Nitrate Removal Rate Research Articles

Impact of tetracycline on mixotrophic denitrification process under different sulfur to nitrogen ratios

The sulfur-based autotrophic-heterotrophic denitrification, i.e., mixotrophic denitrification, is suitable for the nitrate and antibiotics removal in aquaculture tailwater at a low COD to nitrogen (C/N) ratio. This study focused on the effect of tetracycline (TC) on mixotrophic denitrification under different S/N ratios. Two bioreactors were simultaneously operated with or without dosing tetracycline under different sulfur to nitrogen (S/N) ratios of 3.94, 4.64 and 5.94. The results showed that the removal rate of total inorganic nitrogen (TIN) increased from 0.25 to 0.69 mg N L−1 min−1 with the rise of S/N ratio, while TC dosage significantly declined the removal efficiency of TIN. Dissimilatory nitrogen reduction to ammonia (DNRA) bacteria was detected when exposing to TC, indicating that DNRA presented more resistance to TC. The removal efficiency of TC in the denitrification system reached the maximum of 22.87 % at S/N of 4.64. Meanwhile the genus Marinicella was detected at this phase, which was conducive to the degradation of organic pollutants. This study found that TC promoted the accumulation of ammonia nitrogen, and had a great effect on sulfur autotrophic bacteria at S/N of 5.94. The removal of TC mainly depended on microbial co-metabolism, and there was a significant correlation between the reduction of TC concentration and the decrease of sulfur compounds (p < 0.05). 4.64 is the best S/N ratio for the mixotrophic denitrification process, which revealed maximum nitrate and TC removal rates.

Implementation of an Upflow Fixed Bed Bioreactor for Denitrification Coupled to Methane Oxidation: Performance and Biomass Development Under Anoxic Conditions

Denitrification coupled to methane oxidation (DOM) has been shown to be an appropriate process for wastewater treatment applications, since it can reduce greenhouse gas emissions and nitrogen discharges, making wastewater treatment plants more environmentally sustainable. Study of DOM has focused on laboratory-scale application using membrane biological reactors (MBR) or sequency batch reactors (SBR), which have been shown to be able to retain DOM biomass and therefore appropriate for use with this process. However, it is necessary to expand knowledge of the behavior of this process using other configurations, with a view to scaling up. Therefore, in this study, an upflow fixed bed bioreactor (UFBR) was implemented using plastic carriers such as bioballs and Biochips® to carry out the DOM process under anoxic conditions. The reactor reached stable nitrogen removal conditions after approximately 400 days of continuous operation, forming a biomass composed of denitrifying methane-oxidizing microorganisms where the genus Anaerolinea and Methylocystis predominated. Once the biomass was formed and the DOM process was stabilized, maximum nitrite and nitrate removal rates of 17.6 mgN-NO2−/L-d and 8.9 mgN-NO3−/L-d, respectively, and a removal efficiency of methane up to 77% were obtained. This demonstrates the feasibility of the application of the DOM process under anoxic conditions using fixed bed bioreactors, which is promising for further nitrogen removal from wastewater using a varied reactor configuration easily to scaled-up.

Evaluation of Effluent Quality Trends Before and After Filtration Through the Composite Filter from Shirere Wastewater Treatment Plant to River Isiukhu in Kakamega County, Kenya

