Tetrabromobisphenol A (TBBPA) inhibits denitrification via regulating carbon metabolism to decrease electron donation and bacterial population

Hydrolytic Dehalogenation of Toxic Haloacetic Acids via Carbon Metabolism Regulation during Microbial Denitrification

Microbial denitrification is essential in the nitrogen cycle to enhance the quality of the reclaimed water. In addition to nitrogen removal, it has the potential to control trace pollutants. However, the fates of toxic disinfection byproducts (DBPs) on denitrification remain unelucidated. The current study focused on Paracoccus denitrificans (P. denitrificans) to investigate the response mechanisms of denitrifying microorganisms to HAAs, one of the main categories of DBPs. Upon exposure to 20 μM monoiodoacetic acid (MIAA), the number of extracellular reactive oxygen species in P. denitrificans increased to 2.7 times at 16 h. Concurrently, the specific nitrate reduction rate dropped by 9.3% and the specific growth rate declined by 26.7%, leading to the slowdown of the denitrification process. Nevertheless, P. denitrificans increased the activity of the tricarboxylic acid cycle and electron transport for sustainable denitrification under MIAA stress. Microbial hydrolytic dehalogenation contributed to over 70.0% MIAA removal, and it led to the release of iodine ions. MIAA was detoxified and concerted into low-molecular-weight organic acids, which then participated in carbon metabolism. The removal efficiency of different toxic HAAs was also compared to evaluate the adaptiveness of the DBP control. This research highlighted the interactions between denitrifying microorganisms and DBPs, providing new insights into the ecological safety protection of high-quality reclaimed water.

Impacts of environmentally persistent free radicals on the denitrification toxicity of photoaged tire wear particles in estuarine sediments

The widespread detection of tire wear particles (TWPs) in estuaries has raised concerns about their potential environmental hazards. However, knowledge of photoaging-induced environmentally persistent free radicals (EPFRs) formation on TWPs in estuarine environments and their impact on sediment denitrification remains limited. This study investigated the formation of EPFRs on TWP during photoaging in estuarine environments and evaluated their effects on sediment denitrification and nitrous oxide (N2O) accumulation. Sixty days of illumination increased EPFR concentration on TWPs by 373 %, with the generated EPFRs persisting in sediments for over 20 days. Exposure to pristine TWP (PTWP) reduced denitrification rates by 10.3 ± 5.6 % and increased N2O accumulation by 18.3 ± 4.5 %. Further exposure to photoaged TWP (ATWP) under 10-60 days of illumination expanded denitrification suppression and N2O accumulation to 28.1 ± 7.1-42.5 ± 6.6 % and 18.8 ± 4.3-31.7 ± 4.6 %, respectively. EPFRs exacerbated the accumulation of reactive nitrogen species in sediment and compromised the antioxidant systems. Structural equation modeling confirmed that EPFRs indirectly suppressed denitrification rates by directly impairing microbial processes involved in carbon metabolism and electron transfer. This study is the first to report that the formation of EPFRs enhances the negative effects of ATWP on the sediment's nitrogen cycle, offering valuable insights for assessing the ecological risks associated with TWP.

Using sludge biochar@waste pellet ore@kelp extract to enhance Feammox and removal of heavy metals: Performance evaluation and possible metabolic mechanisms

The co-occurrence of nitrogen (N) and heavy metals (HMs) in industrial wastewater has emerged as a critical environmental challenge. Feammox-mediated N removal remains constrained by limited iron (Fe(III)) bioavailability and microbial susceptibility to HMs toxicity. This study developed a multifunctional composite through strategic integration of sludge-based biochar (SBC), pellet ore (PO), and kelp extract (KE) to overcome these limitations. The optimized bioreactor configuration, leveraging synergistic interactions among three distinct waste-derived resources, achieved exceptional removal efficiencies of 90.1, 96.4, and 93.4% for NH4+, NO3-, and COD, respectively. Immobilization mechanisms of HMs involved bio-Fe precipitate adsorption and Fe-EPS-HMs chelation. Microbial community analysis demonstrated selective enrichment of Fe-cycling genera (Geothrix, Zoogloea, and Aquabacterium) and metal-resistant species (Cloacibacterium, Paenibacillus). These advancements establish a sustainable paradigm for industrial wastewater management, while simultaneously addressing critical challenges of resource recovery and waste valorization within circular economy frameworks.

Regulation of microcurrent on carbon and nitrogen metabolism in denitrification under low carbon-to-nitrogen ratio: Optimizing carbon flux distribution

Synergy of autotrophic and heterotrophic denitrification can achieve low-carbon and high-efficient nitrogen removal. However, it remains unclear how microcurrent-driven hydrogen autotrophic denitrification regulates carbon flux distribution (nitrogen reduction, poly-β-hydroxyalkanoate (PHA) storage, and cell growth) in heterotrophic denitrification. This work compared biofilm reactor with biofilm electrode reactor under different carbon-to-nitrogen (C/N) ratios (10 - 3). At C/N ratio of 3, microcurrent accelerated nitrate reduction rate by 0.35 mg/(L·min) and reduced nitrite accumulation by 10.29 mg/L, thus decreasing nitrogen reduction proportion by 11.21%. Meanwhile, PHA storage and cell growth proportions increased by 0.03% and 11.18%, respectively. PHA was initially synthesized and subsequently utilized for nicotinamide adenine dinucleotide and energy production, while cell growth preferentially utilized limited carbon sources to maintain system stability. Increased abundance of hydrogen autotrophic denitrifiers, heterotrophic denitrifiers, and PHA storage bacteria confirmed optimization of microcurrent on carbon flux distribution. These findings advanced the understanding of microcurrent regulation on carbon flux.

Co-metabolism degradation of tetrabromobisphenol A by the newly isolated Sphingobium sp. strain QY1-1: Multiple metabolic pathways, toxicity evaluation, and mechanisms

Tetrabromobisphenol A (TBBPA), a hydrophobic and persistent brominated flame retardant, has attracted considerable attention due to its potential ecotoxicity. Herein, a newly isolated Sphingobium sp. strain QY1-1 was employed to degrade TBBPA under optimized conditions determined by response surface methodology and kinetic analysis. Complete degradation of TBBPA was achieved by the fourth day under optimal conditions. Five main transformation pathways, i.e., debromination, hydroxylation, O-methylation, sulfation, and glycosylation, were proposed for TBBPA biodegradation based on 19 intermediates including two novel transformation products. The toxicity prediction of TBBPA and its degradation products suggested that the biodegradation of TBBPA by strain QY1-1 could effectively reduce its biotoxicity in aquatic environments. Moreover, transcriptomic analysis revealed significant up-regulation of multiple genes encoding oxidoreductases, lyases, free radical proteins, transporter proteins, and efflux transporters, particularly in the presence of glucose. This indicated that these functional enzymes could be involved in the transmembrane transport and catabolism of TBBPA and its by-products. Additionally, the overexpression of genes encoding chemotactic proteins and antioxidant-defense-related enzymes implied that the addition of glucose could heighten the adaptability of strain QY1-1 to TBBPA stress. This study provides new insights into the biodegradation of TBBPA by Sphingobium sp. and potential strategies for its enhancement.

Synchronous vanadium bio-reduction/detoxification/recovery and nitrogen attenuation in a membrane aerated biofilm reactor

The presence of both pentavalent vanadium [V(Ⅴ)] and nitrogen in wastewaters from vanadium smelting poses significant environmental challenges. However, it remains little in the way of continuous flow biological reactor to concurrently eliminated V(Ⅴ) and nitrogen in wastewaters. Herein, membrane-aerated biofilm reactor (MABR) system was designed to achieve simultaneous nitrification and denitrification (SND) alongside biological reduction, detoxification, and recovery of vanadium. Vanadium and nitrogen removal performances, solid-state characterization, microbial compositions and functional genes, and the mechanism related to the metabolism of vanadium and nitrogen were illuminated. Notably, we identified a potential role for biofilm-derived "secretion" in the transformation of V(Ⅴ) and nitrogen. Our findings revealed that the system achieved SND efficiency of 98.00 ± 0.57 % and removed 91.10 ± 3.60 % of total vanadium (TV) even at high influent V(Ⅴ) concentrations in continuous flow stage. Batch experiments implied that the conversion of NH4+-N was the limiting process of nitrogen removal in MABR system, and the extracellular polymeric substances (EPS) might play an important role in the conversion of V(Ⅴ) and nitrogen. V(Ⅴ) was reduced to V(Ⅳ), which was immobilized to biofilm and "secretion" by microbial surface functional groups, including C-O, O-C=O and -OH. Acinetobacter, Dechlorobacter, Denitratisoma and Nitrospira were verified as microbes associated with V(Ⅴ) and nitrogen metabolism. The abundance of functional genes pertaining to electron donor, electron transport, and electron acceptor systems increased under high V(V) stimulation. Collectively, the cooperation of biofilm and "secretion" ensured the intensive removal of vanadium and nitrogen. This study provides new insights into the concurrent removal of heavy metal and environmental nutrient.

