Microbial production and uptake of nitric oxide in soil

Removal of nitric oxide in bioreactors: a review on the pathways, governing factors and mathematical modelling

The constant surge in nitric oxide in the atmosphere results in severe environmental degradation, negatively impacting human health and ecosystems, and is presently a global concern. Widely used physicochemical technologies for nitric oxide (NO) removal comes with high installation and operational costs and the production of secondary pollutants. Thus, biological treatment has been emphasized over the last two decades, but the poor solubility of NO in water makes it a challenging issue. The present article reviews the various technical aspects of biological treatment of nitric oxide, including the removal pathways and reactor configurations involved in the process. The most widely used technologies in this regard are chemical adsorption processes followed by biological reactors like biofilters, biotrickling filters and membrane bioreactors that enhance NO solubility and offer the flexibility and scope of further improvement in process design. The effect of various experimental and operational parameters on NO removal, including pH, carbon source, gas flow rate, gas residence time and presence of inhibitory components in the flue gas, is also discussed along with the developed mathematical models for predicting NO removal in a biological treatment system. There is an extensive scope of investigation regarding the development of an economical system to remove NO, and an exhaustive model that would optimize the process considering maximum practical parameters encountered during such operation. A detailed discussion made in this article gives a proper insight into all these areas.

Acidity and organic matter promote abiotic nitric oxide production in drying soils

Soils are an important source of NO, particularly in dry lands because of trade-offs that develop between biotic and abiotic NO-producing processes when soils dry out. Understanding how drier climates may offset the balance of these trade-offs as soils transition toward more arid states is, therefore, critical to estimating global NO budgets, especially because drylands are expected to increase in size. We measured NO emission pulses after wetting soils from similar lithologies along an altitudinal gradient in the Sierra Nevada, CA, where mean annual precipitation varied from 670 to 1500 mm. Along the gradient, we measured field NO emissions, and used chloroform in the laboratory to reduce microbial activity and partition between biotic and abiotic NO-producing processes (i.e., chemodenitrification). Field NO emission pulses were lowest in the acidic and SOM-rich soils (4-72 ng NO-N m^-2 s^-1 ), but were highest in the high-elevation barren site (~560 ng NO-N m^-2 s^-1 ). In the laboratory, NO emission pulses were up to 19× greater in chloroform-treated soils than in the controls, and these abiotic pulses increased with elevation as pH decreased (6.2-4.4) and soil organic matter (SOM) increased (18-157 mg C g^-1 ). Drought can shift the balance between the biotic and abiotic processes that produce NO, favoring chemodenitrification during periods when biological processes become stressed. Acidic and SOM-rich soils, which typically develop under mesic conditions, are most vulnerable to N loss via NO as interactions between pH, SOM, and drought stimulate chemodenitrification.

Carbonyl sulfide and dimethyl sulfide fluxes in an urban lawn and adjacent bare soil in Guangzhou, China

Carbonyl sulfide (COS) and dimethyl sulfide (DMS) fluxes from an urban Cynodon dactylon lawn and adjacent bare soil were measured during April-July 2005 in Guangzhou, China. Both the lawn and bare soil acted as sinks for COS and sources for DMS. The mean fluxes of COS and DMS in the lawn (-19.27 and 18.16 pmol/(m2 sec), respectively) were significantly higher than those in the bare soil (-9.89 and 9.35 pmol/(m2 sec), respectively). Fluxes of COS and DMS in mowed lawn were also higher than those in bare soils. Both COS and DMS fluxes showed diurnal variation with detectable but much lower values in the nighttime than in the daytime. COS fluxes were related significantly to temperature and the optimal temperature for COS uptake was 29 degrees C. While positive linear correlations were found between DMS fluxes and temperature. COS fluxes increased linearly with ambient COS mixing ratios, and had a compensation point of 336 ppt.

Consumption of biogenic nitric oxide in hydrated soil

An experimental study was conducted in order to determine the relationship of nitric oxide (NO) consumption to water-filled pore space in soil. A test system that included the capability to blend gases, test soil samples, and analyze off-gases was used to conduct the study. The experimental set consisted of three replicates at five different levels of soil water content and three different levels of soil nitrogen in a sandy loam soil: unamended soil, soil fertilized at 56.2 kg N per ha (50 lb N acre(-1)), and soil fertilized at 112.3 kg N per ha (100 lb N acre(-1)). The average NO consumption rates were 7.1x10(-13) g-NO cm(-3) soil, 3.5x10(-11) g-NO cm(-3) soil, and 1.5x10(-10) g-NO cm(-3) soil, respectively.

Nitrite and nitrosyl compounds in food preservation

Nitrite is consumed in the diet, through vegetables and drinking water. It is also added to meat products as a preservative. The potential risks of this practice are balanced against the unique protective effect against toxin-forming bacteria such as Clostridium botulinum. The chemistry of nitrite, and compounds derived from it, in food systems and bacterial cells are complex. It is known that the bactericidal species is not nitrite itself, but a compound or compounds derived from it during food preparation. Of a range of nitrosyl compounds tested, the anion of Roussin's black salt [Fe4S3(NO)7]- was the most inhibitory to C. sporogenes. This compound is active against both anaerobic and aerobic food-spoilage bacteria, while some other compounds are selective, indicating multiple sites of action. There are numerous possible targets for inhibition in the bacterial cells, including respiratory chains, iron-sulfur proteins and other metalloproteins, membranes and the genetic apparatus.

