N2O Production Rates Research Articles

Steady-State Growth under Inorganic Carbon Limitation Conditions Increases Energy Consumption for Maintenance and Enhances Nitrous Oxide Production in Nitrosomonas europaea.

Nitrosomonas europaea is a chemolithoautotrophic bacterium that oxidizes ammonia (NH3) to obtain energy for growth on carbon dioxide (CO2) and can also produce nitrous oxide (N2O), a greenhouse gas. We interrogated the growth, physiological, and transcriptome responses of N. europaea to conditions of replete (>5.2 mM) and limited inorganic carbon (IC) provided by either 1.0 mM or 0.2 mM sodium carbonate (Na2CO3) supplemented with atmospheric CO2 IC-limited cultures oxidized 25 to 58% of available NH3 to nitrite, depending on the dilution rate and Na2CO3 concentration. IC limitation resulted in a 2.3-fold increase in cellular maintenance energy requirements compared to those for NH3-limited cultures. Rates of N2O production increased 2.5- and 6.3-fold under the two IC-limited conditions, increasing the percentage of oxidized NH3-N that was transformed to N2O-N from 0.5% (replete) up to 4.4% (0.2 mM Na2CO3). Transcriptome analysis showed differential expression (P ≤ 0.05) of 488 genes (20% of inventory) between replete and IC-limited conditions, but few differences were detected between the two IC-limiting treatments. IC-limited conditions resulted in a decreased expression of ammonium/ammonia transporter and ammonia monooxygenase subunits and increased the expression of genes involved in C1 metabolism, including the genes for RuBisCO (cbb gene cluster), carbonic anhydrase, folate-linked metabolism of C1 moieties, and putative C salvage due to oxygenase activity of RuBisCO. Increased expression of nitrite reductase (gene cluster NE0924 to NE0927) correlated with increased production of N2O. Together, these data suggest that N. europaea adapts physiologically during IC-limited steady-state growth, which leads to the uncoupling of NH3 oxidation from growth and increased N2O production. Nitrification, the aerobic oxidation of ammonia to nitrate via nitrite, is an important process in the global nitrogen cycle. This process is generally dependent on ammonia-oxidizing microorganisms and nitrite-oxidizing bacteria. Most nitrifiers are chemolithoautotrophs that fix inorganic carbon (CO2) for growth. Here, we investigate how inorganic carbon limitation modifies the physiology and transcriptome of Nitrosomonas europaea, a model ammonia-oxidizing bacterium, and report on increased production of N2O, a potent greenhouse gas. This study, along with previous work, suggests that inorganic carbon limitation may be an important factor in controlling N2O emissions from nitrification in soils and wastewater treatment.

The effects of temperature and resource availability on denitrification and relative N2O production in boreal lake sediments

Anthropogenic environmental stressors (like atmospheric deposition, land use change, and climate warming) are predicted to increase inorganic nitrogen and organic carbon loading to northern boreal lakes, with potential consequences for denitrification in lakes. However, our ability to predict effects of these changes is currently limited as northern boreal lakes have been largely neglected in denitrification studies. The aim of this study was therefore to assess how maximum potential denitrification and N2O production rates, and the relationship between the two (relative N2O production), is controlled by availability of nitrate (NO3−), carbon (C), phosphorus (P), and temperature. Experiments were performed using the acetylene inhibition technique on sediments from a small, nutrient poor boreal lake in northern Sweden in 2014. Maximum potential denitrification and N2O production rates at 4°C were reached already at NO3− additions of 106–120μg NO3−–N/L, and remained unchanged with higher NO3 amendments. Higher incubation temperatures increased maximum potential denitrification and N2O production rates, and Q10 was somewhat higher for N2O production (1.77) than for denitrification (1.69). The relative N2O production ranged between 13% and 64%, and was not related to NO3− concentration, but the ratio increased when incubations were amended with C and P (from a median of 16% to 27%). Combined, our results suggests that unproductive northern boreal lakes currently have low potential for denitrification but are susceptible to small changes in NO3 loading especially if these are accompanied by enhanced C and P availability, likely promoting higher N2O production relative to N2.

