory ependymal and hypendymal cells of the SCO immunoreacted with antibodies against D₂, D₄, and D₅ receptors, with the last-mentioned rendering the strongest reaction, especially at the ventricular cell pole of the secretory ependymocytes, suggesting that dopamine might reach the SCO via the cerebrospinal fluid. The antibodies against the four subtypes of receptors revealed corresponding bands in immunoblots of striatum and fresh SCO. Although the cultured SCO displayed dopamine receptors, dopamine had no apparent effect on the expression of the SCO-spondin gene/protein or on the release of RF-glycoproteins (SCO-spondin included) by SCO explants, suggesting that dopamine affects the function(s) of the SCO differently from the secretion of RF-glycoproteins.Financial support was provided by grants PI 030756 and Red CIEN, Instituto de Salud Carlos III, Spain (to J.M.P.F.), and 1030265 from Fondecyt, Chile (to E.M.R.)     

Simulation and optical spectroscopy of a DC discharge in a CH<Subscript>4</Subscript>/H<Subscript>2</Subscript>/N<Subscript>2</Subscript> mixture during deposition of nanostructured carbon films

Two-dimensional numerical simulations of a dc discharge in a CH₄/H₂/N₂ mixture in the regime of deposition of nanostructured carbon films are carried out with account of the cathode electron beam effects. The distributions of the gas temperature and species number densities are calculated, and the main plasmachemical kinetic processes governing the distribution of methyl radicals above the substrate are analyzed. It is shown that the number density of methyl radicals above the substrate is several orders of magnitude higher than the number densities of other hydrocarbon radicals, which indicates that the former play a dominant role in the growth of nanostructured carbon films. The model is verified by comparing the measured optical emission profiles of the H(n ≡ 3), C ₂ ^* , CH*, and CN* species and the calculated number densities of excited species, as well as the measured and calculated values of the discharge voltage and heat fluxes onto the electrodes and reactor walls. The key role of ion–electron recombination and dissociative excitation of H₂, C₂H₂, CH₄, and HCN molecules in the generation of emitting species (first of all, in the cold regions adjacent to the electrodes) is revealed.     

Molecular properties of metal difluorides and their interactions with CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O molecules: a DFT investigation

A computational study of metal difluorides (MF₂; M = Ca to Zn) and their interactions with carbon dioxide and water molecules was performed. The structural parameter values obtained and the results of AIM analysis and energy decomposition analysis indicated that the Ca–F bond is weaker and less ionic than the bonds in the transition metal difluorides. A deformation density plot revealed the stablizing influence of the Jahn–Teller effect in nonlinear MF₂ molecules (e.g., where M= Sc, Ti, Cr). An anaysis of the metal K-edge peaks of the difluorides showed that shifts in the edge energy were due to the combined effects of the ionicity, effective nuclear charge, and the spin state of the metal. The interactions of CO₂ with ScF₂ (Scc3 geometry) and TiF₂ (Tic2 geometry) caused CO₂ to shift from its usual linear geometry to a bent geometry (η²(C=O) binding mode), while it retained its linear geometry (η¹(O) binding mode) when it interacted with the other metal difluorides. Energy decomposition analysis showed that, among the various geometries considered, the Scc3 and Tic2 geometries possessed the highest interaction energies and orbital interaction energies. Heavier transition metal difluorides showed stronger affinities for H₂O, whereas the lighter transition metal (Sc and Ti) difluorides preferred CO₂. Overall, the results of this study suggest that fluorides of lighter transition metals with partially filled d orbitals (e.g., Sc and Ti) could be used for CO₂ capture under moist conditions.

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Graphical abstract Interaction of metal difluorides with carbon dioxide and water

     

CO<Subscript>2</Subscript> bio-mitigation using microalgae

Microalgae are a group of unicellular or simple multicellular photosynthetic microorganisms that can fix CO(2) efficiently from different sources, including the atmosphere, industrial exhaust gases, and soluble carbonate salts. Combination of CO(2) fixation, biofuel production, and wastewater treatment may provide a very promising alternative to current CO(2) mitigation strategies.     