Cumulative inhibitory effect of 6:2 chlorinated polyfluorooctane ether sulfonate in anaerobic digestion processes

Microbial anaerobic metabolism is crucial for biogeochemical cycles, impacting both natural and engineered ecosystems. However, the increasing emissions of 6:2 Cl-PFESA, an alternative to PFOS, pose significant risks. In this study, long-term high 6:2 Cl-PFESA concentration level of 10 µg/g TS exposure led to a substantial decrease in methane production from 204.8 ± 4.8 mL/g VS to 143.6 ± 3.5 mL/g VS, indicating a cumulative inhibitory effect on carbohydrate-related anaerobic digestion. Key processes such as polysaccharide release, hydrolysis, acetogenesis, and acetoclastic methanogenesis were contributed by 28.6 %, 9.3 %, 8.9 %, and 11.7 % of significant reduction, respectively, correlating with inhibition in relevant enzymatic activities and gene expressions. Hydrolytic bacteria such as Rectinema and Defluviitoga declined to 11.7 % from 14.3 % and 20.9 % from 23.9 %, reflecting decreased hydrolysis efficiency. Reduced transcription levels of acetogenesis- and acidogenesis-related genes further inhibited these processes. Conversely, methanogens Methanolinea and Methanothrix increased from 35.8 % to 55.7 % and 10.9-40.8 %, suggesting enzyme inhibition rather than methanogen abundance reduction. Additionally, 6:2 Cl-PFESA partially biotransformed into 6:2H-PFESA, facilitated by species like Dechloromonas, unclassified Xanthomonadales, and Betaproteobacteria. These findings confirm that the limited degradation and cumulative inhibitory effects of 6:2 Cl-PFESA during anaerobic digestion highlight its significant disruption to carbon cycling stability within ecosystems.

Multi-omics reveals mechanism of hydroxylamine-enhanced ultimate nitrogen removal in pilot-scale anaerobic/aerobic/anoxic system

Hydroxylamine (HA) dosing is an effective strategy for promoting partial nitrification (PN); however, its impact on endogenous denitrification remains underexplored. In this study, long-term continuous HA dosing (1.4 mg/L) was introduced for over 110 days in a pilot-scale anaerobic/aerobic/anoxic (AOA) system treating municipal wastewater (66.7-75 m3/d). The HA dosing significantly increased the nitrite accumulation ratio to 67.6 ± 5.0 % (p<0.001) and reduced the effluent total inorganic nitrogen concentration from 6.2 ± 2.0 to 2.4 ± 1.1 mg/L (p<0.001), achieving a nitrogen removal efficiency of 87.4 ± 4.5 % (p<0.001) at a hydraulic retention time of 8 h. During the HA dosing, aerobic nitrogen removal contribution increased from 2.4 ± 3.4 % to 25.8 ± 8.1 % (p<0.001), and the anoxic nitrogen removal rate improved from 1.63 ± 0.11 to 2.35 ± 0.13 mg N/(L·h) (p<0.001). Enhanced nitrogen removal was not only achieved through the rapid establishment of PN but also driven by the long-term impact of HA dosing on microbial community dynamics. Multi-omics analyses revealed that HA disrupted the polyphosphate (poly-P) cycle, evidenced by enhanced transcription of ppx (poly-P degradation) and suppressed ppk (poly-P synthesis), thereby reducing energy availability for phosphate-accumulating organisms (PAOs) and shifting the carbon source competition toward glycogen-accumulating organisms (GAOs), with Ca. Competibacter abundance increased from 0.16 % to 1.13 % (p < 0.001). The economic analysis demonstrated that HA reduced sludge production by 11.2 % and saved operating costs by 31.4-42.8 % compared to conventional carbon sources. These findings highlight the potential of HA dosing to achieve sustainable and highly efficient wastewater treatment.

New Insight Into the Mechanism of Nitrite Enhancement on Heterotrophic Nitrification and Aerobic Denitrification Bacterium in Gene Expression

The growth and nitrogen metabolism of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria are affected by nitrite, but the mechanisms underlying this for strain Acinetobacter johnsonii EN-J1 are unclear. In this study, the addition of 10 mg/L nitrite increased the reduction rate of ammonium by 1.0 mg/L/h, and 20 mg/L nitrite increased the reduction rate of nitrate by 3.9 mg/L/h. Compared with the control, the nitrate reductase activity, electron transfer activity, and adenosine triphosphate content of EN-J1 were enhanced by 142.0%, 278.0% and 279.0%, respectively, in the nitrate removal process after the addition of 20 mg/L nitrite. The whole genome was annotated with nitrogen removal genes such as narGHI, narK, nsrR, nirBD, nasA, glnA, gltB, gdhA and amt. Transcriptome analysis showed that nitrite triggered significant upregulation of several key pathways, including nitrogen metabolism, the tricarboxylic acid cycle, and amino acid metabolism for enhancing denitrification. The expression of key denitrification genes (narG, narK, hmp, nirBD, glnA and nasA) was detected by real-time quantitative polymerase chain reaction. These results suggested that nitrite enhances denitrification by increasing the expression of denitrification genes, electron transfer and adenosine triphosphate levels, which is important for elucidating the mechanism of nitrite promotion of biological nitrogen removal efficiency.

Simultaneous deep removal of nitrate and tetrabromobisphenol A in microbial electrochemical system-constructed wetland

Microbial electrochemical system-constructed wetland (MES-CW) is environmentally friendly in removing organic contaminants from wastewater. Tetrabromobisphenol A (TBBPA) is an emerging pollutant that is inefficiently removed in conventional wastewater treatment. The feasibility of deep removal of TBBPA and nitrate and the related mechanism in MES-CW remains unclear. This study demonstrated the enhanced TBBPA detoxification in MES-CW accompanied by nitrate removal. Nitrate significantly suppressed the TBBPA reductive debromination and methane generation. It altered the microbial community and enriched Acinetobacter in the electrode, stimulating the TBBPA hydrolytic debromination and metabolite oxidation. The biocathode supplied electrons for dehalogenators in TBBPA reductive debromination, while the anode served as the electron acceptor for function bacteria in TBBPA metabolite oxidation. Nitrate and anodic electricity optimized the microbial community and provided electron acceptors for TBBPA metabolites oxidation in MES-CW, guiding the deep removal of nitrate and emerging pollutants in wastewater.

Genomic synergistic efficient carbon fixation and nitrogen removal induced by excessive inorganic carbon in the anammox-centered coupling system

Given the significance of HCO3- for autotrophic anammox bacteria (AnAOB), excessive HCO3- was always provided in anammox-related systems and engineering applications. However, its impact mechanism on anammox process at genome-level remains unknown. This study firstly established an anammox-centered coupling system that entails heterotrophic partial denitrification (PD) and hydrolytic acidification (A-PDHA) fed mainly with inorganic carbon (high HCO3- concentration and low C/N ratio). Metagenomic binning and metatranscriptomics analyses indicated that high HCO3- concentration enhanced expression of natural most efficient phosphoenolpyruvate (PEP) carboxylase within AnAOB, by up to 30.59 folds. This further induced AnAOB to achieve high-speed carbon-fixing reaction through cross-feeding of phosphate and PEP precursors with heterotrophs. Additionally, the enhanced activity of transporters and catalytic enzymes (up to 4949-fold) induced by low C/N ratio enabled heterotrophs to eliminate extracellular accumulated energy precursors mainly derived from carbon fixation products of AnAOB. This maintained high-speed carbon-fixing reaction within AnAOB and supplemented heterotrophs with organics. Moreover, assimilated energy precursors stimulated nitrogen metabolism enzymes, especially NO2- reductase (968.14 times), in heterotrophs. This established an energy-saving PD-A process mediated by interspecies NO shuttle. These variation resulted in efficient nitrogen removal (>95 %) and reduced external organic carbon demand (67 %) in A-PDHA system. This study unveils the great potential of an anammox-centered autotrophic-heterotrophic coupling system for achieving cost-effective nitrogen removal and enhancing carbon fixation under excessive HCO3- doses.

Potential effects of Cu2+ stress on nitrogen removal performance, microbial characteristics, and metabolism pathways of biofilm reactor

Sequencing batch biofilm reactors (SBBR) were utilized to investigate the impact of Cu2+ on nitrogen (N) removal and microbial characteristics. The result indicated that the low concentration of Cu2+ (0.5 mg L-1) facilitated the removal of ammonia nitrogen (NH4+-N), total nitrogen (TN), nitrate nitrogen (NO3--N), and chemical oxygen demand (COD). In comparison to the average effluent concentration of the control group, the average effluent concentrations of NH4+-N, NO3--N, COD, and TN were found to decrease by 40.53%, 17.02%, 10.73%, and 15.86%, respectively. Conversely, the high concentration of Cu2+ (5 mg L-1) resulted in an increase of 94.27%, 55.47%, 22.22%, and 14.23% in the aforementioned parameters, compared to the control group. Low concentrations of Cu2+ increased the abundance of nitrifying bacteria (Rhodanobacter, unclassified-o-Sacharimonadales), denitrifying bacteria (Thermomonas, Comamonas), denitrification-associated genes (hao, nosZ, norC, nffA, nirB, nick, and nifD), and heavy-metal-resistant genes related to Cu2+ (pcoB, cutM, cutC, pcoA, copZ) to promote nitrification and denitrification. Conversely, high concentration Cu2+ hindered the interspecies relationship among denitrifying bacteria genera, nitrifying bacteria genera, and other genera, reducing denitrification and nitrification efficiency. Cu2+ involved in the N and tricarboxylic acid (TCA) cycles, as evidenced by changes in the abundance of key enzymes, such as (EC:1.7.99.1), (EC:1.7.2.4), and (EC:1.1.1.42), which initially increased and then decreased with varying concentrations of Cu2+. Conversely, the abundance of EC1.7.2.1, associated with the accumulation of nitrite nitrogen (NO2--N), gradually declined. These findings provided insights into the impact of Cu2+ on biological N removal.

