Nitrite Reduction Rate Research Articles

Role of Denitrification in Selenite Reduction by Azospirillum brasilense with the Formation of Selenium Nanoparticles.

Many bacteria are capable of reducing selenium oxyanions, primarily selenite (SeO32-), in most cases forming selenium(0) nanostructures. The mechanisms of these transformations may vary for different bacterial species and have so far not yet been clarified in detail. Bacteria of the genus Azospirillum, including ubiquitous phytostimulating rhizobacteria, are widely studied and have potential for agricultural biotechnology and bioremediation of excessively seleniferous soils, as they are able to reduce selenite ions. Cultures of A.brasilense Sp7 and its derivatives (mutant strains) were grown on the modified liquid malate salt medium in the presence or absence of selenite. The following methods were used: spectrophotometric monitoring of bacterial growth; inhibition of glutathione (GSH) synthesis in bacteria by L-buthionine-sulfoximine (BSO); optical selenite and nitrite reduction assays; transmission electron microscopy of cells grown with and without BSO and/or selenite. In a set of separate comparative studies of nitrite and selenite reduction by the wild-type strain A.brasilense Sp7 and its three specially selected derivatives (mutant strains) with different rates of nitrite reduction, a direct correlation was found between their nitrite and selenite reduction rates for all the strains used in the study. Moreover, for BSO it has been shown that its presence does not block selenite reduction in A.brasilense Sp7. Evidence has been presented for the first time for bacteria of the genus Azospirillum that the denitrification pathway known to be inherent in these bacteria, including nitrite reductase, is likely to be involved in selenite reduction. The results using BSO also imply that detoxification of selenite through the GSH redox system (which is commonly considered as the primary mechanism of selenite reduction in many bacteria) does not play a significant role in A.brasilense. The acquired knowledge on the mechanisms underlying biogenic transformations of inorganic selenium in A.brasilense is a step forward both in understanding the biogeochemical selenium cycle and to a variety of potential nano- and biotechnological applications.

Investigation of Nitrogen Removal in Flue Gas Desulfurization and Denitrification Wastewater Utilizing Halophilic Activated Sludge.

In the process of flue gas desulfurization and denitrification, the generation of high-sulfate wastewater containing nitrogen is a significant challenge for biological wastewater treatment. In this study, halophilic activated sludge was inoculated in a Sequencing Batch Reactor to remove nitrogen from wastewater with a high sulfate concentration (60 g/L). With the influent concentration of 180 mg/L, the removal rate of total nitrogen was more than 96.7%. The effluent ammonium nitrogen concentration was lower than 1.94 mg/L, and the effluent nitrate nitrogen and nitrite nitrogen concentrations were even lower than 0.77 mg/L. The salt tolerance of activated sludge is mainly related to the increase in the content of ectoine in microbial cells. The Specific Nitrite Oxidation Rate is quite low, while the Specific Nitrite Reduction Rate and Specific Nitrate Reduction Rate are relatively strong. In the system, there are various nitrogen metabolic processes, including aerobic nitrification, anaerobic denitrification, and simultaneous nitrification-denitrification processes. By analyzing the nitrogen metabolic mechanisms and microbial community structure of the reaction system, dominate bacteria can be identified, such as Azoarcus, Thauera, and Halomonas, which have significant nitrogen removal capabilities.

