Effect of sulfide on nitrate reduction in mixed methanogenic cultures

The effect of sulfide on nitrate reduction and methanogenesis was investigated in two mixed, mesophilic (35 degrees C) methanogenic cultures: sulfide-free and sulfide-acclimated (67 mg S/L total sulfide). A mixture of dextrin/peptone served as the carbon/electron donor source for the two stock cultures, as well as in all assays reported here. The sulfide-free enriched culture was amended with both nitrate (75-350 mg N/L) and sulfide (10-100 mg S/L). Denitrification was the predominant pathway at all sulfide levels tested and methanogenesis did not recover in any of the sulfide- and nitrate-amended cultures, except in the 10 mg S/L culture. Accumulation of denitrification intermediates such as NO and N(2)O took place, which irreversibly inhibited the methanogens and resulted in the complete cessation of methane production. In contrast, conversion of nitrate to nitrite and then to ammonia via dissimilatory nitrate reduction to ammonia (DNRA) prevented the accumulation of denitrification intermediates and led to the recovery of methanogenesis in the nitrate-amended, sulfide-acclimated, mixed methanogenic culture. The effect of the COD/N value on nitrate reduction was assessed with the sulfide-acclimated, methanogenic culture at COD/N values of 10, 20, and 60. As the COD/N value increased, the fraction of nitrate reduced through DNRA also increased. The results of this study have significant implications relative to the combined anaerobic treatment of carbon-, nitrogen-, and/or sulfur-bearing wastes.     

Inhibitory effects of nitrogen oxides on a mixed methanogenic culture

The effect of nitrate, nitrite, nitric oxide (NO), and nitrous oxide on a mixed, mesophilic (35 degrees C) methanogenic culture was investigated. Short-term inhibition assays were conducted at a concentration range of 10-350 mg N/L nitrate, 17-500 mg N/L nitrite, 0.02-0.8 mg N/L aqueous NO, and 19-191 mg N/L aqueous nitrous oxide. Simultaneous methane production and N-oxide reduction was observed in 10 and 30 mg N/L nitrate and 0.02 mg N/L aqueous NO-amended cultures. However, addition of N-oxide resulted in immediate cessation of methanogenesis in all other cultures. Methanogenesis completely recovered subsequent to the complete reduction of N-oxides to nitrogen gas in all N-oxide-amended cultures, with the exception of the 500 mg N/L nitrite- and 0.8 mg N/L aqueous NO-amended cultures. Partial recovery of methanogenesis was observed in the 500 mg N/L nitrite-amended culture in contrast to complete inhibition of methanogenesis in the 0.8 mg N/L aqueous NO-amended culture. Accumulation of volatile fatty acids was observed in both cultures at the end of the incubation period. Among all N-oxides, NO exerted the most and nitrate exerted the least inhibitory effect on the fermentative/methanogenic consortia. The effect of multiple additions of nitrate (300 mg N/L) on the same methanogenic culture was also investigated. Long-term exposure of the methanogenic culture to nitrate resulted in an increase of N-oxide reduction rates and decrease of methane production rates, which was attributed to changes in the microbial community structure due to nitrate addition.     

Decolorization kinetics of the azo dye Reactive Red 2 under methanogenic conditions: effect of long-term culture acclimation

The biological decolorization of the textile azo dye Reactive Red 2 was investigated using a mixed, mesophilic methanogenic culture, which was developed with mixed liquor obtained from a mesophilic, municipal anaerobic digester and enriched by feeding a mixture of dextrin/peptone as well as media containing salts, trace metals and vitamins. Batch decolorization assays were conducted with the unacclimated methanogenic culture and dye decolorization kinetics were determined as a function of initial dye, biomass, and carbon source concentrations. Dye decolorization was inhibited at initial dye concentrations higher than 100 mg l^-1 and decolorization kinetics were described based on the Haldane model. The effect of long-term culture exposure to the reactive dye on decolorization kinetics, culture acclimation, as well as possible dye mineralization was tested using two reactors fed weekly for two years with an initial dye concentration of 300 mg l^-1 and a mixture of dextrin/peptone. The maximum dye decolorization rate after a 2-year acclimation at an initial dye concentration of 300 mg l^-1 was more than 10-fold higher as compared to that obtained with the unacclimated culture. Aniline and the o-aminohydroxynaphthalene derivative resulting from the reductive azo bond cleavage of the dye were detected, but further transformation(s) leading to dye mineralization were not observed. Reactive Red 2 did not serve as the carbon and energy source for the mixed culture, and dye decolorization was sustained by the continuous addition of dextrin and peptone. Thus, biological decolorization of reactive azo dyes is feasible under conditions of low redox potential created and maintained in overall methanogenic systems, but supply of a biodegradable carbon source is necessary.     

