Enhancement of denitrification performance with reduction of nitrite accumulation and N2O emission by Shewanella oneidensis MR-1 in microbial denitrifying process

Synergetic effects of biochars and denitrifier on nitrate removal

Nitrate is one of the most common water contaminants and has caused severe environmental problems. This work aimed to investigate the effects of integration of denitrifier with biochars on nitrate removal and understand the underlying mechanisms. The results showed that physiochemical properties of biochars varied according to different feedstocks, which influenced bacteria attachment and nitrate removal through adsorption. However, bacteria could colonize on biochars no matter biochars surface were favorable for bacteria attachment or not. Immobilization of denitrifier on biochars significantly improved nitrate removal efficiencies and reduced lag time. Underlying mechanisms investigation showed that the integration of denitrifier with biochars had synergetic effects on promoting nitrate removal, which improved not only the expression and activity of nitrate reductase, but also the electron transport system activity.

High redox potential promotes oxidation of pyrite under neutral conditions: Implications for optimizing pyrite autotrophic denitrification

Pyrite autotrophic denitrification (PAD) represents an important natural attenuation process of nitrate pollution and plays a pivotal role in linking nitrogen, sulfur, and iron cycles in a variety of anoxic environments. However, there are knowledge gaps about the oxidation mechanism of pyrite under anaerobic neutral conditions. This study explored the performance of PAD in the presence of EDTA and revealed the mechanism of anaerobic pyrite oxidation and microbial mineral transformation. It was demonstrated that ~200 mV was the electrochemical threshold for converting pyrite into bioavailable forms in PAD conditions, and accelerated pyrite oxidation by Fe³⁺-EDTA complexes can improve the performance of PAD effectively. Furthermore, genus related to sulfur and nitrogen cycle (Sulfurimonas, Denitrobacter) were found at higher abundances in cultures containing EDTA. The analysis of metagenomic binning showed that the microbial community in PAD culture with EDTA addition exhibited higher levels of functional diversity and redundancy. These results will further the understanding of the oxidation mechanism of pyrite under anaerobic neutral conditions and the corresponding microbial activities, and provide insights into the practical application of PAD.

Insights into heterotrophic denitrification diversity in wastewater treatment systems: Progress and future prospects based on different carbon sources

Nitrate, as the most stable form of nitrogen pollution, widely exists in aquatic environment, which has great potential threat to ecological environment and human health. Heterotrophic denitrification, as the most economical and effective method to treat nitrate wastewater, has been widely and deeply studied. From the perspective of heterotrophic denitrification, this review discusses nitrate removal in the aquatic environment, and the behaviors of different carbon source types were classified and summarized to explain the cyclical evolution of carbon and nitrogen in global biochemical processes. In addition, the denitrification process, electron transfer as well as denitrifying and hydrolyzing microorganisms among different carbon sources were analyzed and compared, and the commonness and characteristics of the denitrification process with various carbon sources were revealed. This study provides theoretical support and technical guidance for further improvement of denitrification technologies.

Nitrogen species control the interaction between NO3--N reduction and aniline degradation and microbial community structure in the oxic-anoxic transition zone

Contrary to the fact that NO₃^--N can serve as electron acceptor to promote organics degradation, it was also found NO₃^--N reduction does not necessarily promote organics degradation. We speculate nitrogen (N) species may control the interaction between NO₃^--N reduction and organics degradation via shifting related microbial community structure. To prove the hypothesis, oxic-anoxic transition zone (OATZ) microcosms simulated by lake water and sediment were conducted with the addition of N species (NO₃^--N, NO₂^--N, and NH₄⁺-N) and aniline as typical organics. High-throughput sequencing was used to analyze the microbial community structure and functional enzyme in the microcosms. Results show that, NO₂^--N inhibited NO₃^--N reduction while enhanced aniline degradation. For NH₄⁺-N, it promoted NO₃^--N reduction when NH₄⁺-N/NO₃^--N concentration ratio ≤ 2 and inhibited aniline degradation when NH₄⁺-N/aniline concentration ratio ≥ 0.5. The presence of NO₂^--N or NH₄⁺-N weakened the interaction between NO₃^--N reduction and aniline degradation, which might be caused by significant changes in the diversity and abundance of microbial communities controlled by N species. The microbial mechanism indicates that NO₂^--N weakened the interaction by affecting both denitrification enzyme activity and electron transfer capability, while NH₄⁺-N weakened the interaction mainly by affecting electron transfer capability. These results imply that N species, as well as other electron acceptors and donors, in the contaminated OATZ should be fully considered, when performing in situ remediation technology of NO₃^--N reduction.

