Wastewater treatment plants are known to be important point sources for nitrous oxide (N2O) in the anthropogenic N cycle. Biofilm based treatment systems have gained increasing popularity in the treatment of wastewater, but the mechanisms and controls of N2O formation are not fully understood. Here, we review functional groups of microorganism involved in nitrogen (N) transformations during wastewater treatment, with emphasis on potential mechanism of N2O production in biofilms. Biofilms used in wastewater treatment typically harbour aerobic and anaerobic zones, mediating close interactions between different groups of N transforming organisms. Current models of mass transfer and biomass interactions in biofilms are discussed to illustrate the complex regulation of N2O production. Ammonia oxidizing bacteria (AOB) are the prime source for N2O in aerobic zones, while heterotrophic denitrifiers dominate N2O production in anoxic zones. Nitrosative stress ensuing from accumulation of NO2? during partial nitrification or denitrification seems to be one of the most critical factors for enhanced N2O formation. In AOB, N2O production is coupled to nitrifier denitrification triggered by nitrosative stress, low O2 tension or low pH. Chemical N2O production from AOB intermediates (NH2OH, HNO, NO) released during high NH3 turnover seems to be limited to surface-near AOB clusters, since diffusive mass transport resistance for O2 slows down NH3 oxidation rates in deeper biofilm layers. The proportion of N2O among gaseous intermediates (NO, N2O, N2) in heterotrophic denitrification increases when NO or nitrous acid (HNO2) accumulates because of increasing NO2?, or when transient oxygen intrusion impairs complete denitrification. Limited electron donor availability due to mass transport limitation of organic substrates into anoxic biofilm zones is another important factor supporting high N2O/N2 ratios in heterotrophic denitrifiers. Biofilms accommodating Anammox bacteria release less N2O, because Anammox bacteria have no known N2O producing metabolism and reduce NO2? to N2, thereby lowering nitrosative stress to AOB and heterotrophs. 相似文献
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. 相似文献
The M2 protein is a small proton channel found in the influenza A virus that is necessary for viral replication. The M2 channel is the target of a class of drugs called the adamantanes, which block the channel pore and prevent the virus from replicating. In recent decades mutations have arisen in M2 that prevent the adamantanes from binding to the channel pore, with the most prevalent of these mutations being S31N. Here we report the first crystal structure of the S31N mutant crystallized using lipidic cubic phase crystallization techniques and solved to 1.59 Å resolution. The Asn31 residues point directly into the center of the channel pore and form a hydrogen‐bonded network that disrupts the drug‐binding site. Ordered waters in the channel pore form a continuous hydrogen bonding network from Gly34 to His37. 相似文献
Theoretical calculations using the M062X and QCISD methods were performed on the addition reactions of the aluminum germylenoid H2GeAlCl3 with ethylene. The most two stable structures of germylenoid H2GeAlCl3, i.e., the p-complex and three-membered ring structures, respectively, were employed as reactants. The calculated results indicate that, for the p-complex, H2GeAlCl3 there are two pathways, I and II, of which path I involves just one transition state, while path II involves two transition states between reactants and products. Comparing the reaction barrier heights of path I (44.6 kJ mol?1) and II (37.6 kJ mol?1), the two pathways are competitive, with similar barriers under the same conditions, while for the three-membered ring structure, another two pathways, III and IV, also exist. Path III has one transition state; however, in path IV, two transition states exist. By comparing their barrier heights, path III (barrier height 39.2 kJ mol?1) could occur more easily than path IV (barrier height 92.8 kJ mol?1). Considering solvent effects on these addition reactions, the PCM model and CH2Cl2 solvent were used in calculations, and the calculated results demonstrate that CH2Cl2 solvent is unfavorable for the reactions, except for path II. In CH2Cl2 solvent, paths II and III are more favorable than paths I and IV. 相似文献
Molecular dynamics trajectories 2 μs in length have been generated for the pH-activated, tetrameric M2 proton channel of the influenza A virus in all protonation states of the pH sensor located at the His37 tetrad. All simulated structures are in very good agreement with high-resolution structures. Changes in the channel caused by progressive protonation of His37 provide insight into the mechanism of proton transport. The channel is closed at both His37 and Trp41 sites in the singly and doubly protonated states, but it opens at Trp41 upon further protonation. Anions access the charged His37 and by doing so stabilize the protonated states of the channel. The narrow opening at the His37 site, further blocked by anions, is inconsistent with the water-wire mechanism of proton transport. Instead, conformational interconversions of His37 correlated with hydrogen bonding to water molecules indicate that these residues shuttle protons in high-protonation states. Hydrogen bonds between charged and uncharged histidines are rare. The valve at Val27 remains on average quite narrow in all protonation states but fluctuates sufficiently to support water and proton transport. A proton transport mechanism in which the channel, depending on pH, opens at either the histidine or valine gate is only partially supported by the simulations. 相似文献