Enhancement of Biological Nitrogen Removal System Resilience to Chronic Exposure of Zinc Oxide Nanoparticles by Quorum Sensing Modulation: Physiochemical, Microbial, and Metabolic Insights

The effects of three typical N-acyl-homoserine lactones (AHLs) on the tolerance of biological nitrogen removal (BNR) system to chronic exposure of zinc oxide nanoparticles (NPs) were investigated. C4-HSL successfully delayed the crash time of nitrogen removal performances in the NP-stressed system, while C6-HSL and C10-HSL maintained total nitrogen removal efficiencies throughout the 90-day NP exposure. All three AHLs increased NPs' contents captured in extracellular polymeric substances, alleviating membrane damage and preserving floc structure. The activities of tricarboxylic acid cycle-related enzymes and the relative abundances of BNR-related functional genes and genera were significantly enhanced. Besides, C6-HSL and C10-HSL augmented antioxidant enzyme activities and the abundances of functional genes and metabolites related to antioxidation, flagellar assembly, and chemotaxis, which synergistically reduced the reactive oxygen species' excessive accumulation. The tested AHLs effectively enhanced BNR systems' tolerance to chronic NP exposure, providing inspiration for quorum sensing applications in emerging contaminant removal.

Enhanced removal of tetrabromobisphenolA (TBBPA) from e-waste by Fe-S nanoparticles and Fe-S/CuS nanocomposite with response surface methodology (RSM)

TetrabromobisphenolA is a well-known member of the brominated flame retardant group and is widely used as a highly effective fire-retardant additive in electronic and electrical equipment. TBBPA is commonly found in various environmental sources and can be harmful to human health. This study presents a simple approach to preparing a magnetic nanocomposite, offering a straightforward method that results in consistent quality and low resource consumption. The nanocomposite has a high surface-to-volume ratio for the removal of tetrabromobisphenolA. Various characterization techniques, including XRD, FTIR, EDX, FESEM, VSM, TEM, and BET analyses were used to characterize the Fe-S nanoparticles and Fe-S/CuS. The results showed that Fe-S/CuS nanocomposite successfully removed over 97% of the initial TBBPA (15 mg L-1) under optimized conditions determined by RSM, such as a contact time of 15 min, pH of 7, Fe-S/CuS nanocomposite dosage of 0.69 g L-1, and salt concentration of 0.10%. The RSM analysis provided a second-order polynomial model with a confidence level of 93% (F = 29.58; p < 0.0001) to predict the TBBPA removal efficiency at various concentrations. In the adsorption kinetic studies, the second-order kinetic model provided the best fit for the experimental data. Additionally, Fe-S/CuS nanocomposite shows great potential for practical applications and environmental remediation efforts, making it a valuable asset for real-sample use in environmental settings.

How denitrifiers defense ciprofloxacin: Insights from intracellular and extracellular stress response

Overuse of antibiotics has led to their existence in nitrogen-containing water. The impacts of antibiotics on bio-denitrification and the metabolic response of denitrifiers to antibiotics are unclear. We systematically analyzed the effect of ciprofloxacin (CIP) on bio-denitrification and found that 5 mg/L CIP greatly inhibited denitrification with a model denitrifier (Paracoccus denitrificans). Nitrate reduction decreased by 32.89 % and nitrous oxide emission increased by 75.53 %. The balance analysis of carbon and nitrogen metabolism during denitrification showed that CIP exposure blocked electron transfer and reduced the flow of substrate metabolism used for denitrification. Proteomics results showed that CIP exposure induced denitrifiers to use the pentose phosphate pathway more for substrate metabolism. This caused a substrate preference to generate NADPH to prevent cellular damage rather than NADH for denitrification. Notably, despite denitrifiers having antioxidant defenses, they could not completely prevent oxidative damage caused by CIP exposure. The effect of CIP exposure on denitrifiers after removal of extracellular polymeric substances (EPS) demonstrated that EPS around denitrifiers formed a barrier against CIP. Fluorescence and infrared spectroscopy revealed that the binding effect of proteins in EPS to CIP prevented damage. This study shows that denitrifiers resist antibiotic stress through different intracellular and extracellular defense strategies.

Interspecies electron transfer and microbial interactions in a novel Fe(II)-mediated anammox coupled mixotrophic denitrification system

This study effectively coupled anammox and mixotrophic denitrification at a high nitrogen load rate of 6.84 g N/L/d with 40 mg/L Fe(II). Fe(II) enhanced the activity of nitrate reductase, nitrite reductase, and hydrazine dehydrogenase enzymes, facilitating accelerated ATP synthesis. Through electrochemical experiments, interspecies electron transfer processes in coupled system were explored. Fe(II) promoted flavin mononucleotide secretion, enhancing electron-donating and electron-accepting capacity by 2.8 and 1.3 times, respectively. Fe(II) triggered the enrichment of autotrophic denitrifying bacteria (Azospira and Hydrogenophaga), transitioning from single organic nutrient to mixotrophic denitrification. Meanwhile, Fe(II) increased Candidatus_Kuenenia abundance from 35.2 % to 49.0 %, establishing the competitive advantage of anammox bacteria over completed denitrifying bacteria (Comamonas). The synergistic interactions between anammox and various denitrification pathways achieved a nitrogen removal rate of 5.88 g N/L/d, with anammox contribution rate of 88.3 %. This study provides insights into broadening the application of partial denitrification /anammox and electron transfer in multi-bacterial coupling systems.

Effects of nitrate reduction on the biotransformation of 1H-1,2,4-triazole: Mechanism and community evolution

Due to the refractory of 1 H-1,2,4-triazole (TZ), conventional anaerobic biological treatment technology is usually restricted by low removal efficiency and poor system stability. In this study, TZ biodegradation and nitrate reduction was coupled to improve the removal efficiency of TZ from polluted wastewater. Batch assay was performed with pure culture strain Raoultella sp. NJUST42, which was reported to have the capability to degrade TZ in our previous study. Based on batch assay result, complete removal of TZ could be achieved in the presence of nitrate, whereas only 50% of TZ could be removed in the control system. Long-term stability experiment indicated that the relative abundance of microorganisms (Bacteroidetes_vadinHA17, Georgenia, Anaerolinea, etc) was obviously enhanced under nitrate reduction condition. During long-term period, major intermediates for TZ biodegradation such as [1,2,4]Triazolidine-3,5-diol, hydrazine dibasic carboxylic acid and carbamic acid were detected. A novel TZ biotransformation approach via hydration, TZ-ring cleavage, deamination and oxidation was speculated. PICRUSt1 and KEGG pathway analyses indicated that hydration (dch), oxidation (adhD, oah, pucG, fdhA) of TZ and nitrate reduction (Nar, napA, nrfA, nirBK, norB, nosZ) were significantly enhanced in the presence of nitrate. Moreover, the significant enrichment of TCA cycle (gab, sdh, fum, etc.) indicated that carbon and energy metabolism were facilitated with the addition of nitrate, thus improved TZ catabolism. The proposed mechanism demonstrated that TZ biodegradation coupled with nitrate reduction would be a promising approach for efficient treatment of wastewater contaminated by TZ.

Low-voltage stimulated denitrification performance of high-salinity wastewater using halotolerant microorganisms

Nitrate is a common contaminant in high-salinity wastewater, which has adverse effects on both the environment and human health. However, conventional biological treatment exhibits poor denitrification performance due to the high-salinity shock. In this study, an innovative approach using an electrostimulating microbial reactor (EMR) was explored to address this challenge. With a low-voltage input of 1.2 V, the EMR reached nitrate removal kinetic parameter (kNO3-N) of 0.0166-0.0808 h-1 under high-salinities (1.5 %-6.5 %), which was higher than that of the microbial reactor (MR) (0.0125-0.0478 h-1). The mechanisms analysis revealed that low-voltage significantly enhanced microbial salt-in strategy and promoted the secretion of extracellular polymeric substances. Halotolerant denitrification microorganisms (Pseudomonas and Nitratireductor) were also enriched in EMR. Moreover, the EMR achieved a NO3-N removal efficiency of 73.64 % in treating high-salinity wastewater (salinity 4.69 %) over 18-cycles, whereas the MR only reached 54.67 %. In summary, this study offers an innovative solution for denitrification of high-salinity wastewater.