Atlantic cod actively avoid CO<Subscript>2</Subscript> and predator odour,even after long-term CO<Subscript>2</Subscript> exposure

Introduction

The rising atmospheric CO₂ level is continuously driving the dissolution of more CO₂ into the oceans, and some emission scenarios project that the surface waters may reach 1000 μatm by the end of the century. It is not known if fish can detect moderately elevated CO₂ levels, and if they avoid areas with high CO₂. If so, avoidance behaviour to water with high CO₂ could affect movement patterns and migrations of fish in the future. It is also being increasingly recognized that fish behaviour can be altered by exposure to CO₂. Therefore this study investigated how long-term exposure to elevated pCO₂ affects predator avoidance and CO₂ avoidance in juvenile Atlantic cod (Gadus morhua). The fish were exposed to control water or CO₂-enriched water (1000 μatm) for six weeks before being subjected to tests of behaviour.

Results

Despite long term exposure to elevated pCO₂ the cod still strongly avoided the smell of a predator. These data are surprising because several coral reef fish have demonstrated reversal of olfactory responses after CO₂ exposure, turning avoidance of predator cues into preference for predator cues. Fish from both treatment groups also demonstrated strong avoidance of CO₂ when presented with the choice of control or CO₂-acidified water, indicating that habituation to the CO₂ sensory stimuli is negligible.

Conclusions

As Atlantic cod maintained normal behavioural responses to olfactory cues, they may be tolerant to CO₂-induced behavioural changes. The results also suggest that despite the long-term exposure to CO₂-acidified water, the fish still preferred the control water over CO₂-acidified water. Therefore, in the future, fish may alter their movements and migrations in search of waters with a lower CO₂ content.

     

Foliar quality influences tree-herbivore-parasitoid interactions: effects of elevated CO<Subscript>2</Subscript>, O<Subscript>3</Subscript>, and plant genotype

This study examined the effects of carbon dioxide (CO₂)-, ozone (O₃)-, and genotype-mediated changes in quaking aspen (Populus tremuloides) chemistry on performance of the forest tent caterpillar (Malacosoma disstria) and its dipteran parasitoid (Compsilura concinnata) at the Aspen Free-Air CO₂ Enrichment (FACE) site. Parasitized and non-parasitized forest tent caterpillars were reared on two aspen genotypes under elevated levels of CO₂ and O₃, alone and in combination. Foliage was collected for determination of the chemical composition of leaves fed upon by forest tent caterpillars during the period of endoparasitoid larval development. Elevated CO₂ decreased nitrogen levels but had no effect on concentrations of carbon-based compounds. In contrast, elevated O₃ decreased nitrogen and phenolic glycoside levels, but increased concentrations of starch and condensed tannins. Foliar chemistry also differed between aspen genotypes. CO₂, O₃, genotype, and their interactions altered forest tent caterpillar performance, and differentially so between sexes. In general, enriched CO₂ had little effect on forest tent caterpillar performance under ambient O₃, but reduced performance (for insects on one aspen genotype) under elevated O₃. Conversely, elevated O₃ improved forest tent caterpillar performance under ambient, but not elevated, CO₂. Parasitoid larval survivorship decreased under elevated O₃, depending upon levels of CO₂ and aspen genotype. Additionally, larval performance and masses of mature female parasitoids differed between aspen genotypes. These results suggest that host-parasitoid interactions in forest systems may be altered by atmospheric conditions anticipated for the future, and that the degree of change may be influenced by plant genotype.     

Comparison of CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O fluxes over grassland vegetations measured by the eddy-covariance technique and by open system chamber

Measurements of CO₂ and H₂O fluxes were carried out using two different techniques—eddy-covariance (EC) and open system gas exchange chamber (OC)—during two-years’ period (2003–2004) at three different grassland sites. OC measurements were made during fourteen measurement campaigns. We found good agreement between the OC and EC CO₂ flux values (n = 63, r ² = 0.5323, OC F_CO2 = −0.6408+0.9508 EC F_CO2). The OC F_H2O values were consistently lower than those measured by the EC technique, probably caused by the air stream difference inside and outside the chamber. Adjusting flow rate within the chamber to the natural conditions would be necessary in future OC measurements. In comparison with EC, the OC proved to be a good tool for gas exchange measurements in grassland ecosystems.     

Putrescine protects hulless barley from damage due to UV-B stress via H<Subscript>2</Subscript>S- and H<Subscript>2</Subscript>O<Subscript>2</Subscript>-mediated signaling pathways

Key message

In hulless barley, H ₂ S mediated increases in H ₂ O ₂ induced by putrescine, and their interaction enhanced tolerance to UV-B by maintaining redox homeostasis and promoting the accumulation of UV-absorbing compounds.