The current stabilization ponds as wastewater treatment practices in urban areas have proven insufficient with continued discharge of untreated wastes into water bodies. Their challenge comes from inappropriate system selection and maintenance, improper design, construction mistakes, physical damage and hydraulic overload. Appropriate infrastructural technologies for waste removal that can be adopted in the drainage channels of effluents into water bodies are scarce. This study incorporates a reactor based composite filter of pumice and sand as an innovative approach for removing residual waste in effluents discharged from Shirere Wastewater Treatment Plant into River Isiukhu, Kakamega Municipality. The objective of the study was to evaluate the trend of effluent quality from Shirere wastewater treatment plant upto river Isiukhu before and after installation of composite granular filter. Effluents, drinking water from Shirere WWTP, Shikoye stream, River Isiukhu and protected spring along Shikoye stream, were collected using presterilized water sampling containers for microbial quality analysis at MMUST and KACWASCO laboratories. The measurements were carried out using UV-VIS spectrophotometer at 752 nanometer wavelengths. Research design was experimental. The average reduction of COD in the mid-season of June to August was 42.2 ±4.6%, being the highest. Concomitantly, the BOD removal by the filter in the season of June to August was19.6±7% and 15.6 ±3.5% for September to November. The average rate of TSS removal in June to August was 19.3±4.5% followed by 16.6±3.8% in September to November and 11.6±7% in March to May. The average rate of Nitrate removal in June to August was 41.8±7.6% followed by 30.0±2.2% for March to May and 25±8.6% for September to November. Phosphates had an average rate of removal in June to August at 31.9±2.7% followed by 20.6±4.8% for September to November and 20.0 ±4.3% for March to May. Specifically, for the first season of March – May 2021 at 200 mm filtration depth were carried out at effluent flow rate of 0.0032 and volume, 0.234 Concentrations of most parameters were above NEMA standards, like COD was 322mg/l yet maximum should be 100 mg/l. Therefore, it was concluded that silica pumice composite filter performance was achieved by big variations in the concentrations of COD, BOD, TSS, Phosphates and Nitrates at Shirere WWTP after filtration which was attributed to effective removing capacity. The effluent concentrations from sampling sites S1-S3 and S5-S7 were found to be above the NEEMA standards implying the high risk of using Isiukhu water and catchment area. Thus, this study recommended that, the composite filter reduced concentrations of all the parameters (COD, BOD, TSS, PO3, NO3) significantly from Shirere WWTP along Shikoye stream up to the confluence of river Isiukhu. Most of the parameters after filtration were ranging within the required standards of NEMA. The requisite measure of adopting new technology of composite filtration should be sustained.

Model the evolutionary pattern of N species and pool size in groundwater continuum by utilizing measured source and sink rates of nitrate and ammonium

Nitrate and ammonium are primary nitrogen (N) contaminants in groundwater and effective restoration strategies depend on understanding the interactions of N transformation processes along redox gradients. Utilizing the 15N tracing technique, we assess nitrate removal rates, focusing on denitrification and anammox in a N-rich groundwater of the Hetao Basin, a typical semiarid region in western China. Results showed that N removal rate (0.36–22.01 µM N d−1) was composed mainly of denitrification (73 ± 18 %), with rates increasing from upstream oxidizing environment to downstream reducing areas. In reducing downstream, both denitrification and anammox adhered to substrate-driven Michaelis-Menten kinetics. Integrating data on all source and sink rates of nitrate and ammonium pools (denitrification, anammox, dissimilatory nitrate reduction ammonia, nitrification, mineralization), we constructed a N-transfer-dynamics model based on chemical stoichiometry. This model effectively captured the observed spatial N transfer patterns and highlighted that the balance of oxidants and biodegradable organic N inputs influences N species retention and removal in groundwater. Our combined experimental and modeling approach underscores the importance of reducing organic N and/or adding oxidants to mitigate groundwater N pollution. These findings provide crucial insights for optimizing high N groundwater remediation strategies and potentially inform for wastewater management practices.

Effect of plant species on wastewater treatment performance of a subsurface vertical-flow constructed wetland with step-feeding at low temperature

To improve the treatment performance of constructed wetlands under low-temperature conditions, this study investigated the effects of plant species on wastewater treatment performance at low temperature and the associated microbiological characteristics in a subsurface vertical-flow constructed wetland (VFCW) with step-feeding. The results showed that the redox microenvironment in the VFCW filter with step-feeding could be restored and optimized by planting appropriate species that can tolerate low temperature, ensuring a high nitrification performance for the system. Correspondingly, the abundance and activity of three functional microbes (namely nitrifiers, denitrifiers, and anammox bacteria) increased to different degrees in the system, eventually ensuring ideal nitrogen removal by the VFCW. Compared with the VFCW planted with Phragmites australis and Acorus gramineus, the operation performance of the VFCW planted with Iris wilsonii could be recovered at low temperature, and its chemical oxygen demand, total phosphorus, total nitrogen, and ammonium nitrate removal rates could respectively reach 95.7%, 99.2%, 93.0%, and 94.4%, respectively. Moreover, nitrogen removal in the system relied on the nitrification/denitrification and partial denitrification - anaerobic ammonium oxidation processes. Nitrosomonas, Nitrospira, Thauera, and Candidatus Brocadia were the four dominant bacterial genera in the filter layer.