Complete biodegradation of tetrabromobisphenol A through sequential anaerobic reductive dehalogenation and aerobic oxidation

Tetrabromobisphenol A (TBBPA), a common brominated flame retardant and a notorious pollutant in anaerobic environments, resists aerobic degradation but can undergo reductive dehalogenation to produce bisphenol A (BPA), an endocrine disruptor. Conversely, BPA is resistant to anaerobic biodegradation but susceptible to aerobic degradation. Microbial degradation of TBBPA via anoxic/oxic processes is scarcely documented. We established an anaerobic microcosm for TBBPA dehalogenation to BPA facilitated by humin. Dehalobacter species increased with a growth yield of 1.5 × 108 cells per μmol Br- released, suggesting their role in TBBPA dehalogenation. We innovatively achieved complete and sustainable biodegradation of TBBPA in sand/soil columns columns, synergizing TBBPA reductive dehalogenation by anaerobic functional microbiota and BPA aerobic oxidation by Sphingomonas sp. strain TTNP3. Over 42 days, 95.11 % of the injected TBBPA in three batches was debrominated to BPA. Following injection of strain TTNP3 cells, 85.57 % of BPA was aerobically degraded. Aerobic BPA degradation column experiments also indicated that aeration and cell colonization significantly increased degradation rates. This treatment strategy provides valuable technical insights for complete TBBPA biodegradation and analogous contaminants.

Critical role of multiple antibiotics on the denitrification rate in groundwater: Field investigative proof

The impact of antibiotics on denitrification has emerged as a significant topic; however, there is a dearth of mechanistic understanding regarding the effects of multiple antibiotics at the ng/L level on denitrification in groundwater. This study conducted five field samplings between March 2019 and July 2021 at two representative monitoring wells. The investigation utilized metagenomic sequencing to unveil the antibiotic mechanisms influencing denitrification. Results revealed the detection of 16 out of 64 antibiotics, with a maximum detection frequency and total concentration of 100 % and 187 ng/L, respectively. Additionally, both nitrate and total antibiotic concentrations exhibited a gradual decrease along the groundwater flow direction. Metagenomic evidence indicated that denitrification served as the dominant biogeochemical process controlling nitrate attenuation in groundwater. However, the denitrification capacity experienced significant inhibition in the presence of multiple antibiotics at the ng/L level. This inhibition was attributed to decreases in the relative abundance of dominant denitrifying bacteria (Candidatus_Scalindua, Herminiimonas and unclassified_p_Planctomycetes) and denitrifying functional genes (narGH, nirKS and norB), signifying the pressure exerted by antibiotics on denitrifying bacteria. The variation in antibiotic concentration (∆Cantibiotics) indicated a change in antibiotic pressure on denitrifying bacteria. A larger ∆Cantibiotics corresponded to a greater rebound in the relative abundance of denitrifying functional genes, resulting in a faster denitrification rate (Kdenitrification). Field observations further demonstrated a positive correlation between Kdenitrification and ∆Cantibiotics. Comparatively, a higher Kdenitrification observed at higher ∆Cantibiotics was primarily due to the enrichment of more nondominant denitrifying bacteria carrying key denitrifying functional genes. In conclusion, this study underscores that multiple antibiotics at the ng/L level in groundwater inhibited denitrification, and the degree of inhibition was closely related to ∆Cantibiotics.

Metabolomic insight into regulatory mechanism of heterotrophic bacteria nitrification-aerobic denitrification bacteria to high-strength ammonium wastewater treatment

This work aimed to elucidate the metabolic mechanism of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria influenced by varying concentrations of ammonium nitrogen (NH4+-N) in high-strength synthetic wastewater treatment. The results showed that the removal rates of NH4+-N and total nitrogen, along with enzymatic activities related to nitrification and denitrification, increased with rising NH4+-N concentrations (N500:500 mg/L, N1000:1000 mg/L and N2000:2000 mg/L). The relative abundances of HN-AD bacteria were 50 %, 62 % and 82 % in the three groups. In the N2000 group, the cAMP signaling pathway, glycerophospholipid metabolites, purines and pyrimidines related to DNA/RNA synthesis, electron donor NAD+-related energy, the tricarboxylic acid (TCA) cycle and glutamate metabolism were upregulated. Therefore, influent NH4+-N at 2000 mg/L promoted glutamate metabolism to accelerate the TCA cycle, and enhanced cellular energy and advanced denitrification activity of bacteria for HN-AD. This mechanism, in turn, enhanced microbial growth and the carbon and nitrogen metabolism of bacteria for HN-AD.

Efficient nitrogen removal from municipal wastewater by an autotrophic-heterotrophic coupled anammox system: The up-regulation of key functional genes

The metabolic pathways based on key functional genes were innovatively revealed in the autotrophic-heterotrophic coupled anammox system for real municipal wastewater treatment. The nitrogen removal performance of the system was stabilized at 88.40 ± 3.39 % during the treatment of real municipal wastewater. The relative abundances of the nitrification functional genes ammonia oxidase (amoA/B/C), hydroxylamine oxidoreductase (hao), and nitrite oxidoreductases (nxrA/B) were increased by 1.2-2.4 times, and these three nitrification functional genes were mostly contributed by Nitrospira that dominated the efficient nitrification of the system. The relative abundance of anammox bacteria Candidatus Brocadia augmented from 0.35 % to 0.75 %, accompanied with the increased expression of hydrazine synthase (hzs) and hydrazine dehydrogenase (hdh), resulting in the major role of anammox (81.24 %) for nitrogen removal. The expression enhancement of the functional genes nitrite reductase (narG/H, napA/B) that promoted partial denitrification (PD) of the system weakened the adverse effects of the sharp decline in the population of PD microbe Thauera (from 5.7 % to 2.2 %). The metabolic module analysis indicated that the carbon metabolism pathways of the system mainly included CO2 fixation and organic carbon metabolism, and the stable enrichment of autotrophic bacteria ensured stable CO2 fixation. Furthermore, the enhanced expression of the glucokinases (glk, GCK, HK, ppgk) and the abundant pyruvate kinase (PK) achieved stable hydrolysis ability of organic carbon metabolism function of the system. This study offers research basics to practical application of the mainstream anammox process.

Moderate anthropogenic disturbance stimulates versatile microbial taxa contributing to denitrification and aromatic compound degradation

Wastewater treatment plants (WWTPs) effluent often contains a significant amount of residual organic pollutants and nutrients, causing disturbance to the coastal effluent receiving areas (ERA). Microbial communities in coastal ERA sediments may benefit from the coexistence of organic pollutants and nutrients, promoting the emergence of versatile taxa that are capable of eliminating these substances simultaneously. However, the identification and exploration of versatile taxa in natural environments under anthropogenic disturbances remain largely uncharted territory. In this study, we specifically focused on the versatile taxa coupled by the degradation of aromatic compounds (ACs) and denitrification, using Hangzhou Bay in China as our study area. We explored how WWTPs effluent disturbance would affect the versatile taxa, and particularly examined the role of disturbance intensity in shaping their composition. Intriguingly, we found that versatile taxa were mainly derived from denitrifiers like Pseudomonas, suggesting the fulfilled potential of denitrifiers regarding ACs degradation. We also discovered that moderate disturbance stimulated the diversity of versatile taxa, resulting in strengthened functional redundancy. Through correlation network analysis, we further demonstrated that moderate disturbance enhanced the community-level cooperation. Thus, moderate disturbance serves as a catalyst for versatile taxa to maintain community function, making them more resilient to effluent disturbances. Additionally, we identified COD and NO3--N concentrations as significant environmental factors influencing the versatile taxa. Overall, our findings reveal the role of effluent disturbances in the promotion of versatile taxa, and highlight moderate disturbance can foster more robust versatile taxa that are better equipped to handle effluent disturbances.

Marine toxin domoic acid alters nitrogen cycling in sediments

As a red tide algal toxin with intense neurotoxicity distributed worldwide, domoic acid (DA) has attracted increasing concerns. In this work, the integrative analysis of metagenome and metabolome are applied to investigate the impact of DA on nitrogen cycling in coastal sediments. Here we show that DA can act as a stressor to induce the variation of nitrogen (N) cycling by altering the abundance of functional genes and electron supply. Moreover, microecology theory revealed that DA can increase the role of deterministic assembly in microbial dynamic succession, resulting in the shift of niches and, ultimately, the alteration in N cycling. Notably, denitrification and Anammox, the important process for sediment N removal, are markedly limited by DA. Also, variation of N cycling implies the modification in cycles of other associated elements. Overall, DA is capable of ecosystem-level effects, which require further evaluation of its potential cascading effects.