Insights into chronic zinc oxide nanoparticle stress responses of biological nitrogen removal system with nitrous oxide emission and its recovery potential

The nitrogen transformation performances and greenhouse gas nitrous oxide (N₂O) emissions in a sequencing batch reactor under chronic exposure to zinc oxide nanoparticles (ZnO NPs) were quantified and the system's self-recovery potentials were assessed. ZnO NPs posed a dose-dependent depression effect on the removal efficiencies of ammonia nitrogen (NH₄⁺-N) and total nitrogen (TN), and the N₂O emissions. The suppressed N₂O emissions had a positive relationship with the activity ratios of nitrite/NO reductases and N₂O reductase, and were expected to be caused by the inhibited heterotrophic denitrification process. The inhibition of glucose metabolism key enzymes and electron transport chain activities would be responsible for the heterotrophic denitrification performances deterioration. Furthermore, the removal efficiencies of NH₄⁺-N and TN were recovered to control levels through the nitrite-shunt. However, the N₂O emission increased significantly above the control during the recovery period mainly due to the irreversibility of the depressed nitrite oxidation activities.

Anaerobic degradation of xenobiotic organic contaminants (XOCs): The role of electron flow and potential enhancing strategies

In groundwater, deep soil layer, sediment, the widespread of xenobiotic organic contaminants (XOCs) have been leading to the concern of human health and eco-environment safety, which calls for a better understanding on the fate and remediation of XOCs in anoxic matrices. In the absence of oxygen, bacteria utilize various oxidized substances, e.g. nitrate, sulphate, metallic (hydr)oxides, humic substance, as terminal electron acceptors (TEAs) to fuel anaerobic XOCs degradation. Although there have been increasing anaerobic biodegradation studies focusing on species identification, degrading pathways, community dynamics, systematic reviews on the underlying mechanism of anaerobic contaminants removal from the perspective of electron flow are limited. In this review, we provide the insight on anaerobic biodegradation from electrons aspect - electron production, transport, and consumption. The mechanism of the coupling between TEAs reduction and pollutants degradation is deconstructed in the level of community, pure culture, and cellular biochemistry. Hereby, relevant strategies to promote anaerobic biodegradation are proposed for guiding to an efficient XOCs bioremediation.

The nitrogen removal characterization of a cold-adapted bacterium: Bacillus simplex H-b

The removal efficacy of biological nitrogen removal process is inhibited by low temperatures. Herein, a psychrotrophic bacterium strain, Bacillus simplex H-b, was isolated and identified with the potential to conduct heterotrophic nitrification and aerobic denitrification in the temperature range from 5 to 37 °C. At 10 °C, the removal efficiencies of initial nitrate-N (63 mg/L), nitrite-N (10 mg/L) and ammonium-N (60 mg/L) were 67.29%, 78.69% and 82.16%, with the maximum removal rate of 0.56, 0.18 and 0.74 mg/L/h, respectively. Additionally, both the accumulation level of ATP (adenosine triphosphate) and the formation of extracellular polymeric substances was found to increase with the decrease of temperature from 37 °C to 10 °C, indicating strain H-b might resist low temperature stress through its cellular extreme environment resistant mechanism and further suggesting the newly isolated strain could serve as a promising candidate for nitrogen contaminated wastewater treatment, especially under low-temperature condition.

A fungus-bacterium co-culture synergistically promoted nitrogen removal by enhancing enzyme activity and electron transfer

The fungus Penicillium citrinum WXP-2 and the bacterium Citrobacter freundii WXP-9 were isolated and found to have poor denitrification performance. Surprisingly, co-culture of the two strains which formed fungus-bacterium pellets (FBPs) promoted the removal efficiency of nitrate (NO₃^--N; 95.78%) and total nitrogen (TN; 81.73%). Nitrogen balance analysis showed that excess degraded NO₃^--N was primarily converted to N₂ (77.53%). Moreover, co-culture increased the dry weight to 0.74 g/L. The diameter of pellets and cell viability also increased by 1.49 and 1.78 times, respectively, indicating that the co-culture exerted a synergistic effect to promote growth. The increase in electron-transmission system activity [99.01 mg iodonitrotetrazolium formazan/(g·L)] and nitrate reductase activity [8.65 mg N/(min·mg protein)] were responsible for denitrification promotion. The FBPs also exhibited the highest degradation rate at 2:1 inoculation ratio and 36 h delayed inoculation of strain WXP-9. Finally, recycling experiments of FBP demonstrated that the high steady TN removal rate could be maintained for five cycles.