Simultaneous nitrogen and phosphorus removal by immobilized bacterial particles of denitrifying phosphorus accumulating microorganisms and its application

Enterobacter cloacae G, a novel denitrifying phosphorus-accumulating bacterial strain, was isolated from anaerobic sludge tank of a wastewater treatment plant used for pig farms. It was discovered that a pH of 7, a temperature of 30°C, an initial phosphorus concentration of 8 mg/L, and a C/N ratio of 10 were the strain's ideal growth conditions. To ensure the stability of strain G in wastewater treatment, strain G was immobilized by 5 % polyvinyl alcohol, 2 % sodium alginate, and 0.6 g of biochar and crosslinked for 9 h in 4 % calcium chloride saturated boric acid solution via an orthogonal test. After the immobilized microspheres were introduced into the sequencing batch reactor (SBR), the nitrate and phosphate removal rates achieved were 89.36 % and 65.53 %, respectively, with a hydraulic retention time (HRT) of 8 hours, a pH of 7.5, and a C/N ratio of 4.5. The immobilized microspheres containing strain G demonstrated potential for the treatment of nitrogen-rich and phosphorus-rich wastewater.

Influence of carbon to phosphorus ratio on the performance of single-stage aerobic simultaneous nitrogen and phosphorus removal by bioflocs

Effluents from intensive aquaculture typically contain high nitrate and phosphate concentrations. Biofloc technology has demonstrated the potential for simultaneous removal of nitrate and phosphate without ammonium nitrogen, and further optimization is needed to enhance the nitrogen and phosphorus removal efficiency. In this study, we investigated the efficiency of bioflocs in treating highly concentrated aquacultural wastewater at different carbon to phosphorus (C/P) ratios of 20 (G20), 30 (G30), and 40 (G40). The results showed that the nitrate removal rate in group G40 (1.25±0.07 mgN/gTSS/h) was significantly higher than in groups G20 and G30 (p < 0.05). However, there was no significant difference between the phosphate removal rates of groups G40 and G30, while both exhibited superior G20. The relative abundance of Thauera in G40 was significantly higher (p < 0.05), accounting for 8.74% of the microbial community. Additionally, the copy counts of denitrification-related genes (napA, nirS, nirK, nosZ) and inorganic phosphate transport genes (pqqC) were significantly higher in G40, correlating positively with an increased C/P ratio. Thees results suggest that the excessive carbon source in G40 enhanced denitrification and reduced biofloc assimilation, thus failing to significantly enhance the phosphate removal rate. This study demonstrates that adjusting the C/P ratio alone can improve the efficiency of nitrogen and phosphorus removal by bioflocs, and that a C/P ratio of 30 may be the most appropriate for enhancing the rate of nutrient removal while minimizing the use of carbon source.

Effect of dissolved oxygen on sulfur autotrophic denitrification and how to address it: An experimental and modelling work

Sulfur autotrophic denitrification (SAD) using elemental sulfur as the electron donor has aroused increasing interest of its application in treating secondary effluent from wastewater treatment plants (WWTPs). However, high influent dissolved oxygen (DO) in secondary effluent would limit the SAD process. This study examined the effect of different DO concentrations on SAD. Results revealed that both low (0–0.5 mg/L) and moderate (2.5–3.5 mg/L) DO concentrations would not harm the nitrate removal rate (NRR) (p > 0.05). However, high DO concentration (5.5–6.5 mg/L) significantly decreased the NRR (p < 0.05) through strong competition over the nitrate for electrons and cutting the relative abundance of sulfur-oxidizing bacteria (SOB). Both modeling and experimental results showed that applying internal reflux could serve as a strategy to mitigate the negative effect of high DO concentration, while keeping an appropriate ratio was crucial. When treating real membrane bioreactor (MBR) effluent with high DO concentration (5.5–6.5 mg/L), an internal reflux ratio of 0.5 boosted the NRR by 1.5 times. This study provided potential reference and strategy for dealing with high DO concentration wastewater by applying SAD technology.