Net soil–atmosphere fluxes mask patterns in gross production and consumption of nitrous oxide and methane in a managed ecosystem

Abstract. Nitrous oxide (N2O) and methane (CH4) are potent greenhouse gases that are both produced and consumed in soil. Production and consumption of these gases are driven by different processes, making it difficult to infer their controls when measuring only net fluxes. We used the trace gas pool dilution technique to simultaneously measure gross fluxes of N2O and CH4 throughout the growing season in a cornfield in northern California, USA. Net N2O fluxes ranged 0–4.5 mg N m−2 d−1 with the N2O yield averaging 0.68 ± 0.02. Gross N2O production was best predicted by net nitrogen (N) mineralization, soil moisture, and soil temperature (R2 = 0.60, n = 39, p< 0.001). Gross N2O reduction was correlated with the combination of gross N2O production rates, net N mineralization rates, and CO2 emissions (R2 = 0.74, n = 39, p< 0.001). Overall, net CH4 fluxes averaged −0.03 ± 0.02 mg C m−2 d−1. The methanogenic fraction of carbon mineralization ranged from 0 to 0.27 % and explained 40 % of the variability in gross CH4 production rates (n = 37, p< 0.001). Gross CH4 oxidation exhibited a strong positive relationship with gross CH4 production rates (R2 = 0.67, n = 37, p< 0.001), which reached as high as 5.4 mg C m−2 d−1. Our study is the first to demonstrate the simultaneous in situ measurement of gross N2O and CH4 fluxes, and results highlight that net soil–atmosphere fluxes can mask significant gross production and consumption of these trace gases.

Revisiting nitrification in the Eastern Tropical South Pacific: A focus on controls

AbstractNitrification, the oxidation of ammonium ( ) to nitrite ( ) and to nitrate ( ), is a component of the nitrogen (N) cycle internal to the fixed N pool. In oxygen minimum zones (OMZs), which are hotspots for oceanic fixed N loss, nitrification plays a key role because it directly supplies substrates for denitrification and anaerobic ammonia oxidation (anammox), and may compete for substrates with these same processes. However, the control of oxygen and substrate concentrations on nitrification are not well understood. We performed onboard incubations with 15N‐labeled substrates to measure rates of and oxidation in the eastern tropical South Pacific (ETSP). The spatial and depth distributions of and oxidation rates were primarily controlled by and availability, oxygen concentration, and light. In the euphotic zone, nitrification was partially photoinhibited. In the anoxic layer, oxidation was negligible or below detection, but high rates of oxidation were observed. oxidation displayed extremely high affinity for both and oxygen. The positive linear correlations between oxidation rates and in situ concentrations and ammonia monooxygenase subunit A (amoA) gene abundances in the upper oxycline indicate that the natural assemblage of ammonia oxidizers responds to in situ concentrations or supply by adjusting their population size, which determines the oxidation potential. The depth distribution of archaeal and bacterial amoA gene abundances and N2O concentration, along with independently reported simultaneous direct N2O production rate measurements, suggests that AOA were predominantly responsible for oxidation, which was a major source of N2O production at oxygen concentrations > 5 µM.

Abundance, composition and activity of denitrifier communities in metal polluted paddy soils.

Denitrification is one of the most important soil microbial processes leading to the production of nitrous oxide (N2O). The potential changes with metal pollution in soil microbial community for N2O production and reduction are not well addressed. In this study, topsoil samples were collected both from polluted and non-polluted rice paddy fields and denitrifier communities were characterized with molecular fingerprinting procedures. All the retrieved nirK sequences could be grouped into neither α- nor β- proteobacteria, while most of the nosZ sequences were affiliated with α-proteobacteria. The abundances of the nirK and nosZ genes were reduced significantly in the two polluted soils. Thus, metal pollution markedly affected composition of both nirK and nosZ denitrifiers. While the total denitrifying activity and N2O production rate were both reduced under heavy metal pollution of the two sites, the N2O reduction rate showed no significant change. These findings suggest that N2O production activity could be sensitive to heavy metal pollution, which could potentially lead to a decrease in N2O emission in polluted paddies. Therefore, metal pollution could have potential impacts on soil N transformation and thus on N2O emission from paddy soils.