Performance evaluation, enzymatic activity change and metagenomic analysis of sequencing batch reactor under divalent zinc stress

Divalent zinc (Zn2+) are widely detected in domestic and industrial wastewater, and it is essential to evaluate the effect of Zn2+ on wastewater biological treatment process due to its bio-toxicity. In this study, the nitrogen removal rates and their corresponding enzymatic activities of sequencing batch reactor decreased with the increase of Zn2+ concentration. The Zn2+ accumulation in activated sludge caused significant antioxidant response, and the reactive oxygen species (ROS) production and antioxidant enzymatic activities were positively correlated with Zn2+ concentration. The presence of Zn2+ inhibited the metabolic pathways related to energy production and electron transport. The abundance decreases of nitrification and denitrification functional genes led to the deterioration of nitrogen removal performance under Zn2+ stress. The correlation analysis between functional gene modules and microbial genera revealed that Zoogloea had obvious Zn2+ resistance. This study can provide the insights into the influencing mechanism of Zn2+ on the biological nitrogen removal process.

Exploring influence mechanism of small-molecule carbon source on heterotrophic nitrification-aerobic denitrification process from carbon metabolism, nitrogen metabolism and electron transport process

The heterotrophic nitrification-aerobic denitrification (HNAD) process can remove nitrogen and organic carbon under aerobic conditions. To get the in-depth mechanism of the HAND process, a strain named Acinetobacter johnsonii ZHL01 was isolated, and enzyme activity, electron transport, energy production, and gene expression of the strain were studied with small-molecule carbon sources, including sodium citrate, sodium acetate, sodium fumarate, and sodium succinate. The HNAD pathway of ZHL01 was NH4+→NH2OH → NO, and nitrogen balance analysis shows that ZHL01 could assimilate and denitrify 58.29 ± 1.05 % and 16.58 ± 1.07 % of nitrogen, respectively. The assimilation, the nitrification/denitrification, and the respiration processes were regulated by the concentration of reduced nicotinamide adenine dinucleotide (NADH) produced from the different metabolic pathways of small-molecule carbon sources. The HNAD process occurs to reduce intracellular redox levels related to NADH concentrations. This discovery provides a theoretical basis for the practical application of HAND bacteria.

Effect and microbial mechanism of suspended sediments particle size on nitrous oxide emission in eutrophic lakes

Suspended sediment (SPS) is an important environmental factor in eutrophic lakes, where they may play a significant role in the microbial nitrogen cycle and thus affect the N2O source and sink function. This study investigated the correlation and corresponding microbial mechanisms between N2O emission fluxes and SPS particle sizes. N2O emission characteristics were investigated in four parallel operated lab-scale microcosmic systems, in which different sizes of SPS particles were inoculated (i.e., <75, 75-150, 150-300, and >300 μm). The results show that, N2O emission fluxes in the eutrophic lakes were exponentially correlated with the lake trophic level index (TLI) (R2 = 0.94, p < 0.01) and the specific surface area of the SPS (R2 = 0.38, p < 0.05). In the microcosmic systems, SPS with 75-150 μm particles had the highest N2O emission rate of 5.94 ± 0.007 μg N/L/d, which was 2.6 times that of the <75 μm particle size system. The microcosmic system with particle size >300 μm had the highest N2O reduction rate (Vmax) of 6.776 μmol/L/h, which was 16-50 times that of the other three groups. Larger particle size SPS have a smaller specific surface area, which could affect the microenvironment on SPS surface and thus affect the microbe functions. The microbial community structure results indicated that the dominant microorganisms on the SPS surface were denitrifying bacteria. The maximum (nirS + nirK)/nosZ ratio was 30.2 for the 75-150 μm system, which was nearly 2 times higher than the other systems. The >300 μm system had the highest nosZ abundance, indicating a strong ability to reduce N2O. The co-occurrence networks analysis indicated that the cooperation and competition among nitrifiers and denitrifiers determined N2O emissions. These results provide fundamental insights into the influence of SPS size on N2O emissions in eutrophic lakes.

Electron transfer mechanism of intracellular carbon-dependent DNRA inside anammox bacteria

Generally, anaerobic ammonium oxidation (anammox) converts nitrite (NO2-) and ammonium (NH4+) to nitrogen gas (N2) but generates some nitrate (NO3-) (equivalent to 11% of inlet total nitrogen (TN)). Although it reported that anammox bacteria could degrade NO3- via dissimilatory nitrate reduction to ammonium (DNRA) pathway using the intracellular carbon as the electron donor, it is still unclear the specific electron transfer mechanism in this intracellular carbon-dependent DNRA inside anammox bacteria, and whether the sole anammox bacteria could achieve higher TN removal efficiency more than the theoretical maximum of 89%. In this study, transcriptome analysis and metabolic inhibitor experiments demonstrated that NADH generated from the decomposition of the intracellular carbon (glycogen) supplied electrons for the NO3-conversion; the electrons were transferred from NADH to nitrate reductase (Nar) and nitrite reductase forming ammonium (NrfA) from ubiquinone (UQ) and complex III, respectively. Combining the intracellular carbon-dependent DNRA with normal anammox process, an average TN removal efficiency of 95% was achieved by the sole anammox bacteria in a sequencing batch reactor. Fluorescent in situ hybridization (FISH) images and real-time fluorescence quantitative PCR (qPCR) results illustrated anammox bacteria could survive and proliferate in the SBR. Our work improved the understanding of the electron transfer mechanism inside anammox bacteria, and further exploit its potential in nitrogen pollutants removal.

How β-Cyclodextrin-Functionalized Biochar Enhanced Biodenitrification in Low C/N Conditions via Regulating Substrate Metabolism and Electron Utilization

Biodenitrification plays a vital role in the remediation of nitrogen-contaminated water. However, influent with a low C/N ratio limits the efficiency of denitrification and causes the accumulation/emission of noxious intermediates. In this study, β-cyclodextrin-functionalized biochar (BC@β-CD) was synthesized and applied to promote the denitrification performance of Paracoccus denitrificans when the C/N was only 4, accompanied by increased nitrate reduction efficiency and lower nitrite accumulation and nitrous oxide emission. Transcriptomic and enzymatic activity analyses showed BC@β-CD enhanced glucose degradation by promoting the activities of glycolysis (EMP), the pentose phosphate pathway (PPP), and the tricarboxylic acid (TCA) cycle. Notably, BC@β-CD drove a great generation of electron donors by stimulating the TCA cycle, causing a greater supply of substrate metabolism to denitrification. Meanwhile, the promotional effect of BC@β-CD on oxidative phosphorylation accelerates electron transfer and ATP synthesis. Moreover, the presence of BC@β-CD increased the intracellular iron level, causing further improved electron utilization in denitrification. BC@β-CD helped to remove metabolites and induced positive feedback on the metabolism of P. denitrificans. Collectively, these effects elevated the glucose utilization for supporting denitrification from 36.37% to 51.19%. This study reveals the great potential of BC@β-CD for enhancing denitrification under low C/N conditions and illustrates a potential application approach for β-CD in wastewater bioremediation.

Characteristics and comparisons of the aerobic and anaerobic denitrification of a Klebsiella oxytoca strain: Performance, electron transfer pathway, and mechanism

The performance and electron (e^-) transfer mechanisms of anaerobic and aerobic denitrification by strain Klebsiella were investigated in this study. The RT-PCR results demonstrated that the membrane bound nitrate reductase gene (narG) and Cu-nitrite reductase gene (nirK) were responsible for both aerobic and anerobic denitrification. The extreme low gene relative abundance of nirK might be responsible for the severe accumulation of NO₂^--N (nitrogen in the form of NO₂^- ion) under anaerobic condition. Moreover, the nitrite reductase (Nir) activity was 0.31 μg NO₂^--N min^-1 mg^-1 protein under anaerobic conditions, which was lower than that under aerobic conditions (0.38 μg NO₂^--N min^-1 mg^-1 protein). By using respiration chain inhibitors, the e^- transfer pathways of anaerobic and aerobic denitrification of Klebsiella strain were constructed. Fe-S protein and Complex III were the core components under anaerobic conditions, while Coenzyme Q (CoQ), Complexes I and III played a key role in aerobic denitrification. Nitrogen assimilation was found to be the main way to generate NH₄⁺-N (nitrogen in the form of NH₄⁺ ion) during anaerobic denitrification, and also served as the primary nitrogen removal way under aerobic condition. The results of this study may help to improve the understanding of the core components of strain Klebsiella during aerobic and anaerobic denitrifications, and may suggest potential applications of the strain for nitrogen-containing wastewater.