Exploring the impact of fulvic acid on electrochemical hydrogen-driven autotrophic denitrification system: Performance, microbial characteristics and mechanism

In this study, the effect of modulating fulvic acid (FA) concentrations (0, 25 and 50 mg/L) on nitrogen removal in a bioelectrochemical hydrogen autotrophic denitrification system (BHDS) was investigated. Results showed that FA increased the nitrate (NO3–-N) removal rate of the BHDSs from 37.8 to 46.2 and 45.2 mg N/(L·d) with a current intensity of 40 mA. The metagenomic analysis revealed that R2 (25 mg/L) was predominantly populated by autotrophic denitrifying microorganisms, which enhanced denitrification performance by facilitating electron transfer. Conversely, R3 (50 mg/L) exhibited an increase in genes related to the heterotrophic process, which improved the denitrification performance through the collaborative action of both autotrophic and heterotrophic denitrification pathways. Besides, the study also identified a potential for nitrogen removal in Serpentinimonas, which have been rarely studied. The interesting set of findings provide valuable reference for optimizing BHDS for nitrogen removal and promoting specific denitrifying genera within the system.

Optimizing carbon sources on performance for enhanced efficacy in single-stage aerobic simultaneous nitrogen and phosphorus removal via biofloc technology

Bioflocs can efficiently achieve simultaneous nitrate and phosphate removal through a single-stage aerobic process, provided they are continuously supplemented with an organic carbon source. This study investigated the effects of different carbon sources on this process. Results revealed that phosphate removal rate in the glucose group was 0.61 ± 0.02 mg/L/h, significantly higher than those in the acetate (0.28 ± 0.01 mg/L/h) and propionate (0.29 ± 0.03 mg/L/h) groups (p < 0.05). However, the three groups observed no significant differences in nitrate removal rates (p > 0.05). The superior performance of the glucose group in simultaneous nitrogen and phosphorus removal is likely due to the higher biomass synthesis. In contrast, nitrate removal in the acetate and propionate groups was primarily driven by denitrification, resulting in lower sludge production and reduced phosphate uptake. For practical application of bioflocs in simultaneous nitrogen and phosphorus removal, glucose is recommended as the optimal carbon source.

Unraveling the impact of perfluorooctanoic acid on sulfur-based autotrophic denitrification process

PFOA has garnered heightened scrutiny for its impact on denitrification, especially given its frequent detection in secondary effluent discharged from wastewater treatment plants. However, it is still unclear what potential risk PFOA release poses to a typical advanced treatment process, especially the sulfur-based autotrophic denitrification (SAD) process. In this study, different PFOA concentration were tested to explore their impact on denitrification kinetics and microbial dynamic responses of the SAD process. The results showed that an increase PFOA concentration from 0 to 1000 μg/L resulted in a decrease in nitrate removal rate from 9.52 to 7.73 mg-N/L·h. At the same time, it increased nitrite accumulation and N2O emission by 6.11 and 2.03 times, respectively. The inhibitory effect of PFOA on nitrate and nitrite reductase activity in the SAD process was linked to the observed fluctuations in nitrate and nitrite levels. It is noteworthy that nitrite reductase was more vulnerable to the influence of PFOA than nitrate reductase. Furthermore, PFOA showed a significant impact on gene expression and microbial community. Metabolic function prediction revealed a notable decrease in nitrogen metabolism and an increase in sulfur metabolism under PFOA exposure. This study highlights that PFOA has a considerable inhibitory effect on SAD performance.