A comprehensive model of simultaneous denitrification and methanogenic fermentation processes

The denitrification process was incorporated into the IWA Anaerobic Digestion Model No. 1 (ADM1) in order to account for the effect of denitrification on the methanogenic fermentation process. The model was calibrated and optimized using previously published experimental data and kinetic parameter values obtained with a mixed, mesophilic (35°C) methanogenic culture. Model simulations were used to predict the effect of nitrate reduction on the methanogenic fermentation process in batch, semi‐continuous, and continuous flow reactors experiencing operational changes and/or system disturbances. The extended model clearly revealed the importance of substrate competition between denitrifiers and non‐denitrifiers as well as the impact of N‐oxide inhibition on process interactions between fermentation, methanogenesis, and denitrification. Biotechnol. Bioeng. 2010;105: 98–108. © 2009 Wiley Periodicals, Inc.     

Methane production from propionate by methanogenic mixed culture

Abstract The association of Desulfobulbus sp. with Methanosarcina barkeri 227 was able to produce CH₄ from propionate in the presence of sulfate, if a sufficient amount of ferrous iron was added to the media in order to trap the soluble sulfides produced from sulfate. In the absence of ferrous iron, soluble sulfides inhibited the acetoclastic reaction. Attempts to cultivate Desulfobulbus sp. with H₂-utilising methanogenic bacteria in the absence of sulfate did not succeed.     

Dissimilatory nitrate reduction by Pseudomonas alcaliphila with an electrode as the sole electron donor

Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were considered two alternative pathways of dissimilatory nitrate reduction. In this study, we firstly reported that both denitrification and DNRA occurred in Pseudomonas alcaliphila strain MBR with an electrode as the sole electron donor in a double chamber bio‐electrochemical system (BES). The initial concentration of nitrate appeared as a factor determining the type of nitrate reduction with electrode as the sole electron donor at the same potential (?500 mV). As the initial concentration of nitrate increased, the fraction of nitrate reduced through denitrification also increased. While nitrite (1.38 ± 0.04 mM) was used as electron acceptor instead of nitrate, the electrons recovery via DNRA and denitrification were 43.06 ± 1.02% and 50.51 ± 1.37%, respectively. The electrochemical activities and surface topography of the working electrode catalyzed by strain MBR were evaluated by cyclic voltammetry and scanning electron microscopy. The results suggested that cells of strain MBR were adhered to the electrode, playing the role of electron transfer media for nitrate and nitrite reduction. Thus, for the first time, the results that DNRA and denitrification occurred simultaneously were confirmed by powering the strain with electricity. The study further expanded the range of metabolic reactions and had potential value for the recognization of dissimilatory nitrate reduction in various ecosystems. Biotechnol. Bioeng. 2012; 109: 2904–2910. © 2012 Wiley Periodicals, Inc.     

Oxygen and nitrate reduction kinetics of a nonflocculating strain of Zoogloea ramigera

The oxygen and nitrate reduction kinetics of a nonflocculating strain of Zoogloea ramigera were determined. Axenic, nitrate-reducing bacterial suspensions were acclimated to various oxygen levels in a chemostat while measuring nitrate reduction in the presence of high ammonium nitrogen concentrations. Significant nitrate reduction was observed at oxygen concentrations up to 8 mg L^–1. Oxygen consumption was inhibited by oxygen concentrations in excess of 2 mg L^–1.     