BioDeNOx process, which combines the advantages of the chemical absorption and biological reduction processes, is regarded as a promising candidate for NO removal from the flue gas. In the BioDeNOx, N2O was accumulated in the process of the biological reduction of FeII(EDTA)-NO. In this work, the pathway of the FeII(EDTA)-NO reduction was investigated and a mathematic model was developed to evaluate and predict the accumulation of N2O. Furthermore, parametric tests such as the effects of the C/N ratio (molar ratio of carbon/nitrogen), electron donor, and sulfite concentrations on N2O accumulation were investigated. Experimental results revealed that N2O accumulation was inhibited with a high C/N ratio (2.4), sufficient electron donor, and a low sulfite concentration. In addition, compared with the inorganic electron donor (FeII(EDTA)), the organic electron donor (glucose) was beneficial for microorganism metabolism and N2O accumulation inhibition. This work will provide significant insight into the inhibition of N2O accumulation during the operation of BioDeNOx and advance this novel process for the industrial application. 相似文献
Molecular dynamics trajectories 2 μs in length have been generated for the pH-activated, tetrameric M2 proton channel of the influenza A virus in all protonation states of the pH sensor located at the His37 tetrad. All simulated structures are in very good agreement with high-resolution structures. Changes in the channel caused by progressive protonation of His37 provide insight into the mechanism of proton transport. The channel is closed at both His37 and Trp41 sites in the singly and doubly protonated states, but it opens at Trp41 upon further protonation. Anions access the charged His37 and by doing so stabilize the protonated states of the channel. The narrow opening at the His37 site, further blocked by anions, is inconsistent with the water-wire mechanism of proton transport. Instead, conformational interconversions of His37 correlated with hydrogen bonding to water molecules indicate that these residues shuttle protons in high-protonation states. Hydrogen bonds between charged and uncharged histidines are rare. The valve at Val27 remains on average quite narrow in all protonation states but fluctuates sufficiently to support water and proton transport. A proton transport mechanism in which the channel, depending on pH, opens at either the histidine or valine gate is only partially supported by the simulations. 相似文献
Heavy water (H218O) has been used to label DNA of soil microorganisms in stable isotope probing experiments, yet no measurements have been
reported for the 18O content of DNA from soil incubated with heavy water. Here we present the first measurements of atom% 18O for DNA extracted from soil incubated with the addition of H218O. Four experiments were conducted to test how the atom% 18O of DNA, extracted from Ponderosa Pine forest soil incubated with heavy water, was affected by the following variables: (1)
time, (2) nutrients, (3) soil moisture, and (4) atom% 18O of added H2O. In the time series experiment, the atom% 18O of DNA increased linearly (R2 = 0.994, p < 0.01) over the first 72 h of incubation. In the nutrient addition experiment, there was a positive correlation (R2 = 0.991, p = 0.006) between the log10 of the amount of tryptic soy broth, a complex nutrient broth, added to soil and the log10 of the atom% 18O of DNA. For the experiment where soil moisture was manipulated, the atom% 18O of DNA increased with higher soil moisture until soil moisture reached 30%, above which 18O enrichment of DNA declined as soils became more saturated. When the atom% 18O for H2O added was varied, there was a positive linear relationship between the atom% 18O of the added water and the atom% 18O of the DNA. Results indicate that quantification of 18O incorporated into DNA from H218O has potential to be used as a proxy for microbial growth in soil. 相似文献
The M2 protein form influenza A virus forms a tetrameric ion channel, which enables proton passage across biological membranes when the N-terminal side is acidified. Among the amino acid residues in the transmembrane domain of the M2 protein, His37 and Trp41 are essential for the pH-regulated proton conductance. Current knowledge about the structures and interactions of His37 and Trp41 suggests a model for the M2 ion channel, in which the channel is closed by a network of His37 hydrogen bonds at neutral pH and is opened by a His37-Trp41 cation-pi interaction at acidic pH. 相似文献
The cooperativity effects of the H-bonding interactions in HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane)???HMX???FA (formamide), HMX???HMX???H2O and HMX???HMX???HMX complexes involving the chair and chair–chair HMX are investigated by using the ONIOM2 (CAM-B3LYP/6–31++G(d,p):PM3) and ONIOM2 (M06-2X/6–31++G(d,p):PM3) methods. The solvent effect of FA or H2O on the cooperativity effect in HMX???HMX???HMX are evaluated by the integral equation formalism polarized continuum model. The results show that the cooperativity and anti-cooperativity effects are not notable in all the systems. Although the effect of solvation on the binding energy of ternary system HMX???HMX???HMX is not large, that on the cooperativity of H-bonds is notable, which leads to the mutually strengthened H-bonding interaction in solution. This is perhaps the reason for the formation of different conformation of HMX in different solvent. Surface electrostatic potential and reduced density gradient are used to reveal the nature of the solvent effect on cooperativity effect in HMX???HMX???HMX.