Denitrification and nitric oxide reduction in an aerobic toluene-treating biofilter

The presence of significant denitrification activity in an aerobic toluene-treating biofilter was demonstrated under batch and flow-through conditions. N2O concentrations of 9.2 ppmv were produced by denitrifying bacteria in the presence of 15% acetylene, in a flow-through system with a bulk gas phase O2 concentration of >17%. The carbon source for denitrification was not toluene but a byproduct or metabolite of toluene catabolism. Denitrification conditions were successfully used for the reduction of 60 ppmv nitric oxide to 15 ppmv at a flow rate of 3 L min-1 (EBRT of 3 min) in a fully aerated, 17% v/v O2 (superficially aerobic) biofilter. Higher NO removal efficiency (97%) was obtained by increasing the toluene supply to the biofilter.

Microbial and plant metabolism of NO

During microbial denitrification, NO is produced by reduction of nitrite by either the reduced high spin d1 hemes in a unique reductase (NIR) or at the expense of a blue copper protein that transfers electrons that move first to a type I copper and then to a type II copper in a unique trimeric NIR. This latter type of NIR is also produced by several denitrifying filamentous fungi. Reduction of NO is then carried out by either a specific cytochrome be complex NOR in denitrifying bacteria or a unique cytochrome P-450 in denitrifying filamentous fungi. NO is also produced by an anomalous reaction of a molybdoprotein, nitrate reductase (NAR), acting on an odd substrate, NO2-. NO is also reduced by a multiheme NIR that serves physiologically for reduction of NO2- to NH3. This type NIR reduces NO to either N2O, if only partially reduced, or NH3, if fully reduced, when it encounters NO. This multiheme NIR is very sensitive to cyanide. Transcription of the genes for NIR and NOR production in a denitrifier is activated by NO, a process that also requires the presence of the gene product, a transcriptional activator, NnrR.

Effect of ionophores on denitrification inFlexibacter canadensis

Denitrification by Flexibacter canadensis was investigated by measuring the production and (or) consumption of nitrite, nitric oxide (NO), and nitrous oxide (N2O) under anaerobic conditions. Carbonyl cyanide m-chlorophenylhydrazone (CCCP), carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), 2,4-dinitrophenol, and nigericin, but not valinomycin-K+inhibited the production of nitrite and N2O from nitrate by intact cells. However, CCCP, FCCP, 2,4-dinitrophenol, nigericin, and valinomycin-K+did not affect nitrite production from nitrate by cell-free extracts. These results suggest that nitrate transport was dependent on the transmembrane pH gradient but not on the membrane potential. CCCP, FCCP, and nigericin but not 2,4-dinitrophenol and valinomycin-K+caused NO accumulation during the reduction of nitrite, and also inhibited NO consumption and N2O production from nitrite by intact cells. These results preclude an explanation for NO accumulation based on the collapse of the proton motive force by ionophores, and imply that CCCP, FCCP, and nigericin perhaps dissociated a nitrite reductase–nitric oxide reductase complex, and (or) inhibited nitric oxide reductase specifically. 2,4-Dinitrophenol and CCCP did not inhibit the reduction of N2O to dinitrogen. Addition of ≤ 1.16 μM dissolved NO did not affect the production of nitrite from nitrate, or the disappearance of nitrite or N2O. The rate of NO consumption was linear with concentrations of dissolved NO up to 67 nM. Above 67 nM NO, NO consumption was inhibited, suggesting that NO is toxic to nitric oxide reductase.Key words: ionophores, denitrification, nitric oxide, Flexibacter canadensis.

Laboratory study of the emission of NO and N2O from some Belgian soils

The NO, NO2 and N2O emission was measured, upon application of nitrate, ammonium and both, to four Belgian soils with different characteristics. The addition of NH 4 (+) caused higher NO and N2O emissions than the addition of no nitrogen, or the addition of NO 3 (-) . In contrast to the two soils with a pH of approximately 8 the two soils with a pH around 6 showed a considerable delay in production of both NO and N2O upon the application of the ammonium, probably due to the lag-period of nitrification. The soils with a pH of 8 gave higher emissions on the application of NH 4 (+) than the soils with a pH of 6. The emission of NO2 was found to be considerably lower than the NO emission from the soils. The NO/NO2 ratio varied between 5-25 at considerable NO emissions (>50 nmol kg(-1)). In the controls of soil 1 and soil 2, which showed very low NO emissions ratios of <1 were observed. The N2O/NO ratios varied between 5-20 when NO emissions were considerable (>50 nmol kg(-1)). Soil 3 and 4 gave lower N2O/NO ratios than soil 1 and 2. In the controls of soil 1 and soil 2, at low NO emissions, N2O/NO ratios of >300 were observed. Soil 3 and 4 gave higher NO/NO2 and lower N2O/NO ratios than soil 1 and 2.

The production and utilization of nitric oxide by a new, denitrifying strain of Pseudomonas aeruginosa

When a new strain of Pseudomonas aeruginosa was grown aerobically and then transferred to anaerobic conditions, cells reduced NO3- quantitatively to NO2- in NO3(-)-respiration. In the absence of nitrate, NO2- was immediately reduced to NO or N2O but not to N2 indicating that NO2(-)-reductase but not N2O-reductase was active. The formation of the products NO or N2O depended on the pH in the medium and the concentration of NO2- present. When P. aeruginosa was grown anaerobically for at least three days N2O-reductase was also active. Such cells reduced NO to N2 via N2O. The new strain generated at H(+)-gradient and grew by reducing N2O to N2 but not by converting NO to N2O. For comparison, Azospirillum brasilense Sp7 showed the same pattern of NO-reduction. In contrast, Paracoccus denitrificans formed 3.5 H+/NO during the reduction of NO to N2O in oxidant pulse experiments but could not grow in the presence of NO. Thus the NO-reduction pattern in P. denitrificans on one side and P. aeruginosa and A. brasilense on the other was very different. The mechanistic implications of such differences are discussed.