Core-shell bioactive capsule for aquaculture wastewater denitrification

The lack of denitrifying bacteria and organic carbon sources, and the inhibition of dissolved oxygen (DO) result in nitrate accumulation in aquaculture wastewater. In order to solve this problem, an encapsulation method was introduced to prepare a novel bioactive capsule, which can provide organic carbon source, denitrifying bacteria, and anoxic microenvironment for aquaculture wastewater denitrification. And can reduce the recovery time of the enclosed denitrifying bacteria. The morphology of the capsule, its nitrate removal rate, and nitrogen conversion pathway in synthetic aquaculture wastewater were investigated. The capsule had a porous surface and the pore diameter ranged from 150.0 nm to 300.0 nm. The enclosed denitrifying bacteria had a reduced recovery time and excellent denitrification performance. The nitrate removal rate reached 86.2 % on the first day and was maintained at 99.7 %. Nitrogen conversion pathways in the capsule include denitrification, assimilatory/dissimilatory nitrate reduction, and nitrogen fixation. The denitrifying capsule has short recovery time and good denitrification performance, which would help to achieve denitrification in aquaculture wastewater or other low C/N wastewater.

Calcined pyrite accelerates sulfur metabolic and electron transfer in driving targeted microbial fuel cell denitrification

The sulfur powder as electron donor in driving dual-chamber microbial fuel cell denitrification (S) process has the advantages in economy and pollution-free to treat nitrate-contained groundwater. However, the low efficiency of electron utilization in sulfur oxidation (ACE) is the bottleneck to this method. In this study, the addition of calcined pyrite to the S system (SCP) accelerated electron generation and intra/extracellular transfer efficiency, thereby improving ACE and denitrification performance. The highest nitrate removal rate reached to 3.55 ± 0.01 mg N/L/h in SCP system, and the ACE was 103 % higher than that in S system. More importantly, calcined pyrite enhanced the enrichment of functional bacteria (Burkholderiales, Thiomonas and Sulfurovum) and functional genes which related to sulfur metabolism and electron transfer. This study was more effective in removing nitrate from groundwater without compromising the water quality.

Simultaneous aerobic nitrogen and phosphorus removal by novel halotolerant fungus Mucor circinelloides SNDM1: Function and metabolism pathway

Fungi capable of simultaneous nitrogen and phosphorus removal from wastewater is rarely found. Here, a novel fungal strain (SNDM1) performing heterotrophic nitrification, aerobic denitrification, and phosphate removal was isolated and identified as Mucor circinelloides. The favorable nutrient removal conditions by the strain using glucose were C/N ratios of 25–30, salinities of 0 %–3 %, and pH of 7.5. Strain SNDM1 achieved ammonium, nitrite, nitrate, and phosphate removal rates of 5.23, 10.08, 4.88, and 0.97 mg/L/h. Nitrogen balance indicated that gaseous (18.60 %–24.55 %) and intracellular nitrogen (43.76 %–70.63 %) were primary fate of initial nitrogen. Enzyme activity revealed that ammonium removal occurred through heterotrophic nitrification and aerobic denitrification. Removed phosphorus was mainly transformed into cell membranes (56 %–64 %) and extracellular polymeric substances (20 %–26 %). Orthophosphate was the major intracellular phosphorus species, while polyphosphate and pyrophosphate existed extracellularly. These findings highlight the potential of this fungal strain for bioremediating polluted wastewater.

A novel coupling process to replace the traditional multi-stage anammox process-sulfur autotrophic denitrification coupled anammox system.