Nitrosyl hemoglobin formation from nitrite in normal and sickle blood

Sickle cell anemia is caused by a single mutation in the gene encoding the beta subunit of hemoglobin. Due to this mutation, sickle cell hemoglobin (HbS) polymerizes under hypoxic conditions, decreasing red blood cell deformability and leading to multiple pathological effects that cause substantial morbidity and mortality. Several pre-clinical and human studies have demonstrated that the anion nitrite has potential therapeutic benefits for patients with sickle cell disease. Nitrite is reduced to nitric oxide (NO) by deoxygenated hemoglobin contributing to vasodilation, decreasing platelet activation, decreasing cellular adhesion to activated endothelium, and decreasing red cell hemolysis; all of which could ameliorate patient morbidities. Previous work on extracellular hemoglobin has shown that solution phase HbS reduces nitrite to NO faster than normal adult hemoglobin (HbA), while polymerized HbS reduces nitrite slower than HbA. In this work, we compared the rate of nitrite reduction to NO measured by the formation of nitrosyl hemoglobin in sickle and normal red blood cells at varying hemoglobin oxygen saturations. We found the overall rate of nitrite reduction between normal and sickle red blood cells was similar and confirmed this result under partially oxygenated conditions, but normal red blood cells reduced nitrite faster than sickle red blood cells under anoxia where HbS polymerization is maximal. These results are consistent with previous work using extracellular hemoglobin where the rate of reduction by solution phase HbS makes up for the slower reduction by polymer phase HbS under partially oxygenated conditions, but the polymer phase kinetics dominates in the complete absence of oxygen.

Enhancing nitrogen removal in low C/N wastewater with recycled sludge-derived biochar: A sustainable solution

Denitrification is an important biological process in wastewater treatment plants (WWTPs). However, a low carbon-to-nitrogen (C/N) ratio limits the availability of organic carbon, potentially reducing denitrification efficiency. This study investigates the impact of sludge-derived biochar on the nitrogen removal of activated sludge for low C/N ratio municipal wastewater. Sludge-based biochar was characterized by its physicochemical properties, revealing that biochar prepared at 400 °C exhibited the highest specific surface area and the most favorable surface functional groups for electron transfer. The results from batch tests showed that adding 4 g/L of biochar dosage enhanced denitrification rates and total nitrogen (TN) removal efficiency the most. Sequencing batch reactors (SBRs) experiments further confirmed that biochar dosgae improved the removal efficiencies of COD, NH4+-N, and TN, achieving stable values of 97.2 ± 1.2 %, 99.2 ± 0.6 %, and 83.8 ± 2.4 %, respectively. Metabolic and electrochemical analyses revealed that biochar addition enhanced the activity of denitrification enzymes, increasing the ammonia oxidation rate by 12.9 ± 0.7 %, nitrite oxidation rate by 14.7 ± 1.2 %, nitrate reduction rate by 36.9 ± 1.5 %, and nitrite reduction rate by 16.4 ± 0.8 %. The relative abundance of denitrification functional genes (amoA, nirS, nirK, narG, nosZ) increased, and the activities of the corresponding enzymes (AMO, NXR, NAP, NIR) rose by 23±6 %, 53±5 %, 260±15 %, and 55±7 %, respectively. This increase in enzyme activity suggested enhanced denitrification processes, which was further supported by the 60.1 ± 3.7 % increase in electron transfer system activity (ETSA), indicating that biochar acted as an electron shuttle. This study proposes a potential sustainable approach for sludge recycling and enhanced wastewater nitrogen removal under low C/N conditions.

Nitrate and nitrite utilization during denitrifying phosphorus removal: Electron acceptor preference and feasible process combinations

Denitrifying phosphorus removal (DPR), which is dominated by denitrifying polyphosphate-accumulating organisms (DPAOs), is a promising process for nitrogen and phosphorus removal. Denitrifying glycogen-accumulating organisms (DGAOs) and DPAOs typically coexist in the DPR sludge, complicating the study of DPAOs’ denitrification capacity. In this study, two reactors were fed with nitrate and nitrite during the anoxic phase to cultivate nitrate-DPR and nitrite-DPR sludge. Both reactors yielded high and low DGAO abundance sludges, enabling the evaluation of the denitrification capacity of DPAOs. For the nitrate-DPR sludge, the nitrite reduction rate was 1.63 times higher than the nitrate reduction rate when DPAOs were the primary denitrifiers. For the nitrite-DPR sludge, the reduction rate of nitrite was more than three times that of nitrate, irrespective of DGAO abundance. These findings indicated that DPAOs preferred nitrite to nitrate and were well suited to reduce nitrite rather than reduce nitrate to supply nitrite.