Simultaneous nitrate and sulfate biotransformation driven by different substrates: comparison of carbon sources and metabolic pathways at different C/N ratios

Nitrate (NO₃^-) and sulfate (SO₄^2-) often coexist in organic wastewater. The effects of different substrates on NO₃^- and SO₄^2- biotransformation pathways at various C/N ratios were investigated in this study. This study used an activated sludge process for simultaneous desulfurization and denitrification in an integrated sequencing batch bioreactor. The results revealed that the most complete removals of NO₃^- and SO₄^2- were achieved at a C/N ratio of 5 in integrated simultaneous desulfurization and denitrification (ISDD). Reactor Rb (sodium succinate) displayed a higher SO₄^2- removal efficiency (93.79%) with lower chemical oxygen demand (COD) consumption (85.72%) than reactor Ra (sodium acetate) on account of almost 100% removal of NO₃^- in both Ra and Rb. Ra produced more S^2- (5.96 mg L^-1) and H₂S (25 mg L^-1) than Rb, which regulated the biotransformation of NO₃^- from denitrification to dissimilatory nitrate reduction to ammonium (DNRA), whereas almost no H₂S accumulated in Rb which can avoid secondary pollution. Sodium acetate-supported systems were found to favor the growth of DNRA bacteria (Desulfovibrio); although denitrifying bacteria (DNB) and sulfate-reducing bacteria (SRB) were found to co-exist in both systems, Rb has a greater keystone taxa diversity. Furthermore, the potential carbon metabolic pathways of the two carbon sources have been predicted. Both succinate and acetate could be generated in reactor Rb through the citrate cycle and the acetyl-CoA pathway. The high prevalence of four-carbon metabolism in Ra suggests that the carbon metabolism of sodium acetate is significantly improved at a C/N ratio of 5. This study has clarified the biotransformation mechanisms of NO₃^- and SO₄^2- in the presence of different substrates and the potential carbon metabolism pathway, which is expected to provide new ideas for the simultaneous removal of NO₃^- and SO₄^2- from different media.

Nitrite-resistance mechanisms on wastewater treatment in denitrifying phosphorus removal process revealed by machine learning, co-occurrence, and metagenomics analysis

Nitrite is a key intermediate in nitrogen metabolism that determines microbial transformations of N and P, greenhouse gas (N₂O) emissions, and system nutrient removal efficiency. However, nitrite also exerts toxic effects on microorganisms. A lack of understanding of high nitrite-resistance mechanisms at community- and genome-scale resolutions hinders the optimization for robustness of wastewater treatment systems. Here, we established nitrite-dependent denitrifying and phosphorus removal (DPR) systems under a gradient concentration of nitrite (0, 5, 10, 15, 20, and 25 mg N/L), relying on 16S rRNA gene amplicon and metagenomics to explore high nitrite-resistance mechanism. The results demonstrated that specific taxa were adopted to change the metabolic relationship of the community through phenotypic evolution to resist toxic nitrite contributing to the enhancement of denitrification and inhibition of nitrification and phosphorus removal. The key specific species, Thauera enhanced denitrification, whereas Candidatus Nitrotoga decreased in abundance to maintain partial nitrification. The extinction of Candidatus Nitrotoga induced a simpler restructuring-community, forcing high nitrite-stimulating microbiome to establish a more focused denitrification rather than nitrification or P metabolism in response to nitrite toxicity. Our work provides insights for understanding microbiome adaptation to toxic nitrite and giving theoretical support for operation strategy of nitrite-based wastewater treatment technology.

Electroactive microorganism-assisted remediation of groundwater contamination: Advances and challenges

Groundwater contamination has become increasingly prominent, therefore, the development of efficient remediation technology is crucial for improving groundwater quality. Bioremediation is cost-effective and environmentally friendly, while coexisting pollutant stress can affect microbial processes, and the heterogeneous character of groundwater medium can induce bioavailability limitations and electron donor/acceptor imbalances. Electroactive microorganisms (EAMs) are advantageous in contaminated groundwater because of their unique bidirectional electron transfer mechanism, which allows them to use solid electrodes as electron donors/acceptors. However, the relatively low-conductivity groundwater environment is unfavorable for electron transfer, which becomes a bottleneck problem that limits the remediation efficiency of EAMs. Therefore, this study reviews the recent advances and challenges of EAMs applied in the groundwater environment with complex coexisting ions, heterogeneity, and low conductivity and proposes corresponding future directions.

Complementary Biotransformation of Antimicrobial Triclocarban Obviously Mitigates Nitrous Oxide Emission toward Sustainable Microbial Denitrification

Sustainable nitrogen cycle is an essential biogeochemical process that ensures ecosystem safety and byproduct greenhouse gas nitrous oxide reduction. Antimicrobials are always co-occurring with anthropogenic reactive nitrogen sources. However, their impacts on the ecological safety of microbial nitrogen cycle remain poorly understood. Here, a denitrifying bacterial strain Paracoccus denitrificans PD1222 was exposed to a widespread broad-spectrum antimicrobial triclocarban (TCC) at environmental concentrations. The denitrification was hindered by TCC at 25 μg L^-1 and was completely inhibited once the TCC concentration exceeded 50 μg L^-1. Importantly, the accumulation of N₂O at 25 μg L^-1 of TCC was 813 times as much as the control group without TCC, which attributed to the significantly downregulated expression of nitrous oxide reductase and the genes related to electron transfer, iron, and sulfur metabolism under TCC stress. Interestingly, combining TCC-degrading denitrifying Ochrobactrum sp. TCC-2 with strain PD1222 promoted the denitrification process and mitigated N₂O emission by 2 orders of magnitude. We further consolidated the importance of complementary detoxification by introducing a TCC-hydrolyzing amidase gene tccA from strain TCC-2 into strain PD1222, which successfully protected strain PD1222 against the TCC stress. This study highlights an important link between TCC detoxification and sustainable denitrification and suggests a necessity to assess the ecological risks of antimicrobials in the context of climate change and ecosystem safety.

A Review on Tetrabromobisphenol A: Human Biomonitoring, Toxicity, Detection and Treatment in the Environment

Tetrabromobisphenol A (TBBPA) is a known endocrine disruptor employed in a range of consumer products and has been predominantly found in different environments through industrial processes and in human samples. In this review, we aimed to summarize published scientific evidence on human biomonitoring, toxic effects and mode of action of TBBPA in humans. Interestingly, an overview of various pretreatment methods, emerging detection methods, and treatment methods was elucidated. Studies on exposure routes in humans, a combination of detection methods, adsorbent-based treatments and degradation of TBBPA are in the preliminary phase and have several limitations. Therefore, in-depth studies on these subjects should be considered to enhance the accurate body load of non-invasive matrix, external exposure levels, optimal design of combined detection techniques, and degrading technology of TBBPA. Overall, this review will improve the scientific comprehension of TBBPA in humans as well as the environment, and the breakthrough for treating waste products containing TBBPA.

A hormesis-like effect of FeS on heterotrophic denitrification and its mechanisms

To alleviate the insufficiency of carbon source in sewage, many sulfur-containing inorganic electron donors were added into traditional heterotrophic denitrification process. However, the effects of extraneous inorganic electron donors on heterotrophic denitrification were still largely unknown. In this study, a hormesis-like effect of ferrous sulfide (FeS, a representative inorganic electron donors) on Paracoccus denitrificans was observed. Total nitrogen (TN) removal efficiency of P. denitrificans rose by 15% with the increase of FeS dosage from 0 to 0.3 g L^-1 (low level), whereas the TN removal significantly decreased to 53% as the dosage of FeS mounted up to 5.0 g L^-1 (high level). Furthermore, the impacts of FeS on glucose utilization and bacterial growth exhibited hormesis-like effects. A subsequent mechanistic study revealed that above influences were caused by its released ions (Fe²⁺, Fe³⁺, and S^2-) rather than particle size. Further study illustrated that low dosage of FeS released a small amount of Fe²⁺ and Fe³⁺, which provided sufficient electrons via promoting glucose utilization, then improved denitrification. Conversely, FeS with high dosage inhibited denitrification via its released S^2-, which suppressed the activity of key denitrifying enzymes rather than influenced glucose metabolism and electron provision. Our results provide an insight into improving denitrification efficiency of the mixotrophic process coexisting with autotrophic and heterotrophic denitrifiers.

Molecular mechanisms through which different carbon sources affect denitrification by Thauera linaloolentis: Electron generation, transfer, and competition

Characterizing the molecular mechanism through which different carbon sources affect the denitrification process would provide a basis for the proper selection of carbon sources, thus avoiding excessive carbon source dosing and secondary pollution while also improving denitrification efficiency. Here, we selected Thauera linaloolentis as a model organism of denitrification, whose genomic information was elucidated by draft genome sequencing and KEGG annotations, to investigate the growth kinetics, denitrification performances and characteristics of metabolic pathways under diverse carbon source conditions. We reconstructed a metabolic network of Thauera linaloolentis based on genomic analysis to help develop a systematic method of researching electron pathways. Our findings indicated that carbon sources with simple metabolic pathways (e.g., ethanol and sodium acetate) promoted the reproduction of Thauera linaloolentis, and its maximum growth density reached OD₆₀₀ = 0.36 and maximum specific growth rate reached 0.145 h^-1. These carbon sources also accelerated the denitrification process without the accumulation of intermediates. Nitrate could be reduced completely under any carbon source condition; but in the "glucose group", the maximum accumulation of nitrite was 117.00 mg/L (1.51 times more than that in the "ethanol group", which was 77.41 mg/L), the maximum accumulation of nitric oxide was 363.02 μg/L (7.35 times more than that in the "ethanol group", which was 49.40 μg/L), and the maximum accumulation of nitrous oxide was 22.58 mg/L (26.56 times more than that in the "ethanol group", which was 0.85 mg/L). Molecular biological analyses demonstrated that diverse types of carbon sources directly induced different carbon metabolic activities, resulting in variations in electron generation efficiency. Furthermore, the activities of the electron transport system were positively correlated with different carbon metabolic activities. Finally, these differences were reflected in the phenomenon of electronic competition between denitrifying reductases. Thus we concluded that this was the main molecular mechanism through which the carbon source type affected the denitrification process. In brief, carbon sources with simple metabolic pathways induced higher efficiency of electron generation, transfer, and competition, which promoted rapid proliferation and complete denitrification; otherwise Thauera linaloolentis would grow slowly and intermediate products would accumulate seriously. Our study established a method to evaluate and optimize carbon source utilization efficiency based on confirmed molecular mechanisms.