A novel coupling process to replace the traditional multi-stage anammox process-sulfur autotrophic denitrification (SAD) coupled anaerobic ammonium oxidation (anammox) system was designed, which solved problems of nitrate produced in anammox process and low nitrate conversion rate caused by nitrite accumulation in SAD process. Different filter structures (SAD filter and anammox granular sludge) were investigated to further explore the excellent performance of the novel integrated reactor. The results of sequential batch experiments indicated that nitrite accumulation occurred during SAD, which inhibited the conversion of nitrate to dinitrogen gas. When SAD filter and anammox granular sludge were added to packed bed reactor simultaneously, the nitrate removal rate increased by 37.21% and effluent nitrite concentration decreased by 100% compared to that achieved using SAD. The stratified filter structure solved groove flow. Different proportion influence of SAD filter and anammox granular sludge on the stratified filter structure was evaluated. More suitable ratio of SAD filter to anammox granular sludge was 2:1. Proteobacteria (57.26%), Bacteroidetes (20.12%) and Chloroflexi (9.95%) were the main phyla. The dominant genera of denitrification functional bacteria were Thiobacillus (39.80%), Chlorobaculum (3.99%), norank_f_PHOs-HE36 (2.90%) and Ignavibacterium (2.64%). The dominant genus of anammox bacterium was Candidatus_Kuenenia (3.05%).

Hydro‐Biogeochemical Controls on Nitrate Removal: Insights From Artificial Emergent Vegetation Experiments in a Recirculating Flume Mesocosm

AbstractEnvironments with aquatic vegetation can mitigate excess nitrogen (N) loads to downstream waters. However, complex interactions between multiple hydro‐biogeochemical processes control N removal within these environments and thus complicate implementation of aquatic vegetation as a management solution. Here, we conducted controlled experiments using a canopy of artificial rigid emergent vegetation in a recirculating flume mesocosm to quantify differences in rates of mass transport and nitrate (NO3−N) removal between the open channel‐canopy interface across a range in nominal water velocities. We found NO3−N removal rates were 86% greater with the canopy present compared to no canopy control experiments and were always greatest at intermediate velocity (6 cms−1). With the canopy present, a hydrodynamically distinct mixing layer formed at the open channel‐canopy interface, and resources, such as carbon (C), CN ratios, and dissolved oxygen, differed between open channel and vegetated canopy. The dimensionless Damköhler (Da) number indicated NO3−N removal rates were reaction limited (Da &lt;&lt; 1) for all canopy experiments, yet across all velocities NO3−N removal was more reaction limited in the open channel than the canopy due to higher rates of mixing and less contact time with reactive surfaces. We found significant relationships between NO3−N removal rates and Da with hydrodynamic metrics (mixing zone width and Reynolds number, respectively), suggesting that NO3−N removal in the presence of rigid vegetation can be enhanced by manipulating flow conditions. These findings demonstrate that rigid emergent vegetation‐open channel interfaces create conditions conducive for NO3−N removal and with effective management can improve overall water quality.

Sulfur-driven microbial fuel cells denitrification system for targeted treatment of nitrate-containing groundwater: Performance and mechanism

Sulfur (S0)-driven autotrophic denitrification system has obvious advantages in economy and performance in treating the low carbon/nitrogen groundwater. However, the by-products produced from S0 oxidation will cause disturbance to the water quality. In this study, a dual-chamber microbial fuel cell was driven by S0 (S0-MFC) to separate the S0 and nitrate, the electrons generated by the oxidation of S0 in the anodic chamber transferred to the cathode for nitrate reduction. Under the action of the anode microorganisms, the S0 was directly oxidized to sulfate. The maximum nitrate removal rate (NRR) in the cathodic chamber was 2.31 ± 0.03 mg N/L/h, and the anode electron utilization rate could reached to 19.69 %. Electrochemical results showed that flavin and c-type cytochrome could acted as electron mediators promote the electron transfer processes. The presence of loop current contributed to the enhancement of microbial metabolic activity, thus improving the stability of sludge and NRR. The denitrifiers (Thiobacillus and Denitratisoma) and sulfur autotrophs (Desulfocapsa and Acidithiobacillus) played an important role in the high NRR. A low-cost and pollution-free method for removing nitrate from groundwater was realized in this study.