Biochar modulates intracellular electron transfer for nitrate reduction in denitrifying anaerobic methane oxidizing archaea

Denitrifying anaerobic methane oxidizing (DAMO) archaea plays a significant role in simultaneously nitrogen removal and methane mitigation, yet its limited metabolic activity hinders engineering applications. This study employed biochar to explore its potential for enhancing the metabolic activity and nitrate reduction capacity of DAMO microorganisms. Sawdust biochar (7 g/L) was found to increase the nitrate reduction rate by 2.85 times, although it did not affect the nitrite reduction rate individually. Scanning electron microscopy (SEM) and fluorescence excitation-emission matrix (EEM) analyses revealed that biochar promoted microbial aggregation, and stimulated the secretion of extracellular polymeric substances (EPS). Moreover, biochar bolstered the redox capacity and conductivity of the biofilm, notably enhancing the activity of the electron transfer system by 1.65 times. Key genes involved in intracellular electron transport (Hdr, MHC, Rnf) and membrane transport proteins (BBP, ABC, NDH) of archaea were significantly up-regulated. These findings suggest that biochar regulates electrons generated by reverse methanogenesis to the membrane for nitrate reduction.

Sequestration of Labile Organic Matter by Secondary Fe Minerals from Chemodenitrification: Insight into Mineral Protection Mechanisms.

Labile organic matter (OM) immobilized by secondary iron (Fe) minerals from chemodenitrification may be an effective way to immobilize organic carbon (OC). However, the underlying mechanisms of coupled chemodenitrification and OC sequestration are poorly understood. Here, OM immobilization by secondary Fe minerals from chemodenitrification was investigated at different C/Fe ratios. Kinetics of Fe(II) oxidation and nitrite reduction rates decreased with increasing C/Fe ratios. Despite efficient sequestration, the immobilization efficiency of OM by secondary minerals varied with the C/Fe ratios. Higher C/Fe ratios were conducive to the formation of ferrihydrite and lepidocrocite, with defects and nanopores. Three contributions, including inner-core Fe-O and edge- and corner-shared Fe-Fe interactions, constituted the local coordination environment of mineral-organic composites. Microscopic analysis at the molecular scale uncovered that labile OM was more likely to combine with secondary minerals with poor crystallinity to enhance its stability, and OM distributed within nanopores and defects had a higher oxidation state. After chemodenitrification, high molecular weight substances and substances high in unsaturation or O/C ratios including phenols, polycyclic aromatics, and carboxylic compounds exhibited a stronger affinity to Fe minerals in the treatments with lower C/Fe ratios. Collectively, labile OM immobilization can occur during chemodenitrification. The findings on OM sequestration coupled with chemodenitrification have significant implications for understanding the long-term cycling of Fe, C, and N, providing a potential strategy for OM immobilization in anoxic soils and sediments.

Microplastics shaped performance, microbial ecology and community assembly in simultaneous nitrification, denitrification and phosphorus removal process

The widespread use of microplastics (MPs) has led to an increase in their discharge to wastewater treatment plants. However, the knowledge of impact of MPs on macro-performance and micro-ecology in simultaneous nitrification, denitrification, and phosphorus removal (SNDPR) systems is limited, hampering the understanding of potential risks posed by MPs. This study firstly comprehensively investigated the performance, species interactions, and community assembly under polystyrene (PS) and polyvinyl chloride (PVC) exposure in SNDPR systems. The results showed under PS (1, 10 mg/L) and PVC (1, 10 mg/L) exposure, total nitrogen removal was reduced by 3.38–10.15 %. PS and PVC restrained the specific rates of nitrite and nitrate reduction (SNIRR, SNRR), as well as the activities of nitrite and nitrate reductase enzymes (NIR, NR). The specific ammonia oxidation rate (SAOR) and activity of ammonia oxidase enzyme (AMO) were reduced only at 10 mg/L PVC. PS and PVC enhanced the size of co-occurrence networks, niche breadth, and number of key species while decreasing microbial cooperation by 5.85–13.48 %. Heterogeneous selection dominated microbial community assembly, and PS and PVC strengthened the contribution of stochastic processes. PICRUSt prediction further revealed some important pathways were blocked by PS and PVC. Together, the reduced TN removal under PS and PVC exposure can be attributed to the inhibition of SAOR, SNRR, and SNIRR, the restrained activities of NIR, NR, and AMO, the changes in species interactions and community assembly mechanisms, and the suppression of some essential metabolic pathways. This paper offers a new perspective on comprehending the effects of MPs on SNDPR systems.