Investigation of NO formation in premixed adiabatic laminar flames of H2/CO syngas and air by saturated laser-induced fluorescence and kinetic modeling

A series of experiments were performed to investigate the relationship between syngas composition (H2/CO/CO2/N2/O2) and NO formation. Laser-saturated fluorescence measurements of NO in a premixed syngas flat flame from a heat flux burner were recorded for various fuel compositions, dilution ratios and equivalence ratios. Quantitative measurements were compared with the predictions from CHEMKIN software using four different chemical mechanisms, i.e., GRI mech 3.0, Chemical Reaction Engineering and Chemical Kinetics 1407, GDF-Kin 3.0, and the Mendiara and Glarborg mechanism; each mechanism included reaction subsets for NOx formation. These models were validated against a CH4 flame; the GDF-Kin 3.0 mechanism produced the best agreement with measurements. NO measurements for CO/H2 syngas fuel with 50% CO2 dilution at various equivalence ratios found the maximum NO production near stoichiometry. The majority of NO produced for these conditions is predicted to come from the combination of the NNH and N2O pathways. Investigation of NO production during combustion of different CO/H2 ratio syngas diluted with 60% N2 showed that increasing the H2 fraction decreased the total NO concentration. However, normalizing the NO production rate by the fuel mass consumption rate showed H2 produces more NO per gram of fuel consumed. Predictions of temperature and NO rate of production along the flame axis showed that the syngas with high H2 content had a flame front closer to the burner exit and NO production rate greater than the high CO case. Dilution of CO/H2 syngas fuel with N2 and CO2 showed that CO2 has a stronger reduction in NO emission than N2. GDF-Kin 3.0 predicted that dilution with CO2 caused a greater reduction in flame temperature than the same volume of N2. Predicted NO rate of production and reaction sensitivity analysis predicted that the Zel’dovich pathway was dominant for undiluted syngas. However, as the flame temperature reduced, the Zel’dovich pathway was inhibited to a greater extent than the N2O and NNH pathways so that, at high dilution, NO production was driven by the N2O and NNH pathways.

Nitrous oxide production by nitrification and denitrification in the Eastern Tropical South Pacific oxygen minimum zone

AbstractThe Eastern Tropical South Pacific oxygen minimum zone (ETSP‐OMZ) is a site of intense nitrous oxide (N2O) flux to the atmosphere. This flux results from production of N2O by nitrification and denitrification, but the contribution of the two processes is unknown. The rates of these pathways and their distributions were measured directly using 15N tracers. The highest N2O production rates occurred at the depth of peak N2O concentrations at the oxic‐anoxic interface above the oxygen deficient zone (ODZ) because slightly oxygenated waters allowed (1) N2O production from both nitrification and denitrification and (2) higher nitrous oxide production yields from nitrification. Within the ODZ proper (i.e., anoxia), the only source of N2O was denitrification (i.e., nitrite and nitrate reduction), the rates of which were reflected in the abundance of nirS genes (encoding nitrite reductase). Overall, denitrification was the dominant pathway contributing the N2O production in the ETSP‐OMZ.

Nitrous Oxide (N2O) Emissions by Termites: Does the Feeding Guild Matter?

In the tropics, termites are major players in the mineralization of organic matter leading to the production of greenhouse gases including nitrous oxide (N2O). Termites have a wide trophic diversity and their N-metabolism depends on the feeding guild. This study assessed the extent to which N2O emission levels were determined by termite feeding guild and tested the hypothesis that termite species feeding on a diet rich in N emit higher levels of N2O than those feeding on a diet low in N. An in-vitro incubation approach was used to determine the levels of N2O production in 14 termite species belonging to different feeding guilds, collected from a wide range of biomes. Fungus-growing and soil-feeding termites emit N2O. The N2O production levels varied considerably, ranging from 13.14 to 117.62 ng N2O-N d-1 (g dry wt.)-1 for soil-feeding species, with Cubitermes spp. having the highest production levels, and from 39.61 to 65.61 ng N2O-N d-1 (g dry wt.)-1 for fungus-growing species. Wood-feeding termites were net N2O consumers rather than N2O producers with a consumption ranging from 16.09 to 45.22 ng N2O-N d-1 (g dry wt.)-1. Incubating live termites together with their mound increased the levels of N2O production by between 6 and 13 fold for soil-feeders, with the highest increase in Capritermes capricornis, and between 14 and 34 fold for fungus-growers, with the highest increase in Macrotermes muelleri. Ammonia-oxidizing (amoA-AOB and amoA-AOA) and denitrifying (nirK, nirS, nosZ) gene markers were detected in the guts of all termite species studied. No correlation was found between the abundance of these marker genes and the levels of N2O production from different feeding guilds. Overall, these results support the hypothesis that N2O production rates were higher in termites feeding on substrates with higher N content, such as soil and fungi, compared to those feeding on N-poor wood.