Effect of humic acid on aerobic denitrification by Achromobacter sp. strain GAD-3

Humic acid (HA), a common natural organic matter, could affect conventional anoxic denitrification. Aim of this study was to investigate effect of HA on the process of aerobic denitrification in Achromobacter sp. GAD-3, an aerobic denitrifying strain. The findings demonstrated that an increase in HA concentrations (≥5mgL-1) promoted the aerobic denitrification process (excluding N2O reduction), manifesting as higher rates of nitrate removal (6.67-11.1mgL-1h-1) and lower levels of nitrite accumulation (30.2-20.7mgL-1). This was attributed to the increased electron transfer activities and denitrifying reductase activities (including NAR, NIR and NOR) facilitated by HA. Accordingly, the expression of denitrification genes such as napA, cnorB, and nirS was enhanced by HA. Nonetheless, the nosZ gene and N2OR activity underwent suppression by HA, which was accountable for N2O emission. It is crucial to understand the HA mechanism towards aerobic denitrifiers for wastewater treatment plants to enhance nitrogen removal.

Anaerobic manganese oxidation coupled to denitrification by novel autotrophic microbial consortium

Human activities have more than doubled the pre-industrial supply of reactive nitrogen to the biosphere, while manganese is the fifth most abundant metal in the crust of Earth with main redox states of Mn(II) and Mn(IV), which could provide electrons for denitrification driving nitrogen cycling. Although few microorganisms performing nitrate-dependent anaerobic manganese oxidation (NDMO) were identified, their active metabolic pathways have not been confirmed. Here, a microbial consortium was enriched from the mixture of freshwater sediment and activated sludge with nitrate and dissolved divalent manganese. After 528 days of enrichment, the nitrate and Mn(II) removal rate was stable at 7.54 mg N L−1 d−1 and 19.60 mg Mn L−1 d−1, respectively. The mass balance showed that nitrate was reduced into N2, and Mn(II) was converted into manganese oxides. Three novel bacteria were uniquely identified in this microbial consortium: Candidatus Nitratireduceret manganoxidans (17 %) and Candidatus Nitratoreduceret manganoxydans (3.27 %) within the order of Anaerolineales, Candidatus Nitrooxidireductor manganoxidant (6.33 %) affiliated to the order of Phycisphaerales. Ca. N. manganoxidans and Ca. N. manganoxydans encode cotA and moxA for Mn(II) oxidation and denitrification gene for NO3- reduction to N2O, while genes in Ca. N. manganoxidant, moxA, and nosZ only encoded manganese oxidase and nitrous-oxide reductase, which convert N2O further to N2. The two orders of bacteria cooperated in completing a NDMO process, which could be a profound link between nitrogen and manganese cycling in environments that should be investigated in future research. SynopsisThree novel bacteria cooperate to perform autotrophic denitrification coupled with manganese oxidation, and metagenomic analysis revealed their metabolic pathway.

Effects of Weak Electric Fields on the Denitrification Performance of Pseudomonas stutzeri: Insights into Enzymes and Metabolic Pathways.

Enhanced denitrification has been reported under weak electric fields. However, it is difficult to investigate the mechanism of enhanced denitrification due to the complex interspecific interactions of mixed-culture systems. In this study, Pseudomonas stutzeri, capable of denitrification under anaerobic conditions, was selected for treating low COD/N (2.0, ratio between concentration of chemical oxygen demand and NO3--N) artificial wastewater under constant external voltages of 0.2, 0.4, and 0.6 V. The results revealed that P. stutzeri exhibited the highest efficiency in nitrate reduction at 0.2 V. Moreover, the maximum nitrate removal rate was 15.96 mg/(L·h) among the closed-circuit groups, 19.39% higher than that under the open-circuit group. Additionally, a notable reduction in nitrite accumulation was observed under weak electric fields. Enzyme activity analysis showed that the nitrate reductase activities were significantly increased among the closed-circuit groups, while nitrite reductase activities were inhibited. Transcriptomic analysis indicated that amino acid metabolism, carbohydrate metabolism, and energy metabolism were increased, enhancing the resistance of P. stutzeri to environmental stress and the efficiency of carbon source utilization for denitrification. The current study examined the impacts of weak electric fields on enzyme activities and microbial metabolic pathways and offers valuable insights into the mechanism by which denitrification is enhanced by weak electric fields.