Nitrite reduction by a mixed culture under conditions relevant to shortcut biological nitrogen removal

Dissimilative reduction of nitrite by nitrite-acclimated cellswas investigated in a batch reactor under various environmental conditions that can beencountered in shortcut biological nitrogen removal (SBNR: ammonia to nitrite andnitrite to nitrogen gas). The maximum specific nitrite reduction rate was as much as 4.3 times faster than the rate of nitrate reduction when individually tested, but the reaction was inhibited in the presence of nitrate when the initial nitrate concentration was greater than approximately 25 mg-N/l or the initialNO ₃ ^- N/NO ₂ ^- N ratio was larger than 0.5. Nitrite reduction was also inhibited by nitrite itself when theconcentration was higher than that to which the cells had been acclimated. Therefore, it was desirable to avoid excessively high nitrite and nitrate concentrations in a denitrification reactor. Nitrite reduction, however, was not affected by an alkaline pH (in the range of 7–9) or a high concentration of FA (in the range of 16–39 mg/l), which can be common in SBNR processes. The chemical oxygen demand (COD) requirement for nitrite reduction was approximately 22–38% lower than that for nitrate reduction, demonstrating that the SBNR process can be economical. The specific consumption,measured as the ratio of COD consumed to nitrogen removed, was affected by the availability of COD and the physiological state of the cells. The ratio increased when the cells grew rapidly and were storing carbon and electrons.     

Denitrifying sulfide removal and carbon methanogenesis in a mesophilic, methanogenic culture

The inhibitory effects of 90-189 mg l⁻¹ of sulfide and 25-75 mg-N l⁻¹ of nitrate on methanogenesis were investigated in a mixed methanogenic culture using butyrate as carbon source. In the initial phase of 90 mg l⁻¹ S²⁻ test, autotrophic denitrification of nitrate occurred with sulfide as the electron donor. Then the sulfate-reducing strains converted the produced sulfur back to sulfide via heterotrophic oxidation pathway. Methanogenesis was not markedly inhibited when 90 mg l⁻¹ of sulfide was dosed alone. When 25-75 mg-N l⁻¹ of nitrate was presented, initiation of methanogenesis was seriously delayed. Nitrogen oxides (NO_x), the intermediates for nitrate reduction via denitrification pathway, inhibited methanogenesis. The 90 mg l⁻¹ of sulfide favored heterotrophic dissimilatory nitrate reduction to ammonia (DNRA) pathway for nitrate reduction. Possible ways of maximizing methane production from an organic carbon-rich wastewater with high levels of sulfide and nitrate were discussed.     

Inhibitory effect of chlorinated guaiacols on methanogenic activity of anaerobic digester sludge

Of four chlorinated guaiacols, tetrachloroguaiacol at 62 M inhibited acetate methanogenesis, the strongest decreasing activity by 50%. 4,5,6-Trichloroguaiacol, 4,5-dichloroguaiacol, and 4-chloroguaiacol showed 50% inhibition at 0.13, 0.32, and 1.50 mM, respectively. Degradation test results of volatile fatty acids (acetic, propionic, and butyric acid) by anaerobic digester sludge (stored 5 weeks) indicated that syntrophic butyrate degraders of this sludge were more sensitive to tetrachloroguaiacol than acetoclastic methanogens and syntrophic propionate degraders.     

Combined removal of nitrate and carbon in granular sludge: Substrate competition and activities

Granular sludge from an upflow anaerobic sludge blanket reactor treating synthetic waste water containing a mixture of volatile fatty acids and nitrate showed a removal efficiency of nearly 100% for both nitrogen and carbon. This activity was achieved by a combined process of denitrification and methanogenesis under conditions of surplus carbon. Under batch conditions the two processes proceeded clearly separated in time with first denitrification dominating and excluding methanogenesis. However, as soon as nitrate was depleted, methane production was initiated, showing that the inhibition of methanogenesis by nitrate was reversible. Of the volatile fatty acids supplied to the reactor, i.e. acetate, propionate, and butyrate, the denitrifying population clearly preferred butyrate and propionate even though acetate could also be metabolized. Consequently, growth of syntrophic volatile fatty acid degraders was suppressed by the denitrifiers in cases of low C:N ratios in the medium, leaving acetate as the major substrate for methanogenesis.Abbreviations UASB upflow anaerobic sludge blanket - COD chemical oxygen demand - VFA volatile fatty     

The determination of the carbonic anhydrases activators in vitro effect of mixed donor crown ethers