Abstract

This study investigated the possible relationship between putrescence (Put), hydrogen sulfide (H₂S) and hydrogen peroxide (H₂O₂) as well as the underlying mechanism of their interaction in reducing UV-B induced damage. UV-B radiation increased electrolyte leakage (EL) and the levels of malondialdehyde (MDA) and UV-absorbing compounds but reduced antioxidant enzyme activities and glutathione (GSH) and ascorbic acid (AsA) contents. Exogenous application of Put, H₂S or H₂O₂ reduced some of the above-mentioned negative effects, but were enhanced by the addition of Put, H₂S and H₂O₂ inhibitors. Moreover, the protective effect of Put against UV-B radiation-induced damage to hulless barley was diminished by dl-propargylglycine (PAG, a H₂S biosynthesis inhibitor), hydroxylamine (HT, a H₂S scavenger), diphenylene iodonium (DPI, a PM-NADPH oxidase inhibitor) and dimethylthiourea (DMTU, a ROS scavenger), and the effect of Put on H₂O₂ accumulation was abolished by HT. Taken together, as the downstream component of the Put signaling pathway, H₂S mediated H₂O₂ accumulation, and H₂O₂ induced the accumulation of UV-absorbing compounds and maintained redox homeostasis under UV-B stress, thereby increasing the tolerance of hulless barley seedlings to UV-B stress.

     

Selective effects of H<Subscript>2</Subscript>O<Subscript>2</Subscript> on cyanobacterial photosynthesis

The sensitivity of phytoplankton species for hydrogen peroxide (H₂O₂) was analyzed by pulse amplitude modulated (PAM) fluorometry. The inhibition of photosynthesis was more severe in five tested cyanobacterial species than in three green algal species and one diatom species. Hence the inhibitory effect of H₂O₂ is especially pronounced for cyanobacteria. A specific damage of the photosynthetic apparatus was demonstrated by changes in 77 K fluorescence emission spectra. Different handling of oxidative stress and different cell structure are responsible for the different susceptibility to H₂O₂ between cyanobacteria and other phytoplankton species. This principle may be potentially employed in the development of new agents to combat cyanobacterial bloom formation in water reservoirs.     

The control of mitochondrial succinate-dependent H<Subscript>2</Subscript>O<Subscript>2</Subscript> production

In brain mitochondria succinate activates H₂O₂ release, concentration dependently (starting at 15 μM), and in the presence of NAD dependent substrates (glutamate, pyruvate, β-hydroxybutyrate). We report that TCA cycle metabolites (citrate, isocitrate, α-ketoglutarate, fumarate, malate) individually and quickly inhibit H₂O₂ release. When they are present together at physiological concentration (0.2, 0.01, 0.15, 0.12, 0.2 mM respectively) they decrease H₂O₂ production by over 60% at 0.1–0.2 mM succinate. The degree of inhibition depends on the concentration of each metabolite. Acetoacetate is a strong inhibitor of H₂O₂ release, starting at 10 μM and acting quickly. It potentiates the inhibition induced by TCA cycle metabolites. The action of acetoacetate is partially removed by β-hydroxybutyrate. Removal is minimal at 0.1 mM acetoacetate, and is higher at 0.5 mM acetoacetate. We conclude that several inhibitors of H₂O₂ release act jointly and concentration dependently to rapidly set the required level of H₂O₂ generation at each succinate concentration.     

Decades-long effects of high CO<Subscript>2</Subscript> concentration on soil nitrogen dynamics at a natural CO<Subscript>2</Subscript> spring

The effects of high atmospheric CO₂ concentration ([CO₂]) on ecosystem processes have been explored using temporal facilities such as open-top-chambers and free-air CO₂ enrichment. However, the effects of high [CO₂] on soil properties takes decades and may not be captured by short-term experiments. Natural CO₂ springs provide a unique opportunity to study the long-term effects of high [CO₂]. In this study, we investigated soil properties at a natural CO₂ spring. We found that the amounts of total carbon (C) and nitrogen (N) stored in the soil at the high [CO₂] site exceeded those in the reference site by 60 and 30%, respectively. The effects of high [CO₂] were large in the upper slope position where the canopy openness was high and plants grew faster, but no effects were detected in the lowest position where the canopy openness was lower (half of that at the upper slope position). In contrast, effects of high [CO₂] on soil N dynamics, such as N mineralization and nitrification rates, did not exhibit a slope gradient. This suggests that effects of high [CO₂] differed among soil stoichiometric characteristics and N dynamics. These complicated effects of high [CO₂] imply that the future effects of high [CO₂] on ecosystems could vary widely in conjunction with environmental conditions such as light availability and/or topographic conditions.