Susceptibility and resistance to Echinococcus granulosus infection: Associations between mouse strains and early peritoneal immune responses

In helminth infections, there are no easy associations between host susceptibility and immune responses. Interestingly, immunity to cestodes – unlike most helminths – seems to require Th1-type effectors. In this sense, we reported recently that Balb/c and C57Bl/6 mice are high and low susceptible strains, respectively, to experimental infection by Echinococcus granulosus. However, the role of the early cellular peritoneal response in such differential susceptibility is unknown. Here, we analyzed the kinetics of cytokines expression and cellular phenotypes in peritoneal cells from infected Balb/c and C57Bl/6 mice. Additionally, Principal Components Analysis (PCA) were conducted to highlight the most relevant differences between strains. Finally, the anti-parasite activities of peritoneal cells were assessed through in vitro systems. PCAs clustered C57Bl/6 mice by their early mixed IL-5/TNF-α responses and less intense expression of Th2-type cytokines. Moreover, they exhibited lower counts of eosinophils and higher numbers of macrophages and B cells. Functional studies showed that peritoneal cells from infected C57Bl/6 mice displayed greater anti-parasite activities, in accordance with higher rates of NO production and more efficient ADCC responses. In conclusion, mild Th2-responses and active cellular mechanisms are key determinants in murine resistance to E. granulosus infection, supporting the cestode immune exception among helminth parasites.

Hydration and diffusion processes shape microbial community organization and function in model soil aggregates

AbstractThe constantly changing soil hydration status affects gas and nutrient diffusion through soil pores and thus the functioning of soil microbial communities. The conditions within soil aggregates are of particular interest due to limitations to oxygen diffusion into their core, and the presence of organic carbon often acting as binding agent. We developed a model for microbial life in simulated soil aggregates comprising of 3‐D angular pore network model (APNM) that mimics soil hydraulic and transport properties. Within these APNM, we introduced individual motile (flagellated) microbial cells with different physiological traits that grow, disperse, and respond to local nutrients and oxygen concentrations. The model quantifies the dynamics and spatial extent of anoxic regions that vary with hydration conditions, and their role in shaping microbial community size and activity and the spatial (self) segregation of anaerobes and aerobes. Internal carbon source and opposing diffusion directions of oxygen and carbon within an aggregate were essential to emergence of stable coexistence of aerobic and anaerobic communities (anaerobes become extinct when carbon sources are external). The model illustrates a range of hydration conditions that promote or suppress denitrification or decomposition of organic matter and thus affect soil GHG emissions. Model predictions of CO2 and N2O production rates were in good agreement with limited experimental data. These limited tests support the dynamic modeling approach whereby microbial community size, composition, and spatial arrangement emerge from internal interactions within soil aggregates. The upscaling of the results to a population of aggregates of different sizes embedded in a soil profile is underway.

Assessment of nitric oxide (NO) redox reactions contribution to nitrous oxide (N2 O) formation during nitrification using a multispecies metabolic network model.

Over the coming decades nitrous oxide (N2O) is expected to become a dominant greenhouse gas and atmospheric ozone depleting substance. In wastewater treatment systems, N2O is majorly produced by nitrifying microbes through biochemical reduction of nitrite (NO2(-)) and nitric oxide (NO). However it is unknown if the amount of N2O formed is affected by alternative NO redox reactions catalyzed by oxidative nitrite oxidoreductase (NirK), cytochromes (i.e., P460 [CytP460] and 554 [Cyt554 ]) and flavohemoglobins (Hmp) in ammonia- and nitrite-oxidizing bacteria (AOB and NOB, respectively). In this study, a mathematical model is developed to assess how N2O formation is affected by such alternative nitrogen redox transformations. The developed multispecies metabolic network model captures the nitrogen respiratory pathways inferred from genomes of eight AOB and NOB species. The performance of model variants, obtained as different combinations of active NO redox reactions, was assessed against nine experimental datasets for nitrifying cultures producing N2O at different concentration of electron donor and acceptor. Model predicted metabolic fluxes show that only variants that included NO oxidation to NO2(-) by CytP460 and Hmp in AOB gave statistically similar estimates to observed production rates of N2O, NO, NO2(-) and nitrate (NO3(-)), together with fractions of AOB and NOB species in biomass. Simulations showed that NO oxidation to NO2(-) decreased N2O formation by 60% without changing culture's NO2(-) production rate. Model variants including NO reduction to N2O by Cyt554 and cNor in NOB did not improve the accuracy of experimental datasets estimates, suggesting null N2O production by NOB during nitrification. Finally, the analysis shows that in nitrifying cultures transitioning from dissolved oxygen levels above 3.8 ± 0.38 to <1.5 ± 0.8 mg/L, NOB cells can oxidize the NO produced by AOB through reactions catalyzed by oxidative NirK.