Carbonic anhydrases (CAs) play an important function in various physiological and pathological processes. Therefore, many researchers work in this field in order to design and synthesize new drugs. Both inhibitors and activators of CAs, which are associated with the diagnosis and treatment of many diseases, are very important. The emergence of the use of CA activators in the treatment of Alzheimer has led many scholars to work on this issue. In this study, CA activators and inhibitors are determined. The crown ethers compounds ( 1 , 2 , 3 , 6 , 7 , 8 , and 9 ) were found to cause activation on enzyme activities of hCA I and II. The AC₅₀ values on hCA I and II of the compounds are in the range of 4.6565–374.979 μM. The 4 (IC₅₀; 1.301 and 3.215 μM for hCA I and II) and 5 (IC₅₀; 73.96 and 378.5 μM for hCA I and II) compounds were found to cause inhibition on enzyme activities of hCA I and II.     

Denitrification,dissimilatory nitrate reduction to ammonium,and nitrogen fixation along a nitrate concentration gradient in a created freshwater wetland

Wetlands are often highly effective nitrogen (N) sinks. In the Lake Waco Wetland (LWW), near Waco, Texas, USA, nitrate (NO₃⁻) concentrations are reduced by more than 90% in the first 500 m downstream of the inflow, creating a distinct gradient in NO₃⁻ concentration along the flow path of water. The relative importance of sediment denitrification (DNF), dissimilatory NO₃⁻ reduction to ammonium (DNRA), and N₂ fixation were examined along the NO₃⁻ concentration gradient in the LWW. “Potential DNF” (hereafter potDNF) was observed in all months and ranged from 54 to 278 μmol N m⁻² h⁻¹. “Potential DNRA” (hereafter potDNRA) was observed only in summer months and ranged from 1.3 to 33 μmol N m⁻² h⁻¹. Net N₂ flux ranged from 184 (net denitrification) to −270 (net N₂ fixation) μmol N m⁻² h⁻¹. Nitrogen fixation was variable, ranging from 0 to 426 μmol N m⁻² h⁻¹, but high rates ranked among the highest reported for aquatic sediments. On average, summer potDNRA comprised only 5% (±2% SE) of total NO₃⁻ loss through dissimilatory pathways, but was as high as 36% at one site where potDNF was consistently low. Potential DNRA was higher in sediments with higher sediment oxygen demand (r ² = 0.84), and was related to NO₃⁻ concentration in overlying water in one summer (r ² = 0.81). Sediments were a NO₃⁻ sink and accounted for 50% of wetland NO₃⁻ removal (r ² = 0.90). Sediments were an NH₄⁺ source, but the wetland was often a net NH₄⁺ sink. Although DNRA rates in freshwater wetlands may rival those observed in estuarine systems, the importance of DNRA in freshwater sediments appears to be minor relative to DNF. Furthermore, sediment N₂ fixation can be extremely high when NO₃⁻ in overlying water is consistently low. The data suggest that newly fixed N can support sustained N transformation processes such as DNF and DNRA when surface water inorganic N supply rates are low.     

Cometabolic transformation ofo-xylene in a biofilm system under nitrate reducing conditions

The purpose of this work was to investigate the anaerobic transformation ofo-xylene in a laboratory biofilm system with nitrate as an electron acceptor.o-Xylene was degraded cometabolically with toluene as primary carbon source. A mass balance showed thato-xylene was not mineralized but transformed.o-Methyl-benzalcohol ando-methyl-benzaldehyde were identified as intermediates ofo-xylene transformation which resulted in the formation ofo-methyl-benzoic acid as an end product. A cross inhibition phenomenon was observed between toluene ando-xylene. The presence of toluene was necessary for stimulation ofo-xylene transformation, but above a toluene concentration of 1–3 mg/L theo-xylene removal rate dramatically decreased. In returno-xylene inhibited the toluene degradation at concentrations above 2–3 mg/L.     

Effects of electron donors on nitrate removal by nitrate and nitrite reductases

Effects of artificial electron donors to deliver reducing power on enzymic denitrification were investigated using nitrate reductase and nitrite reductase obtained fromOchrobactrum antropi. The activity of nitrite reductase in the soluble portion was almost the same as that in the precipitated portion of the cell extract. Nitrate removal efficiency was higher with benzyl viologen than with methyl viologen or NADH as an artificial electron donor. The turn-over numbers of nitrate and nitrite reductase were 14.1 and 1.9 μmol of nitrogen reduced/min·mg cell extracts, respectively when benzyl viologen was used as an electron donor.     