Hydroxylamine supplementation accelerated the rates of cell growth, aerobic denitrification and nitrous oxide emission of Pseudomonas taiwanensis EN-F2

Hydroxylamine can disrupt the protein translation process of most reported nitrogen-converting bacteria, and thus hinder the reproduction of bacteria and nitrogen conversion capacity. However, the effect of hydroxylamine on the denitrification ability of strain EN-F2 is unclear. In this study, the cell growth, aerobic denitrification ability, and nitrous oxide (N2O) emission by Pseudomonas taiwanensis were carefully investigated by addition of hydroxylamine at different concentrations. The results demonstrated that the rates of nitrate and nitrite reduction were enhanced by 2.51 and 2.78 mg/L/h after the addition of 8.0 and 12.0 mg/L hydroxylamine, respectively. The N2O production from nitrate and nitrite reaction systems were strongly promoted by 4.39 and 8.62 mg/L, respectively, through the simultaneous acceleration of cell growth and both of nitrite and nitrate reduction. Additionally, the enzymatic activities of nitrate reductase and nitrite reductase climbed from 0.13 and 0.01 to 0.22 and 0.04 U/mg protein when hydroxylamine concentration increased from 0 to 6.0 and 12.0 mg/L. This may be the main mechanism for controlling the observed higher denitrification rate and N2O release. Overall, hydroxylamine supplementation supported the EN-F2 strain cell growth, denitrification and N2O emission rates.

Response of aerobic granular sludge under acute inhibition by polystyrene microplastics: Activity, aggregation performance, and microbial analysis

In this study, the activity, aggregation performance, microbial community and functional proteins of aerobic granular sludge (AGS) in response to acute inhibition by different concentrations of polystyrene microplastics (PS-MPs) were investigated. As the PS-MPs concentration increased from 0 mg/L to 200 mg/L, the specific nitrogen removal rate and the activity of enzymes were inhibited. The inhibition of specific nitrite reduction rate (SNIRR) and specific nitrate reduction rate (SNRR) was most obvious at the PS-MPs concentration of 100 mg/L, and that of nitrite reductase (NIR) and nitrate reductase (NR) was most obvious at the concentration of 50 mg/L. But the inhibitory effects were mitigated at the concentration of 200 mg/L. The increase of reactive oxygen species (ROS) and lactate dehydrogenase (LDH) indicated that the cells were damaged with the increase of PS-MPs concentration. The content of proteins and polysaccharides in extracellular polymeric substances (EPS) decreased, especially the polysaccharides were more affected. Analysis of zeta potential, hydrophobicity and surface thermodynamics of AGS revealed that addition of PS-MPs was unfavorable for AGS aggregation. It was also found that bacteria genera associated with EPS secretion and nitrogen removal functions were inhibited, while functions associated with cell metabolism, protein synthesis and cell repair were enhanced. This also confirmed that acute inhibition of PS-MPs had a detrimental effect on the nitrogen removal and aggregation performance of AGS. This study can provide theoretical support for the operation of AGS reactors under microplastics impact load.

Abstract 377: Developing a Novel Oxygen Carrier for Resuscitation and Oxygen Delivery Through Targeted Mutations of Recombinant Heme-Proteins