Nitrous oxide (N2O) is one of the three main biogenic greenhouse gases (GHGs) and agriculture represents close to 30 % of the total N2O net emissions. In agricultural soils, N2O is emitted by two main microbial processes, nitrification and denitrification, both of which can convert synthetic nitrogen fertilizer into N2O. Legume-rhizobia symbiosis could be an effective and environmental-friendly alternative to nitrogen fertilization and hence, to mitigate soil N2O emissions. However, legume crops also contribute to N2O emissions. A better understanding of the environmental factors involved in the emission of N2O from nodules would be instrumental for mitigating the release of this GHG gas. In this work, in vivo N2O emissions from nodulated soybean roots in response to nitrate (0, 1, 2 and 4 mM) and flooding have been measured. To investigate the contribution of rhizobial denitrification in N2O emission from nodules, plants were inoculated with B. japonicum USDA110 and napA and nosZ denitrification mutants. The results showed that nitrate was essential for N2O emissions and its concentration enhanced N2O fluxes showing a statistical linear correlation, being the highest N2O fluxes obtained with 4 mM nitrate. When inoculated plants grown with 4 mM nitrate were subjected to flooding, a 150- and 830-fold induction of N2O emission rates from USDA110 and nosZ nodulated roots, respectively, was observed compared to non-flooded plants, especially during long-term flooding. Under these conditions, N2O emissions from detached nodules produced by the napA mutant were significantly lower (p?<?0.05) than those produced by the wild-type strain (382 versus 1120 nmol N2O h?1 g?1 NFW, respectively). In contrast, nodules from plants inoculated with the nosZ mutant accumulated statistically higher levels of N2O compared to wild-type nodules (2522 versus nmol 1120 N2O h?1 g?1 NFW, p?<?0.05). These results demonstrate that flooding is an important environmental factor for N2O emissions from soybean nodules and that B. japonicum denitrification is involved in such emission. 相似文献
A density functional theory (DFT) study of cct-As, ccc, and cct-CO isomers of the ruthenium dihydride complex RuH2(CO)2(AsMe2Ph)2 is reported (see Scheme for the labeling isomer 34 structures of RuH2(CO)2(AsMe2Ph)2). Complex geometries and relative energies of different isomers have been calculated with both B3LYP and M06-2X functionals. The results show that the B3LYP calculated Boltzmann populations of cct-As, ccc, and cct-CO isomers are 65.5, 34.2, and 0.3%, respectively. These are in better agreement with the experimental data than those calculated at the M06-2X level. However, the calculations of 1H NMR chemical shifts were found to be better described with M06-2X than with B3LYP or with HF level of theories. In addition, a transition state between the two most stable isomers was determined through DFT/(B3LYP or M06-2X) calculations.
We found a two-fold increase in the productivity of baker’s yeast grown on a nutrient mixture prepared in light water with a D2O content (127 ppm) smaller than in the distilled water (150 ppm). The number of water monomers that provides the biosynthetic activity (water transport through membrane channels) of yeast cells with an increased CO2 yield was determined for the first time. We established that the selectivity of cell membrane channels in water of different composition depends not only on the motion of ortho-and para-spin H2O isomers in solution, as was shown earlier, but also on the concentration of D2O. 相似文献
Nitrous oxide (N2O) is a potent greenhouse gas with a high contribution from agricultural soils and emissions that depend on soil type, climate,
crops and management practices. The N2O emissions therefore need to be included as an integral part of environmental assessments of agricultural production systems.
An algorithm for N2O production and emission from agricultural soils was developed and included in the FASSET whole-farm model. The model simulated
carbon and nitrogen (N) turnover on a daily basis. Both nitrification and denitrification was included in the model as sources
for N2O production, and the N2O emissions depended on soil microbial and physical conditions. The model was tested on experimental data of N2O emissions from grasslands in UK, Finland and Denmark, differing in climatic conditions, soil properties and management.