Potential of nitrous oxide recovery from an aerobic/oxic/anoxic SBR process.

A single sequencing batch reactor (SBR) with an operating mode of anaerobic/oxic/anoxic (A/O/A) was developed to determine a simpler process to recover nitrous oxide (N2O) from synthetic wastewater containing ammonia and glucose. This SBR system was initiated in A/O mode to implement nitritation (ammonia to nitrite) and then switched to A/O/A mode. Using measurements of the dissolved N2O concentration and release rate, the total production and conversion rate of N2O were calculated to reveal the potential of producing and recovering N2O in the extended anoxic phase. Results showed that the A/O/A SBR could convert the majority of the nitrite available in the system into N2O by heterotrophic denitritation over longer anoxic periods, and a conversion rate of 77% could be achieved. As a consequence, the A/O/A SBR presents potential ability to produce and recover N2O from wastewater containing ammonia and organic carbon.

NOx Emissions Modeling and Uncertainty From Exhaust-Gas-Diluted Flames

Oxides of nitrogen (NOx) are pollutants emitted by combustion processes during power generation and transportation that are subject to increasingly stringent regulations due to their impact on human health and the environment. One NOx reduction technology being investigated for gas-turbine engines is exhaust-gas recirculation (EGR), either through external exhaust-gas recycling or staged combustion. In this study, the effects of different percentages of EGR on NOx production will be investigated for methane–air and propane–air flames at a selected adiabatic flame temperature of 1800 K. The variability and uncertainty of the results obtained by the gri-mech 3.0 (GRI), San-Diego 2005 (SD), and the CSE thermochemical mechanisms are assessed. It was found that key parameters associated with postflame NO emissions can vary up to 192% for peak CH values, 35% for thermal NO production rate, and 81% for flame speed, depending on the mechanism used for the simulation. A linear uncertainty analysis, including both kinetic and thermodynamic parameters, demonstrates that simulated postflame nitric oxide levels have uncertainties on the order of ±50–60%. The high variability of model predictions, and their relatively high associated uncertainties, motivates future experiments of NOx formation in exhaust-gas-diluted flames under engine-relevant conditions to improve and validate combustion and NOx design tools.

Human land uses enhance sediment denitrification and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O production in Yangtze lakes primarily by influencing lake water quality

Abstract. Sediment denitrification in lakes alleviates the effects of eutrophication through the removal of nitrogen to the atmosphere as N2O and N2. However, N2O contributes notably to the greenhouse effect and global warming. Human land uses (e.g. agricultural and urban areas) strongly affect lake water quality and sediment characteristics, which, in turn, may regulate lake sediment denitrification and N2O production. In this study, we investigated sediment denitrification and N2O production and their relationships to within-lake variables and watershed land uses in 20 lakes from the Yangtze River basin in China. The results indicated that both lake water quality and sediment characteristics were significantly influenced by watershed land uses. N2O production rates increased with increasing background denitrification rates. Background denitrification and N2O production rates were positively related to water nitrogen concentrations but were not significantly correlated with sediment characteristics and plant community structure. A significant positive relationship was observed between background denitrification rate and percentage of human-dominated land uses (HDL) in watersheds. Structural equation modelling revealed that the indirect effects of HDL on sediment denitrification and N2O production in Yangtze lakes were mediated primarily through lake water quality. Our findings also suggest that although sediments in Yangtze lakes can remove large quantities of nitrogen through denitrification, they may also be an important source of N2O, especially in lakes with high nitrogen content.