Growth and cometabolic reduction kinetics of a uranium- and sulfate-reducing Desulfovibrio/Clostridia mixed culture: Temperature effects

Bioremediation of contaminated soils and aquifers is subject to spatial and temporal temperature changes that can alter the kinetics of key microbial processes. This study quantifies temperature effects on the kinetics of an ethanol-fed sulfate-reducing mixed culture derived from a uranium-contaminated aquifer subject to seasonal temperature fluctuations. The mixed culture contains Desulfovibrio sp. and a Clostridia-like organism. Rates of growth, ethanol utilization, decay, and uranium reduction decreased with decreasing temperature. No significant uranium reduction was observed at 10 degrees C. While both Monod saturation kinetics and pseudo second-order kinetics adequately described the rates of growth and utilization of electron donor (ethanol), model parameters for the pseudo second-order expression had smaller uncertainties. Uranium reduction kinetics were best described by pseudo second-order kinetics modified to include a term for inactivation/death of cells.     

Simultaneous nitrification and denitrification in a mixed methanotrophic culture

Simultaneous nitrification and denitrification using a mixed methanotrophic culture was investigated. When both NO₃ ^–-N (108 mg l^–1) and NH₃-N (59 mg l^–1) were added into batch reactors, nitrate removal was complete within 10 h at the rate of 47 mg NO₃ ^–-N g VSS^–1 day^–1 when dissolved oxygen (DO) concentration was maintained at 2 mg DO l^–1. Ammonia removal started simultaneously with nitrate removal at a slower rate of 14 NH₃-N g VSS^–1 day^–1. No significant accumulation of nitrite or nitrate during ammonia utilization suggested the occurrence of simultaneous nitrification and denitrification.     

Nitric oxide reduction in BioDeNOx reactors: kinetics and mechanism

Biological reduction of nitric oxide (NO) to di-nitrogen (N(2)) gas in aqueous Fe(II)EDTA(2-) solutions is a key reaction in BioDeNOx, a novel process for NOx removal from flue gases. The mechanism and kinetics of the first step of NO reduction, that is, the conversion of NO to N(2)O, was determined in batch experiments using various types of inocula. Experiments were performed in Fe(II)EDTA(2-) medium (5-25 mM) under BioDeNOx reactor conditions (55 degrees C, pH 7.2 +/- 0.2) with ethanol as external electron donor. BioDeNOx reactor mixed liquor gave the highest NO reduction rates (+/-0.34 nmol s(-1) mg(prot)(-1)) with an estimated K(m) value for NO lower than 10 nM. The specific NO (to N(2)O) reduction rate depended on the NO (aq) and Fe(II)EDTA(2-) concentration as well as the temperature. The experimental results, complemented with kinetic and thermodynamic considerations, show that Fe(II)EDTA(2-), and not ethanol, is the primary electron donor for NO reduction, that is, the BioDeNOx reactor medium (the redox system Fe(II)EDTA(2-)/Fe(III)EDTA(-)) interferes with the NO reduction electron transfer chain and thus enhances the NO denitrification rate.     

硝态氮异化还原机制及其主导因素研究进展

硝态氮(NO_3~-)异化还原过程通常包含反硝化和异化还原为铵(DNRA)两个方面,是土壤氮素转化的重要途径,其强度大小直接影响着硝态氮的利用和环境效应(如淋溶和氮氧化物气体排放)。反硝化和DNRA过程在反应条件、产物和影响因素等方面常会呈现出协同与竞争的交互作用机制。综述了反硝化和DNRA过程的研究进展及其二者协同竞争的作用机理,并阐述了在NO_3~-、pH、有效C、氧化还原电位(Eh)等环境条件和土壤微生物对其发生强度和产物的影响,提出了今后应在产生机理、土壤环境因素、微生物学过程以及与其他氮素转化过程耦联作用等方面亟需深入研究,以期增进对氮素循环过程的认识以及为加强氮素管理利用提供依据。