Introduction: Hemorrhage is a leading cause of death in trauma patients. Interventions dedicated to hemostatic resuscitation have demonstrated benefit in decreasing mortality due to hemorrhagic injury. There remain significant limitations to donor-based blood transfusions. We have identified a heme-binding protein, the Bacterial Regulator of carbon monoxide metabolism (RcoM), with suitable properties to be a robust artificial oxygen carrier. We have engineered a variant that lacks a DNA binding domain and contains cysteine mutations for improved stability (RcoM HBD-CCC). Hypothesis: We hypothesize that engineered RcoM HBD-CCC can be used as an ideal artificial oxygen carrier due to the optimization of 4 factors: ligand binding, nitrite reduction rate, autoxidation rate, and thermal denaturation. Methods: Spectrophotometric methods were used to characterize RcoM HBD-CCC in terms of its biochemical properties for optimal oxygen transport and release. Specifically, ligand binding was tested by determination of the P50 (oxygen pressure for 50% saturation of the molecule) value indicating the affinity of RcoM HBD-CCC towards oxygen. Nitrite reduction assays were carried out to test the rate at which nitric oxide (NO) forms to counteract potential NO scavenging. The autoxidation rate determines how long a protein stays in the reduced state, where it is able to bind to oxygen. Thermal denaturation (T m ) identifies the resistance of a protein to higher temperatures. Results: We determined that RcoM HBD-CCC’s P50 value was 4.2 mmHg which is within the ideal range for oxygen binding affinity given that optimal values for oxygen carriers to mimic hemoglobin oxygen dissociation are between 1 to 26 mmHg. Nitrite reductase rate was 5.6M -1 s -1 which is well above the rate for Hb (K = 0.12 M -1 s -1 ). RcoM HBD-CCC has a low autoxidation rate at 0.016 min -1 . RcoM HBD-CCC has a T m of 71°C which is well above all ambient environmental temperatures. Conclusion: We have designed and synthesized a variant of RcoM that we have demonstrated in in vitro spectrophotometric assays to have ideal biochemical properties for an artificial oxygen carrier. We next plan to test the efficacy of RcoM HBD-CCC in animal models of hemorrhage and assess safety in animal exposure models.

Engineering neuroglobin nitrite reductase activity based on myoglobin models

Neuroglobin is a hemoprotein expressed in several nervous system cell lineages with yet unknown physiological functions. Neuroglobin presents a very similar structure to that of the related globins hemoglobin and myoglobin, but shows an hexacoordinate heme as compared to the pentacoordinated heme of myoglobin and hemoglobin. While several reactions of neuroglobin have been characterized in vitro, the relative importance of most of those reactions in vivo is yet undefined. Neuroglobin, like other heme proteins, can reduce nitrite to nitric oxide, providing a possible route to generate nitric oxide in vivo in low oxygen conditions. The reaction kinetics are highly dependent on the nature of the distal residue, and replacement of the distal histidine His64(E7) can increase the reaction rate constants by several orders of magnitude. However, mutation of other distal pocket positions such as Phe28(B10) or Val68(E11) has more limited impact on the rates. Computational analysis using myoglobin as template, guided by the structure of dedicated nitrite reductases like cytochrome cd1 nitrite reductase, has pointed out that combined mutations of the residues B10 and CD1 could increase the nitrite reductase activity of myoglobin, by mimicking the environment of the distal heme pocket in cytochrome cd1 nitrite reductase. As neuroglobin shows high sequence and structural homology with myoglobin, we hypothesized that such mutations (F28H and F42Y in neuroglobin) could also modify the nitrite reductase activity of neuroglobin. Here we study the effect of these mutations. Unfortunately, we do not observe in any case an increase in the nitrite reduction rates. Our results provide some further indications of nitrite reductase regulation in neuroglobin and highlight the minor but critical differences between the structure of penta- and hexacoordinate globins.

Development of CuNi immobilized Pt surface to minimize nitrite evolution during electrocatalytic nitrate reduction in neutral medium