Catchment agriculture and local environment affecting the soil denitrification potential and nitrous oxide production of riparian zones in the Han River Basin, China

Riparian zones play an important role in reducing nitrogen (N) loading to rivers and streams primarily through soil denitrification which reduces nitrate (NO3–) to nitrous oxide (N2O) and dinitrogen (N2) gases. Although the relationships between local environments and soil denitrification are well understood, relatively little is known about the indirect effects of landscape factors (e.g., catchment agriculture) on the soil denitrification of riparian zones. In this study, we used the acetylene block technique to measure the denitrification potential and net N2O production of soils collected from 62 riparian sites in 15 subtropical rivers of varying catchment land uses. The results indicated that, among the local factors studied, the soil moisture, organic matter and NO3– concentrations were positively associated with both the denitrification potential and N2O production rate. Agricultural riparian zones had a denitrification potential (2.81±1.01ngNg−1h−1) significantly higher than forested riparian zones (0.66±0.24ngNg−1h−1). Additionally, the riparian denitrification potential increased with the percentage of agriculture in the catchments (R=0.53, P<0.05). Structural equation modeling revealed that the indirect effects of catchment agriculture on the riparian denitrification potential and N2O production rate were mediated primarily through soil NO3–. Our findings suggest that, compared to forested riparian zones, agricultural riparian zones have greater potential to remove N from polluted runoff. The conversion of original vegetation to agricultural lands in catchments may have a profound impact on the soil N cycles and NO3– removal capacity of riparian zones.

Influences of Land Use/Cover Types on Nitrous Oxide Emissions during Freeze-Thaw Periods from Waterlogged Soils in Inner Mongolia.

Nitrous oxide emissions during freeze/thaw periods contribute significantly to annual soil N2O emissions budgets in middle- and high-latitude areas; however, the freeze/thaw-related N2O emissions from waterlogged soils have hardly been studied in the Hulunber Grassland, Inner Mongolia. For this study, the effects of changes in land use/cover types on N2O emissions during freeze–thaw cycles were investigated to more accurately quantify the annual N2O emissions from grasslands. Soil cores from six sites were incubated at varying temperature (ranging from −15 to 10°C) to simulate freeze–thaw cycles. N2O production rates were low in all soil cores during freezing periods, but increased markedly after soil thawed. Mean rates of N2O production differed by vegetation type, and followed the sequence: Leymus chinensis (LC) and Artemisia tanacetifolia (AT) steppes > LC steppes ≥ Stipa baicalensis (SB) steppes. Land use types (mowing and grazing) had differing effects on freeze/thaw-related N2O production. Grazing significantly reduced N2O production by 36.8%, while mowing enhanced production. The production of N2O was related to the rate at which grassland was mowed, in the order: triennially (M3) > once annually (M1) ≥ unmown (UM). Compared with the UM control plot, the M3 and M1 mowing regimes enhanced N2O production by 57.9% and 13.0% respectively. The results of in situ year-round measurements showed that large amounts of N2O were emitted during the freeze–thaw period, and that annual mean fluxes of N2O were 9.21 μg N2O-N m-2 h-1 (ungrazed steppe) and 6.54 μg N2O-N m-2 h-1 (grazed steppe). Our results further the understanding of freeze/thaw events as enhancing N2O production, and confirm that different land use/cover types should be differentiated rather than presumed to be equivalent, regarding nitrous oxide emission. Even so, further research involving multi-year and intensive measurements of N2O emission is still needed.

Production of Nitrous Oxide from Nitrite in Stable Type II Methanotrophic Enrichments.

The coupled aerobic-anoxic nitrous decomposition operation is a new process for wastewater treatment that removes nitrogen from wastewater and recovers energy from the nitrogen in three steps: (1) NH4(+) oxidation to NO2(-), (2) NO2(-) reduction to N2O, and (3) N2O conversion to N2 with energy production. Here, we demonstrate that type II methanotrophic enrichments can mediate step two by coupling oxidation of poly(3-hydroxybutyrate) (P3HB) to NO2(-) reduction. Enrichments grown with NH4(+) and NO2(-) were subject to alternating 48-h aerobic and anoxic periods, in which CH4 and NO2(-) were added together in a "coupled" mode of operation or separately in a "decoupled mode". Community structure was stable in both modes and dominated by Methylocystis. In the coupled mode, production of P3HB and N2O was low. In the decoupled mode, significant P3HB was produced, and oxidation of P3HB drove reduction of NO2(-) to N2O with ∼ 70% conversion for >30 cycles (120 d). In batch tests of wasted cells from the decoupled mode, N2O production rates increased at low O2 or high NO2(-) levels. The results are significant for the development of engineered processes that remove nitrogen from wastewater and for understanding of conditions that favor environmental production of N2O.