Multi-Omics Analysis Reveals the Nitrogen Removal Mechanism Induced by Electron Flow during the Start-up of the Anammox-Centered Process

Significant progress in understanding the key enzymes or species of anammox has been made; however, the nitrogen removal mechanism in complex coupling systems centered on anammox remains limited. In this study, by the combination of metagenomics-metatranscriptomics analyses, the nitrogen removal in the anammox-centered coupling system that entails partial denitrification (PD) and hydrolytic acidification (HA, A-PDHA) was elucidated to be the nitrogen transformation driven by the electron generation-transport-consumption process. The results showed that a total nitrogen (TN) removal efficiency of >98%, with a TN effluence of <1 mg/L and a TN removal contribution via anammox of >98%, was achieved after 59 days under famine operation and alkaline conditions during the start-up process. Further investigation confirmed that famine operation promoted the activity of genes responsible for electron generation in anammox, and increased the abundance or expression of genes related to electron consumption. Alkaline conditions enhanced the electron generation for PD by upregulating the activity of glyceraldehyde 3-phosphate dehydrogenase and strengthened electron transfer by increasing the gene encoding quinone pool. Altogether, these variations in the electron flow led to efficient nitrogen removal. These results improve our understanding of the nitrogen removal mechanism and application of the anammox-centered coupling systems in treating nitrogen wastewater.

Microbial driving mechanism for simultaneous removal of nitrogen and phosphorus in a pure anammox reactor under ferrous ion exposure

The mechanisms of Fe²⁺ on nitrogen and phosphorus removal and functional bacterial competition in anammox systems was investigated. Under 0.12 mM Fe²⁺, the performance of nitrogen and phosphorus removal increased by 10.08 % and 151.91 %, respectively, compared with the control stage. Phosphorus removal was achieved through extracellular polymeric substance (EPS) induced biomineralization to form Fe-P minerals, and functional group COC in EPS played a critical role. T-EPSs was the major nucleation site due to it maintaining the supersaturated state (saturation index > 0) of Fe-P minerals for a long time. Population succession showed that Fe²⁺ weakened the competition between heterotrophic denitrifier (Denitrasoma) and anammox microbe (Candidatus Brocadia) for space and substrates, which was favorable for the enrichment of anammox biomass. Moreover, the variation in gene abundance (such as Hao, Cyt c, and Nir) indicated that Fe²⁺ improved electron behaviors (generation, transport, and consumption) during the nitrogen metabolism of anammox systems.

Co-culturing fungus Penicillium citrinum and strain Citrobacter freundii improved nitrate removal and carbon utilization by promoting glyceride metabolism

Exploring the interaction between denitrifying microbial species is significant for improving denitrification performance. In this study, the effects of co-culturing fungus Penicillium citrinum and strain Citrobacter freundii on denitrification were investigated. Results showed that the maximum nitrate removal and carbon utilization in co-culture were 68.0 and 14.1 mg·L^-1·d^-1, respectively. The total adenosine triphosphatase activity was increased to 9.87 U‧mg^-1 protein in co-culture, and nicotinamide adenine dinucleotide production was 1.7-2.3 times that of monoculture, attributing to increased carbon utilization. Further metabolomics and membrane permeability assay revealed that co-culture increased the metabolism of glycerides, thereby enhancing the membrane permeability of strain Citrobacter freundii and promoting the transmembrane transport of nitrate and glucose, which boosted nitrate reductase activity and nicotinamide adenine dinucleotide production in turn. In summary, co-culturing promoted carbon utilization and enhanced substrate removal efficiency through the metabolism of glycerides, which provided a strategy to enhance denitrification performance in wastewater treatment.

Hormesis-Like Effects of Tetrabromobisphenol A on Anaerobic Digestion: Responses of Metabolic Activity and Microbial Community

Tetrabromobisphenol A (TBBPA) has extensive applications in various fields; its release into ecosystems and the potential toxic effects on organisms are becoming major concerns. Here, we investigated the effects of TBBPA on anaerobic digestion, whose process is closely related to the carbon cycles under anaerobic conditions. The results revealed that TBBPA exhibited dose-dependent hormesis-like effects on methane production from glucose, i.e., the presence of 0.1 mg/L TBBPA increased the methane production rate by 8.79%, but 1.0-4.0 mg/L TBBPA caused 3.45-28.98% of decrement. We found that TBBPA was bound by the tyrosine-like proteins of the extracellular polymeric substances of anaerobes and induced the increase of reactive oxygen species, whose slight accumulation stimulated the metabolism activities but high accumulation increased the apoptosis of anaerobes. Owing to the differences between individual anaerobes in tolerance, TBBPA at 0.1 mg/L stimulated the acidogenesis and hydrogenotrophic methanogenesis, whereas higher levels (i.e., 1.0-4.0 mg/L) severely restrained all of the processes of acidogenesis, acetogenesis, and methanogenesis. Along with the accumulation of bisphenol A (BPA) produced from TBBPA by Longilinea sp. and Pseudomonas sp., the methanogenic pathway was partly shifted from acetate-dependent to hydrogen-dependent direction, and the activities of carbon monoxide dehydrogenase and acetyl-CoA decarbonylase/synthase were inhibited, while acetate kinase and F420 were hormetically affected. These findings elucidated the mechanism of anaerobic syntrophic consortium responses to TBBPA, supplementing the potential environmental risks of brominated flame retardants.

Cysteine reduced the inhibition of CO2 on heterotrophic denitrification: Restoring redox balance, facilitating iron acquisition and carbon metabolism

The direct effect of CO₂ on denitrification has attracted great attention currently. Our previous studies have confirmed that CO₂ inhibited heterotrophic denitrification and caused high nitrite accumulation and nitrous oxide emission. Cysteine is a widely reported bio-accelerator; however, its effect on denitrification under CO₂ exposure remains unknown. In this paper, the effect of cysteine on heterotrophic denitrification and its mechanisms under CO₂ exposure were explored with the model denitrifier, Paracoccus denitrificans. We observed that total nitrogen removal increased from 17.9% to 90.4% as cysteine concentration increased from 0 to 50 μM, probably due to restoration of cell growth and viability. Further study showed that cysteine reduced the inhibition of CO₂ on denitrification due to multiple positive influences: (1) regulating glutathione metabolism to eliminate intracellular reactive nitrogen species (RNS), while reducing extracellular polymeric substances (EPS) levels and altering its composition, ultimately restoring cell membrane integrity (2) facilitating the transport and metabolism of carbon sources to increase NADH production, and (3) increasing intracellular iron and up-regulating the expression of key iron transporters genes (AfuA, AfuB, ExbB and TonB) to restore the transport and consumption of electron. This study suggests that cysteine can be added to recover heterotrophic denitrification performance after inhibition by elevated CO₂.

Antimicrobial peptide antibiotics inhibit aerobic denitrification via affecting electron transportation and remolding carbon metabolism

The harmful effects of antibiotics on biological denitrification have attracted widespread attention due to their excessive usage. Polymyxin B (PMB) as the typical antimicrobial peptides having been regarded as the "last hope" for treatment of multidrug-resistance bacteria, has also been detected in wastewater. However, little is known about the influence of PMB on aerobic denitrification. In this study, the impact of PMB on aerobic denitrification performance was investigated. Results showed 0.50 mg/L PMB decreased nitrate removal efficiency from 97.4% to 85.3%, and drove denitrifiers to transform more nitrate to biomass instead of producing gas-N. The live/dead staining method showed PMB damaged bacterial membrane. Transcriptome analysis further indicated the key enzymes participating in denitrification and aerobic respiratory chains were suppressed by PMB. To resist the PMB stress, denitrifiers formed thicker biofilm to protect cells from PMB damaging and thus remodeling the central carbon metabolism. Further investigation revealed denitrifiers have different preference on various carbon sources when PMB is present. Subsequently, the underlying mechanism of the distinctive carbon sources preference was explored by the combination of transcriptome and metabolism analysis. Overall, our data suggested denitrifiers have distinctive carbon sources preference under PMB treatment conditions, reminding us that carbon source selection should be cautious in practical applications.