Nitrite (NO2−) is one of the major troublemakers in our environment, merits rigorous attention on removal purposes via effective treatments. Meanwhile, often NO2− ions are evolved as stable intermediates during electrocatalytic nitrate reduction reaction (NRR). In the present article, it is shown how the immobilization of Cu and Ni particles on a Pt surface influences both nitrate and nitrite reduction rates in the neutral medium. Using sandwich type membrane reactor, it was observed that NO3− and NO2− ions reduced having a first order rate constant (k) of 26.1×10–3 min−1 and 29.5×10–3 min−1, respectively at a Pt-CuNi cathode surface. Due to larger NO2−reduction rate, NO2− ions did not appear while NO3− ions were reduced at the Pt-CuNi electrode surface. At the end of reaction, NH3 was identified as the only product. Beside the evaluation of reactivity and product selectivity, several kinetic properties of NRR at Pt-CuNi surface were also evaluated. A Pt-Cu electrode requires 10.05 kJmol−1 activation free energy, but this energy is decreased to 5.76 kJmol−1 while Pt-CuNi electrode is used to attain NRR in the neutral medium. All the experimental data revealed that incorporation of Ni into Pt-Cu matrix, significantly improves NRR efficiency. In addition, the NRR process does not compete with hydrogen evolution reaction (HER) process at the Pt-CuNi electrode surface. Finally, Pt-CuNi being the efficient catalyst for attaining NRR, this catalyst was characterized with different electrochemical and spectroscopic techniques to unveil inherent physical properties.

Effect of Cd(II) shock loading on performance, microbial enzymatic activity and microbial community in a sequencing batch reactor

The performance, microbial enzymatic activity and microbial community of a sequencing batch reactor (SBR) were explored under instantaneous Cd(II) shock loading. After a 24-h Cd(II) shock loading of 100 mg/L, the chemical oxygen demand and NH4+-N removal efficiencies decreased significantly from 92.73% and 99.56% on day 22 to 32.73% and 43% on day 24, respectively, and then recovered to the normal values gradually. The specific oxygen utilization rate (SOUR), specific ammonia oxidation rate (SAOR), specific nitrite oxidation rate (SNOR), specific nitrite reduction rate (SNIRR) and specific nitrate reduction rate (SNRR) decreased by 64.81%, 73.28%, 77.77%, 56.84% and 52.46% on day 23 in comparison with the absence of Cd(II) shock loading, respectively, and they gradually returned to the normal levels. The changing trends of their associated microbial enzymatic activities including dehydrogenase, ammonia monooxygenase, nitrite oxidoreductase, nitrite reductase and nitrate reductase were in accordance with SOUR, SAOR, SNOR, SNIRR and SNRR, respectively. Cd(II) shock loading promoted the microbial reactive oxygen species production and lactate dehydrogenase release, indicating that instantaneous shock caused oxidative stress and damaged to cell membranes of the activated sludge. The microbial richness and diversity, and the relative abundance of Nitrosomonas and Thauera obviously decreased under the stress of Cd(II) shock loading. PICRUSt prediction showed that Cd (II) shock loading significantly affected Amino acid biosynthesis, Nucleoside and nucleotide biosynthesis. The present results are conducive to take adequate precautions to reduce the adverse effect on bioreactor performance in wastewater treatment systems.

Feasibility of Achieving Efficient Nitrite Accumulation in Moving Bed Biofilm Reactor: The Influencing Factors, Microbial Structures, and Biofilm Characteristics

Moving bed biofilm reactor (MBBR) is considered as a promising technology for wastewater treatment owing to the high biomass retention and low cost. In this study, the performance of using MBBR for partial denitrification (PD) was investigated. Denitrifying biofilm was successfully formed after 40 days with the biomass and nitrite reduction rate of 40.83 mg VSS/g carriers and 51.52 mg N/(gVSS·h), respectively. Morphology analysis by scanning electron microscope (SEM) showed that the biofilm surface was dominant by cocci, filamentous bacteria, and extracellular polymeric substances (EPS). Investigation about the influencing factors of PD found that the optimal COD/NO3−-N and pH for efficient nitrite production (nitrate to nitrite ratio: 96.49%) was 3 and 9, respectively. Moreover, Saccharimonadales was proved to be dominant functional microbes in the constructed PD systems with different influent conditions because its relative abundance exhibited good correlation with the nitrite accumulation. By analyzing the biofilm characteristics under different conditions, PD was observed to mainly occur in the range of 300–700 μm inside the biofilm, where most of the dissolved oxygen was consumed. This study confirmed the feasibility and superior performance of PD-MBBR system.