Detection of nitric oxide production in cell cultures by luciferin–luciferase chemiluminescence

A chemiluminescent method is proposed for quantitation of NO generation in cell cultures. The method is based on activation of soluble guanylyl cyclase by NO. The product of the guanylyl cyclase reaction, pyrophosphate, is converted to ATP by ATP sulfurylase and ATP is detected in a luciferin–luciferase system. The method has been applied to the measurement of NO generated by activated murine macrophages (RAW 264.7) and bovine aortic endothelial cells. For macrophages activated by lipopolysaccharide and γ-interferon, the rate of NO production is about 100 amol/(cell·min). The rate was confirmed by the measurements of nitrite, the product of NO oxidation. For endothelial cells, the basal rate of NO generation is 5 amol/(cell·min); the rate approximately doubles upon activation by bradykinin, Ca2+ ionophore A23187 or mechanical stress. For both types of cells the measured rate of NO generation is strongly affected by inhibitors of NO synthase. The sensitivity of the method is about 50 pM/min, allowing the registration of NO generated by 102–104 cells. The enzyme-linked chemiluminescent method is two orders of magnitude more sensitive than fluorescent detection using 4-amino-5-methylamino-2′,7′-difluorofluorescein (DAF-FM).

Novel abiotic reactions increase nitrous oxide production during partial nitrification: Modeling and experiments

This research augmented the detailed Activated Sludge Model (ASM) to include abiotic hybrid nitrous oxide (N2O) production due to reactions between hydroxylamine (NH2OH) and nitrite. Model parameters were extracted from a series of abiotic and biological batch tests that showed that abiotic N2O production rates are 1–3 orders of magnitude greater than biological N2O production rates under observed experimental conditions. The mass transfer coefficients (kLa) were 19h−1 and 21h−1 for abiotic and biological tests, respectively. Second order reaction rate constants for N2O indicated that the presence of activated sludge enhanced the rate of abiotic hybrid N2O production. Experimental findings were supported by standard free energy estimates showing that the hybrid reaction (ΔG° hybrid=−538kJ/mol) is more thermodynamically favorable than hydroxylamine oxidoreductase (HAO)-mediated oxidation of NH2OH (i.e. ΔG° −247kJ/mol). The revised model successfully predicted the ammonia-N removal rate (49mgN/L-h) as well as the temporal position of the N2O production observed from independently conducted dynamic loading experiments. Incorporation of abiotic hybrid N2O production should significantly improve N2O emissions estimates from partial nitrification systems but it will also alter the material and energy balance for nitrifiers because these abiotic reactions effectively re-route NH2OH.

N₂O accumulation from denitrification under different temperatures.

The effects of temperature on nitrous oxide (N2O) accumulation during denitrification and denitritation were investigated. Batch experiments were performed to measure N2O accumulation at 25 and 35 °C. More N2O accumulation was observed during denitritation at the higher temperature as compared with full denitrification and low temperature tests. The highest nitrite concentration tested in this study (25 mg/L NO2 (-)N and pH 8.0) did not show inhibitory effect on N2O reduction. It was found that the major cause of more N2O accumulation during denitrification at higher temperature was due to higher N2O production rate and lower N2O solubility. Specific nitrate, nitrite, and N2O reduction rates increased 62, 61, and 41 %, respectively, when temperature rose from 25 to 35 °C. The decrease of N2O solubility in mixed liquor at 35 °C (when compared to 25 °C) resulted in faster diffusing rate of N2O from liquid to gas phase. It was also more difficult for gas phase N2O to be re-dissolved. The diffused N2O was then accumulated in the headspace, which was not available for denitrification by denitrifiers. The results of this study suggest higher temperature may worsen N2O emission from wastewater treatment plants (WWTPs).