Impacts of carbon-based nanomaterials on nutrient removal in constructed wetlands: Microbial community structure, enzyme activities, and metabolism process

The increasing use of raw carbon-based nanomaterials (CBNs) will inevitably affect wastewater treatment systems. Constructed wetlands (CWs) are ecological wastewater treatment facilities and can intercept the vast particles pollutant, including CBNs. However, the impacts of CBNs on the treatment performance of CWs have no available knowledge. Therefore, we systematically inspected the effects of single-walled and multi-walled carbon nanotubes (SWCNTs and MWCNTs) and fullerene nanoparticles (C₆₀) on CW performance under 180-day exposure to 0, 10 and 1000 μg/L concentrations. The results showed that CBNs had marginally adverse impacts on chemical oxygen demand (COD) and total phosphorus (TP) removal, whereas nitrogen removal declined by 24.1 %-42.7 % following long-term exposure to CBNs. MWCNTs had the greatest inhibition effect on nitrogen removal, followed by SWCNTs and C₆₀. The CBNs also induced reactive oxygen species (ROS) overproduction as the increasing concentration, which confirmed that CBNs have biotoxic effects in CWs. The variation of functional microbial community and the inhibition of enzyme activities were the dominant reasons for the decline in nitrogen removal efficiency. Furthermore, predictive functional profiling showed that CBNs affected functional gene abundance, and caused a decline in the enzymes abundance connected to nitrogen removal by the end of the 180-day exposure period.

Microbial community shift via black carbon: Insight into biological nitrogen removal from microbial assemblage and functional patterns

Understanding the ecological relationship of microbial community under external stimulation is crucial for environmental restoration. Black carbon (e.g., biochar) have been widely deemed as a strategy to enhance pollutants removal because of its structure and redox-active property. However, the underlying ecological mechanism of microbial community under black carbon addition is poorly understood. The major purposes of this study were to determine the microbial assemblage and functional patterns via null model test, network analysis, and function prediction. The results showed that the nitrate removal efficiency of modified black carbon system achieved 46.44%. Both deterministic and stochastic processes were significant for mediating the microbial assemblage and the deterministic process dominated (>95%) the assemblage of community. Keystone taxa in the black carbon systems, involving Sulfuricella, Allorhizobium, and Nitrospira, stimulated the shift of community composition regarding the nitrogen removal. The existence of black carbon and the biotic interactions increased biological nitrate utilization and promoted nitrogen removal. Overall, this study presents the mechanism of bacterial community assembly and provides insight into biological nitrogen removal from microbial ecological perspective.

Raney nickel coupled nascent hydrogen as a novel strategy for enhanced reduction of nitrate and nitrite

Raney nickel (R-Ni) is a cost-effective hydrogenation catalyst, and nascent hydrogen (Nas-H₂) generated in situ on the cathode trends to more reactive than commercial hydrogen (Com-H₂). In the present work, nitrate and nitrite (NO_X^-) reduction via R-Ni/Nas-H₂ catalytic system was investigated. The results show that hydrogenation of NO_X^- (C₀ = 3.0 mM) follows pseudo-first-order reaction kinetics with kinetic constants of 5.18 × 10^-2 min^-1 (NO₃^-) and 6.46 × 10^-2 min^-1 (NO₂^-). The saturation demand for Nas-H₂ is only 0.8 mL min^-1 at a fixed R-Ni dosage of 1.0 g L^-1. The experiments reveal that both Nas-H₂ and hydrogen adatoms (H_ads∗) can drive the reduction of NO_X^-. The improved reduction ratios of NO_X^- are attributed to two aspects: (1) the micro/nano-sized Nas-H₂ bubbles exhibits increased reactivity due to the fine dispersion of the hydrogen molecules; (2) the alkaline environment formed by the cathode positively maintain R-Ni activity, thus, Nas-H₂ bubbles were more readily activated to generate powerful H_ads∗. The results give insight into NO_X^- hydrogenation via introducing fine hydrogen resource, and can develop an efficient catalytic hydrogenation technique without noble metals.