Nitrogen removal characteristics of biofilms in each area of a full-scale AAO oxidation ditch process.

Plastic carriers were installed in different areas of a full-scale anaerobic/anoxic/aerobic (AAO) oxidation ditch process, and the dynamics of nitrogen removal, biofilm morphologies, and microorganism species were investigated. The results showed that the biofilm at the front of the aerobic area (dissolved oxygen [DO]=0.93mgL-1) provided the best denitrification, with specific nitrate and nitrite reduction rates of 10.16 and 3.78mg·(g·h)-1, respectively. The biofilm in the middle of the aerobic area (DO=1.27mgL-1) exhibited the best nitrification performance, with a maximum specific ammonia oxidation rate of 3.21mg·(g·h)-1. The biofilm at the end of the aerobic area (DO=0.01mgL-1) exhibited the highest anammox potential with a maximum specific anammox rate of 0.67mg·(g·h)-1. No correlation was observed between the specific nitrogen removal rates and abundance of nitrogen-removing microorganisms at the genus level. Biofilm denitrification during the process was primarily completed by heterotrophic denitrifying bacteria (Thauera, Acinetobacter, Hyphomicrobium, and Thermomonas). Aerobic denitrifying bacteria (0.19% Thauera and 0.34% Hyphomicrobium) were identified as the main denitrifying bacteria in the middle of the aerobic area. The dominant nitrifying microorganisms in the middle of the aerobic area were Nitrosomonas (0.50%) and Nitrospira (1.04%). A biofilm in the end of the aerobic area exhibited specific anammox potential, which may have been related to the dominance of 0.024% Candidatus Brocadia. Kinetic analysis revealed that adding plastic carriers to the front and middle of the aerobic area contributed to stable nitrogen removal efficiency.

Regulation of nitrite reductase and lipid binding properties of cytoglobin by surface and distal histidine mutations

Cytoglobin is a hemoprotein widely expressed in fibroblasts and related cell lineages with yet undefined physiological function. Cytoglobin, as other heme proteins, can reduce nitrite to nitric oxide (NO) providing a route to generate NO in vivo in low oxygen conditions. In addition, cytoglobin can also bind lipids such as oleic acid and cardiolipin with high affinity. These two processes are potentially relevant to cytoglobin function. Little is known about how specific amino acids contribute to nitrite reduction and lipid binding. Here we investigate the role of the distal histidine His81 (E7) and several surface residues on the regulation of nitrite reduction and lipid binding. We observe that the replacement of His81 (E7) greatly increases heme reactivity towards nitrite, with nitrite reduction rate constants of up to 1100 M−1s−1 for the His81Ala mutant. His81 (E7) mutation causes a small decrease in lipid binding affinity, however experiments on the presence of imidazole indicate that His81 (E7) does not compete with the lipid for the binding site. Mutations of the surface residues Arg84 and Lys116 largely impair lipid binding. Our results suggest that dissociation of His81 (E7) from the heme mediates the formation of a hydrophobic cavity in the proximal heme side that can accommodate the lipid, with important contributions of the hydrophobic patch around residues Thr91, Val105, and Leu108, whereas the positive charges from Arg84 and Lys116 stabilize the carboxyl group of the fatty acid. Gain and loss-of-function mutations described here can serve as tools to study in vivo the physiological role of these putative cytoglobin functions.

Response difference of simultaneous sulfide and nitrite removal process to different cooling modes

The effects of various cooling modes (sudden cooling (25℃→10℃) and step cooling (25℃→20℃→15℃→10℃)) on the performance of simultaneous sulfide and nitrite removal process were reported. Regardless of cooling mode adopted, the process maintained good sulfide removal performance, and removal percentage was 100.00%. Considering nitrite removal percentage, the process was more sensitive to step cooling mode (k = 0.06707) in comparison to sudden cooling mode (k = 0.02760). Lowering temperature promoted the transformation from sulfate to elemental sulfur, and it was easier to increase the proportion of elemental sulfur (79.90%) by means of step cooling. The sulfide oxidation rate and nitrite reduction rate were 0.01540 mg /(L∙min) and 0.00354 mg /(L∙min), respectively, in the sudden cooling mode, and 0.01168 mg /(L∙min) and 0.00138 mg /(L∙min), respectively, in the step cooling mode. Low temperature reduced the diversity of microbial community, and Sulfurovum was still a dominant bacterial member in both cooling modes.