High concentration powder carrier bio-fluidized bed process: A new perspective for domestic wastewater treatment

The nitrogen removal mechanism of the high concentration powder carrier bio-fluidized bed (HPB) process was investigated with actual domestic wastewater. The micron-size (10-70 μm) powder carriers were diatomite and Fe-C. Results showed diatomite enriched the relative abundances of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria, accordingly increasing the rate of nitrification. Even a 100% increase of genes associated with the ammonia oxidation was achieved. Fe-C enhanced the rate of substrate utilization mainly by increasing the activity of the electron transfer system. Hydrocyclone separator, as a key device of HPB, was able to recover the carriers with high efficiency (recovery efficiency of 72.66 ─ 82.50% after 75 days), thus, indirectly improving the functionality of the carriers. Furthermore, it could renew the surface of microbial aggregations, consequently improving the adsorption capacity to substrates. HPB could provide the feasibility of shortening the hydraulic retention time and expanding the capacity of wastewater treatment plants.

Bioinspired facilitation of intrinsically conductive polymers: Mediating intra/extracellular electron transfer and microbial metabolism in denitrification

Intrinsically conductive polymers, polyaniline and polyaniline sulfonate (PASAni) were used to explore their effect on denitrification. Denitrification was accelerated 1.90 times by 2 mM PASAni and the possible mechanisms were mainly attributed to the accelerated electron transfer and the enhanced microbial metabolism activity. Intracellular electron transfer was accelerated by PASAni and the acceleration sites were from NADH to coenzyme Q (CoQ), quinone loop, from Complex II to CoQ and from QH₂ to Cyt. c₁. Extracellular electron transfer was accelerated because PASAni promoted more secretion of redox species and PASAni embedded in extracellular polymeric substance (EPS). Moreover, PASAni itselfprovided more electron transfer pathways as redox species. Microbial metabolism activity was also enhanced by PASAni, which was reflected in the increased nitrate/nitrite reductase activity (236.13/155.43%), electron transfer system activity (112.49%), adenosine triphosphate level (133.41%) and EPS content (189.06%). Besides, the enriched Proteobacteria in PASAni supplement system was also conducive to denitrification. This work provided fundamental information for conductive polymers mediating microbial electron transfer and enhancing contaminants biotransformation.

Visible light promoted the removal of tetrabromobisphenol A from water by humic acid-FeS colloid

Ferrous sulfide (FeS) and humic acid (HA) are typical black substances in black bloom water. Based on the strong reduction ability of FeS and the photosensitivity of HA, the transformation of toxic organic pollutants by the combination of FeS and HA (HA-FeS) is not clear. In order to explore this issue, the stability of HA-FeS was analyzed by measuring the hydrodynamic diameter and zeta potential of HA-FeS, and then the removal mechanism and possible degradation pathway of tetrabromobisphenol A (TBBPA) by HA-FeS under continuous illumination were discussed. The results showed that the hydrodynamic diameter of FeS was reduced and the stability of FeS was improved, and it was easily suspended after FeS combined with the HA in the water. The combination of HA and FeS promoted the removal of TBBPA in water, no matter it was in the presence or absence of light. Besides, compared with the absence of light, the removal efficiency of TBBPA was improved by HA-FeS with continuous light. There were two reasons for the increase in the removal efficiency of TBBPA by HA-FeS. On the one hand, Fe²⁺ and S^2- of HA-FeS had more stable chemical valence and obtained better reducibility under continuous light than that in the dark. On the other hand, light induced the release of active species (O₂^-, ¹O₂, and OH) in the HA-FeS composite colloid and further promoted the degradation of organic pollutants. Therefore, the black substances (FeS) of black blooms may play a beneficial role in the removal of pollutants under sunlight.

Short- and long-term effects of decabromodiphenyl ether (BDE-209) on sediment denitrification using a semi-continuous microcosm

The widespread use of decabromodiphenyl ether (BDE-209) resulted in its deposition in environmental media and biological matrices. However, to date, few studies focused on the effect of BDE-209 on microorganisms, and those available were investigated via an enclosed system completely cutting off the communication between testing system and its native environment. Herein, 4.0 mg/g BDE-209 acute exposure induced a 20% decline of NO_X-N (the sum of NO₃^--N and NO₂^--N) removal efficiency and a significant accumulation of NO₂^--N and N₂O. These inhibitory effects presented in a BDE-209 concentration-dependent manner. Using a semi-continuous microcosm, the inhibitory effects of BDE-209 on denitrification were observed to be significantly enhanced with the extending of exposure duration. Denitrifying genes assay illustrated that BDE-209 has an insignificant effect on the global abundance of denitrifying bacteria because of microbial exchange with its overlying water. But the utilization of electron donor (carbon substrate), the activity of electron transport system and denitrifying enzymes were significantly inhibited by BDE-209 exposure in a exposure-duration-dependent manner. Finally, insufficient electron donor and lower efficiency of electron transport and utilization on denitrifying enzymes deteriorated the denitrification performance. These results provided a new insight into BDE-209 influence on denitrification in the natural environment.

Perturbation of clopyralid on bio-denitrification and nitrite accumulation: Long-term performance and biological mechanism

The contaminant of herbicide clopyralid (3,6-dichloro-2- pyridine-carboxylic acid, CLP) poses a potential threat to the ecological system. However, there is a general lack of research devoted to the perturbation of CLP to the bio-denitrification process, and its biological response mechanism remains unclear. Herein, long-term exposure to CLP was systematically investigated to explore its influences on denitrification performance and dynamic microbial responses. Results showed that low-concentration of CLP (<15 mg/L) caused severe nitrite accumulation initially, while higher concentrations (35-60 mg/L) of CLP had no further effect after long-term acclimation. The mechanistic study demonstrated that CLP reduced nitrite reductase (NIR) activity and inhibited metabolic activity (carbon metabolism and nitrogen metabolism) by causing oxidative stress and membrane damage, resulting in nitrite accumulation. However, after more than 80 days of acclimation, almost no nitrite accumulation was found at 60 mg/L CLP. It was proposed that the secretion of extracellular polymeric substances (EPS) increased from 75.03 mg/g VSS at 15 mg/L CLP to 109.97 mg/g VSS at 60 mg/L CLP, which strengthened the protection of microbial cells and improved NIR activity and metabolic activities. Additionally, the biodiversity and richness of the microbial community experienced a U-shaped process. The relative abundance of denitrification- and carbon metabolism-associated microorganisms decreased initially and then recovered with the enrichment of microorganisms related to the secretion of EPS and N-acyl-homoserine lactones (AHLs). These microorganisms protected microbe from toxic substances and regulated their interactions among inter- and intra-species. This study revealed the biological response mechanism of denitrification after successive exposure to CLP and provided proper guidance for analyzing and treating herbicide-containing wastewater.

High-temperature biofilm system based on heterotrophic nitrification and aerobic denitrification treating high-strength ammonia wastewater: Nitrogen removal performances and temperature-regulated metabolic pathways

Conventional autotrophic nitrification process is difficult to treat high-temperature wastewater with high-strength ammonia. In this study, a high-temperature (50 °C) biofilm system based on heterotrophic nitrification and aerobic denitrification (HNAD) was established. The results showed that the HNAD process was high temperature resistant, and the nitrogen removal performance, pathway and microbial mechanism varied remarkably at different temperatures. The high-temperature system showed excellent nitrogen and COD removal capacities at 50 °C. Ammonia oxidation was mainly undertaken by heterotrophic nitrification, while anoxic and aerobic pathways worked in concert for denitrification. High-throughput sequencing indicated that heterotrophic nitrifying bacteria (8.58%) and denitrifying bacteria (52.88%) were dominant at 50 °C. Metagenomic analysis further suggested that the carbon metabolism was up-regulated in response to the increasing temperature, so more energy was generated, thereby promoting the HNAD-related nitrogen removal pathways. The study revealed the microbial mechanism of HNAD at high temperature and provided new insights into high-temperature biological nitrogen removal.

How Does Chitosan Affect Methane Production in Anaerobic Digestion?

The expanding use of chitosan in sewage and sludge treatment processes raises concerns about its potential environmental impacts. However, investigations of the impacts of chitosan on sewage sludge anaerobic digestion where chitosan is present at substantial levels are sparse. This study therefore aims to fill this knowledge gap through both long-term and batch tests. The results showed that 4 g/kg total suspended solid (TSS) chitosan had no acute effects on methane production, but chitosan at 8-32 g/kg TSS inhibited methane production by 7.2-30.3%. Mass balance and metabolism of organic analyses indicated that chitosan restrained the transfer of organic substrates from solid phase to liquid phase, macromolecules to micromolecules, and finally to methane. Further exploration revealed that chitosan suppressed the secretion of extracellular polymeric substances of anaerobes by occupying the connection sites of indigenous carbohydrates and increased the mass transfer resistance between anaerobes and substrates, which thereby lowered the metabolic activities of anaerobes. Although chitosan could be partly degraded by anaerobes, it is much more persistent to be degraded compared with indigenous organics in sludge. Microbial community and key enzyme encoding gene analyses further revealed that the inhibition of chitosan to CO₂-dependent methanogenesis was much severer than that to acetate-dependent methanogenesis.