Efficient nitrate removal by Pseudomonas mendocina GL6 immobilized on biochar

The performance of nitrate removal by Pseudomonas mendocina GL6 cells immobilized on bamboo biochar was investigated. The results showed that immobilized bacterial cells performed better nitrate removal than the free bacterial cells, and the nitrate removal rate increased from 6.51 mg/(L·h) of free cells to 8.34 mg/(L·h) of immobilized cells. The nitrate removal of immobilized bacterial cells fitted well to the zero-order kinetics model. Moreover, bath experiments showed that immobilized bacterial cells displayed more nitrate removal capacity under different conditions than free bacterial cells due to the protection of biochar carrier. The subsequent mechanistic study suggested that biochar promoted the expression level of denitrification functional genes (napA and nirK) and electron transfer genes involved in denitrification (napB and napC), which resulted in the increase of nitrate removal efficiency. Thus, biochar-immobilized P. mendocina GL6 has much potential to remove nitrate from wastewater via aerobic denitrification.

Temperature and oxygen level determine N2 O respiration activities of heterotrophic N2 O-reducing bacteria: Biokinetic study

Nitrous oxide (N₂ O), a potent greenhouse gas, is reduced to N₂ gas by N₂ O-reducing bacteria (N₂ ORB), a process which represents an N₂ O sink in natural and engineered ecosystems. The N₂ O sink activity by N₂ ORB depends on temperature and O₂ exposure, yet the specifics are not yet understood. This study explores the effects of temperature and oxygen exposure on biokinetics of pure culture N₂ ORB. Four N₂ ORB, representing either clade I type nosZ (Pseudomonas stutzeri JCM5965 and Paracoccus denitrificans NBRC102528) or clade II type nosZ (Azospira sp. strains I09 and I13), were individually tested. The higher activation energy for N₂ O by Azospira sp. strain I13 (114.0 ± 22.6 kJ mol^-1 ) compared with the other tested N₂ ORB (38.3-60.1 kJ mol^-1 ) indicates that N₂ ORB can adapt to different temperatures. The O₂ inhibition constants (K_I ) of Azospira sp. strain I09 and Ps. stutzeri JCM5965 increased from 0.06 ± 0.05 and 0.05 ± 0.02 μmol L^-1 to 0.92 ± 0.24 and 0.84 ± 0.31 μmol L^-1 , respectively, as the temperature increased from 15°C to 35°C, while that of Azospira sp. strain I13 was temperature-independent (p = 0.106). Within the range of temperatures examined, Azospira sp. strain I13 had a faster recovery after O₂ exposure compared with Azospira sp. strain I09 and Ps. stutzeri JCM5965 (p < 0.05). These results suggest that temperature and O₂ exposure result in the growth of ecophysiologically distinct N₂ ORB as N₂ O sinks. This knowledge can help develop a suitable N₂ O mitigation strategy according to the physiologies of the predominant N₂ ORB.

Aerobic denitrifying bacterial communities drive nitrate removal: Performance, metabolic activity, dynamics and interactions of core species

Three novel mix-cultured aerobic denitrifying bacteria (Mix-CADB) consortia named D14, X21, and CL exhibited excellent total organic carbon (TOC) removal and aerobic denitrification capacities. The TOC and nitrate removal efficiencies were higher than 93.00% and 98.00%. The results of Biolog demonstrated that three communities displayed high carbon metabolic activity. nirS gene sequencing and ecological network model revealed that Pseudomonas stutzeri, Paracoccus sp., and Paracoccus denitrificans dominated in the D14, X21, and CL communities. The dynamics and co-existence of core species in communities drove the nutrient removal. Response surface methodology showed the predicted total nitrogen removal efficiency reached 99.43% for D14 community. The three Mix-CADB consortia have great potential for nitrogen-polluted aquatic water treatment because of their strong adaptability and removal performance. These results will provide new understanding of co-existence, interaction and dynamics of Mix-CADB consortia for nitrogen removal in nitrogen-polluted aquatic ecosystems.

Effect of heterotrophic anodic denitrification on anolyte pH control and bioelectricity generation enhancement of bufferless microbial fuel cells

Heterotrophic anodic denitrification (HAD) in the single-chamber microbial fuel cell (MFC) is a promising nitrogen removal technology. In this paper, the benefit (anolyte pH increase) and challenge (substrate consumption) brought by the heterotrophic anodic denitrification process for the electricity generation of bufferless MFCs were studied for the first time. Substrate anaerobic hydrolysis dramatically decreased the anolyte pH to 5.1, which seriously restricted the electric power output of the Control. The anolyte pH of the heterotrophic anodic denitrification MFCs (HADMFCs) with 60 mg/L (HADMFC-60), 90 mg/L (HADMFC-90), and 120 mg/L (HADMFC-120) nitrate nitrogen (NO₃^--N), retained above 6.0, 6.5, and 6.8 in every running cycles, due to the protons (H⁺) consumption by nitrate reduction. In the HADMFC-60 and HADMFC-90, 17.6% and 26.1% of the total organic carbons (TOC) were used for the nitrate reduction, but their electric power output significantly increased. The maximum power densities of the HADMFC-60 and HADMFC-90 were 3.3 and 5.4 times higher than that of the Control. However, when the proportion of TOC consumption for nitrate reduction increased to 35.8%, substrate insufficiency became a serious limitation for the electricity generation. The P_max of the HADMFC-120 dramatically decreased to 17.3 mW/m². Dysgonomonas was the dominant electro-active genus, and Petrimonas, Acidovorax and Devosia appeared as the denitrifying bacteria genera.