The model simulated the general time course of N2O emissions and captured the observed effects of fertiliser and manure management on emissions. Scenario analyses for grazed
and cut grasslands were conducted to evaluate the effects of soil texture, climatic conditions, grassland management and N
fertilisation on N2O emissions. The soils varied from coarse sand to sandy loam and the climatic variation was taken to represent the climatic
variation within Denmark. N fertiliser rates were varied from 0 to 500 kg N ha−1. The simulated N2O emissions showed a non-linear response to increasing N rates with increasing emission factors at higher N rates. The simulated
emissions increased with increasing soil clay contents. N2O emissions were slightly increased at higher temperatures, whereas increasing annual rainfall generally lead to decreasing
emissions. Emissions were slightly higher from grazed grasslands compared with cut grasslands at similar rates of total N
input (fertiliser and animal excreta). The results indicate higher emission factors and thus higher potentials for reducing
N2O emissions for intensively grazed grasslands on fine textured soils than for extensive cut-based grasslands on sandy soils. 相似文献
The present study deals with the decomposition of CF3OCF2O radical formed from a hydrofluoroether, CF3OCHF2 (HFE-125), in the atmosphere. The study is performed using ab initio quantum mechanical methods. Two plausible pathways of
decomposition of the titled species have been considered, one involving C-O bond scission and the other occurring via F atom
elimination. The geometries of the reactant, products and transition states involved in the decomposition pathways are optimized
and characterized at DFT (B3LYP) level of theory using 6-311G(d,p) basis set. Single point energy calculations have been performed
at G2M(CC,MP2) level of theory. Out of the two prominent decomposition channels considered, the C-O bond scission is found
to be dominant involving a barrier height of 15.3 kcal mol−1 whereas the F-elimination path proceeds with a barrier of 26.1 kcal mol−1. The thermal rate constants for the above two decomposition pathways are evaluated using canonical transition state theory
(CTST) and these are found to be 1.78 × 106 s−1 and 2.83 × 10−7 s−1 for C-O bond scission and F-elimination respectively at 298 K and 1 atm pressure. Transition states are searched on the potential
energy surfaces involved during the decomposition channels and each of the transition states is characterized. The existence
of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate
(IRC) calculation. 相似文献
H+-FOF1-ATP synthase (F-ATPase, F-type ATPase, FOF1 complex) catalyzes ATP synthesis from ADP and inorganic phosphate in eubacteria, mitochondria, chloroplasts, and some archaea. ATP synthesis is powered by the transmembrane proton transport driven by the proton motive force (PMF) generated by the respiratory or photosynthetic electron transport chains. When the PMF is decreased or absent, ATP synthase catalyzes the reverse reaction, working as an ATP-dependent proton pump. The ATPase activity of the enzyme is regulated by several mechanisms, of which the most conserved is the non-competitive inhibition by the MgADP complex (ADP-inhibition). When ADP binds to the catalytic site without phosphate, the enzyme may undergo conformational changes that lock bound ADP, resulting in enzyme inactivation. PMF can induce release of inhibitory ADP and reactivate ATP synthase; the threshold PMF value required for enzyme reactivation might exceed the PMF for ATP synthesis. Moreover, membrane energization increases the catalytic site affinity to phosphate, thereby reducing the probability of ADP binding without phosphate and preventing enzyme transition to the ADP-inhibited state. Besides phosphate, oxyanions (e.g., sulfite and bicarbonate), alcohols, lauryldimethylamine oxide, and a number of other detergents can weaken ADP-inhibition and increase ATPase activity of the enzyme. In this paper, we review the data on ADP-inhibition of ATP synthases from different organisms and discuss the in vivo role of this phenomenon and its relationship with other regulatory mechanisms, such as ATPase activity inhibition by subunit ε and nucleotide binding in the noncatalytic sites of the enzyme. It should be noted that in Escherichia coli enzyme, ADP-inhibition is relatively weak and rather enhanced than prevented by phosphate. 相似文献
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. 相似文献
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. 相似文献
The effect of exogenous hydrogen peroxide (H2O2) on mitotic activity and chromosomal aberrations in root tip meristems of barley (Hordeum vulgare L. var. Tokak 157/37) germinated under salinity was analyzed. The inhibitory effect of salinity on mitotic index and the
frequency of chromosomal aberrations increased with increasing salt concentration (0.00 control, 0.35, 0.40, 0.45 M, molal
NaCl). The frequency of chromosomal aberrations of seeds germinated in medium with 0.40 M NaCl after pretreatment with H2O2 (30 μM, micromolal) was significantly higher than the control group. The highest concentration of NaCl (0.45 M) together
with H2O2 caused total inhibition of germination. In this study, the intention was to determine the performance of H2O2 in alleviating detrimental effect of salt stress on mitotic activity and chromosomal aberrations. However, H2O2 did not reduce the detrimental effect of NaCl on these parameters. Also, it caused higher chromotoxic effect compared to
those of control groups. 相似文献