Mitigating nitrite accumulation during S0-based autotrophic denitrification: Balancing nitrate-nitrite reduction rate with thiosulfate as external electron donor

This study was carried out to determine the effect of influent nitrate loading on nitrite accumulation during elemental-sulfur based denitrification process, and proposed to enhance the nitrogen removal efficiency by mitigating nitrite accumulation with thiosulfate as external electron donor. Along with increasing the nitrate influent loading (from 0.09 kg N/m3/d to 1.73 kg N/m3/d) by shortening the empty bed contact time (EBCT) (from 5 h to 0.25 h), the nitrate removal loading increased from 0.08 to 0.83 kg N/m3/d. Meanwhile, the raise of the nitrate influent loading obviously aggravated the nitrite accumulation. Herein, nitrite began to accumulate since the nitrate influent loading was over 0.86 kg N/m3/d, and a maximum nitrite accumulation of 2.39 mg/L was observed under the 0.25 h of EBCT and 15 mg/L of nitrate influent concentration condition. Thiosulfate was used as the external electron donor to accelerate the nitrite reduction rate in order to mitigate the nitrite accumulation. As a result, the nitrite accumulation significantly decreased from 2.39 mg/L to 0.17 mg/L with the thiosulfate dosage of 13.36 mg/L. However, the nitrite accumulation bounced with the on-going increase of the thiosulfate dosage, indicating that the nitrate reduction rate and nitrite reduction rate were accelerated alternatively. After dosing thiosulfate, the relative abundances of sulfurimonas and ferritrophicum grew up significantly.

Human and rodent red blood cells do not demonstrate xanthine oxidase activity or XO-catalyzed nitrite reduction to NO

A number of molybdopterin enzymes, including xanthine oxidoreductase (XOR), aldehyde oxidase (AO), sulfite oxidase (SO), and mitochondrial amidoxime reducing component (mARC), have been identified as nitrate and nitrite reductases. Of these enzymes, XOR has been the most extensively studied and reported to be a substantive source of nitric oxide (NO) under inflammatory/hypoxic conditions that limit the catalytic activity of the canonical NOS pathway. It has also been postulated that XOR nitrite reductase activity extends to red blood cell (RBCs) membranes where it has been immunohistochemically identified. These findings, when combined with countervailing reports of XOR activity in RBCs, incentivized our current study to critically evaluate XOR protein abundance/enzymatic activity in/on RBCs from human, mouse, and rat sources. Using various protein concentrations of RBC homogenates for both human and rodent samples, neither XOR protein nor enzymatic activity (xanthine → uric acid) was detectable. In addition, potential loading of RBC-associated glycosaminoglycans (GAGs) by exposing RBC preparations to purified XO before washing did not solicit detectable enzymatic activity (xanthine → uric acid) or alter NO generation profiles. To ensure these observations extended to absence of XOR-mediated contributions to overall RBC-associated nitrite reduction, we examined the nitrite reductase activity of washed and lysed RBC preparations via enhanced chemiluminescence in the presence or absence of the XOR-specific inhibitor febuxostat (Uloric®). Neither addition of inhibitor nor the presence of the XOR substrate xanthine significantly altered the rates of nitrite reduction to NO by RBC preparations from either human or rodent sources confirming the absence of XO enzymatic activity. Furthermore, examination of the influence of the age (young cells vs. old cells) of human RBCs on XO activity also failed to demonstrate detectable XO protein. Combined, these data suggest: 1) that XO does not contribute to nitrite reduction in/on human and rodent erythrocytes, 2) care should be taken to validate immuno-detectable XO by demonstrating enzymatic activity, and 3) XO does not associate with human erythrocytic glycosaminoglycans or participate in nonspecific binding.