Effect of the C/N Ratio on Biodegradation of Ciprofloxacin and Denitrification from Low C/N Wastewater as Assessed by a Novel 3D-BER System

Emerging pollutants in the form of pharmaceuticals have drawn international attention during the past few decades. Ciprofloxacin (CIP) is a common drug widely found in effluents from hospitals, industrial and different wastewater treatment plants, as well as rivers. In this work, the lab-scale 3D-BER system was established, and more than 90% of the antibiotic CIP was removed from Low C/N wastewater. The best results were obtained with the current intensity being taken into account, and a different C/N ratio significantly improved the removal of CIP and nitrates when the ideal conditions were C/N = 1.5–3.5, pH = 7.0–7.5 and I = 60 mA. The highest removal efficiency occurred when CIP = 94.2%, NO3−-N = 95.5% and total nitrogen (TN) = 84.3%, respectively. In this novel system, the autotrophic-heterotrophic denitrifying bacteria played a vital role in the removal of CIP and an enhanced denitrification process. Thus, autotrophic denitrifying bacteria uses CO2 and H2 as carbon sources to reduce nitrates to N2. This system has the assortment and prosperous community revealed at the current intensity of 60 mA, and the analysis of bacterial community structure in effluent samples fluctuates under different conditions of C/N ratios. Based on the results of LC-MS/MS analysis, the intermediate products were proposed after efficient biodegradation of CIP. The microbial community on biodegrading was mostly found at phylum, and the class level was dominantly responsible for the NO3−-N and biodegradation of CIP. This work can provide some new insights towards the biodegradation of CIP and the efficient removal of nitrates from low C/N wastewater treatment through the novel 3D-BER system.

Nitrate reduction by Paracoccus thiophilus strain LSL 251 under aerobic condition: Performance and intracellular central carbon flux pathways

The intracellular carbon metabolic flux pathways of denitrifying bacteria under aerobic conditions remain unclear. Here, a newly strain LSL251 was identified as Paracoccus thiophilus. Strain LSL251 removed 94.79% and 98.78% of total organic carbon and nitrate. 74.66% of nitrogen in culture system was lost as gaseous nitrogen. Moreover, ¹³C stable isotopic labeling and metabolic flux analyses revealed that the primary intracellular carbon metabolic pathways were the Entner-Doudoroff pathway and the tricarboxylic acid (TCA) cycle. Electrons are primarily donated as direct electron donor-NADH through the TCA cycle. Furthermore, response surface methodology modeled that the highest total nitrogen removal efficiency was 98.43%, where the optimal parameters were C/N ratio of 8.00, 32.98 °C, 50.18 rpm, and initial pH of 7.73. All together, these results have shed new lights on intracellular central carbon metabolic distribution and flux pathways of aerobic denitrifying bacteria.

The Structure and Diversity of Nitrogen Functional Groups from Different Cropping Systems in Yellow River Delta

The Yellow River Delta (YRD) region is an important production base in Shandong Province. It encompasses an array of diversified crop systems, including the corn-wheat rotation system (Wheat-Corn), soybean-corn rotation system (Soybean-Corn), fruits or vegetables system (Fruit), cotton system (Cotton) and rice system (Rice). In this study, the communities of ammonia oxidizer-, denitrifier- and nitrogen (N)-fixing bacteria in those cropping systems were investigated by Illumina Miseq sequencing. We found that Rice soil exhibited significantly higher diversity indices of investigated N-cycling microbial communities than other crop soils, possibly due to its high soil water content. Wheat-Corn soils had higher abundances of nitrification gene amoA and denitrification genes nirK and nirS, and exhibited higher soil potential nitrification rate (PNR), compared with Soybean-Corn, Cotton and Fruit soils. Consistently, redundancy analysis (RDA) showed that soil water content (SWC), electrical conductivity (EC), and total nitrogen (TN) were the most important influencing factors of the diversity and structure of the investigated N-cycling microbial.