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

Groundwater flow regime shapes nitrogen functional traits by affecting microbial community assembly processes in the subsurface

The complex nitrogen (N) cycle in groundwater systems is affected by both biological and environmental factors. The interactions between hydrogeological conditions and the microbial community assembly processes that impact N-cycling processes remain poorly understood. We explored the assembly patterns of N-cycling microbial communities along the groundwater flow path. The environmental heterogeneity in different hydrological phases increased along the flow path (mean Ed: 0.16-0.49), accompanied by different microbial community assembly patterns. The assembly patterns that engaged in dissimilatory nitrate reduction to ammonium (DNRA) and denitrification changed across the water-sediment phases. Nitrifying microorganisms in the discharge area were mainly influenced by heterogeneous selection (41-69 %), and were closely correlated with dissolved oxygen (DO) concentrations. Homogeneity along flow-through increased stochastic assemblies, such as downstream drift of anammox bacterial (AnAOB) communities. Thus, the N removal pathway changed from "nitrification-denitrification" in the recharge area to "partial nitrification-anammox" in the discharge area. The increasing environmental heterogeneity brought more deterministic assembly patterns of N-cycling communities, linked to higher community turnover along the groundwater flow path. This study indicated that groundwater flow regime determined microbial community assembly patterns, providing valuable insight into the response of N transitions to environmental variations in groundwater systems.

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

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.

The mechanism of microbial sulfate reduction in high concentration sulfate wastewater enhanced by maifanite

Excessive sulfate levels in water bodies pose a dual threat to the ecological environment and human health. The microbial removal of sulfate encounters challenges, particularly in environments with high sulfate concentrations, where the gradual accumulation of sulfide hampers microbial activity. This study focuses on elucidating the mechanisms underlying the enhancement of microbial sulfate reduction in high-concentration sulfate wastewater through a comparative analysis of maifanite and zeolite biostimulants. The investigation reveals that zeolite primarily facilitates microbial growth by providing attachment sites, while maifanite augments sulfate-reducing bacteria (SRB) activity through the release of active substances such as Mo, Ca, and Cu. The addition of maifanite proves instrumental in enhancing microbial activity, manifesting as increased microbial load and protein production, augmented extracellular polymer generation, accelerated electron transfer, and facilitated microbial growth and biofilm formation. Noteworthy is the observation that the combined application of maifanite and zeolite exhibited a synergistic effect, resulting in a 167 % and 68 % increase in sulfate reduction rate compared to the utilization of maifanite (0.12 d-1) or zeolite (0.19 d-1) in isolation. Within this synergistic context, the relative abundance of Desulfobacteraceae reaches a peak of 15.4 %. The outcomes of this study corroborate the distinct promotion mechanisms of maifanite and zeolite in microbial sulfate reduction, offering novel insights into the application of maifanite in the context of high-concentration sulfate removal.

Effects of antibiotics on microbial nitrogen cycling and N2O emissions: A review

Sulfonamides, quinolones, tetracyclines, and macrolides are the most prevalent classes of antibiotics used in both medical treatment and agriculture. The misuse of antibiotics leads to their extensive dissemination in the environment. These antibiotics can modify the structure and functionality of microbial communities, consequently impacting microbial-mediated nitrogen cycling processes including nitrification, denitrification, and anammox. They can change the relative abundance of nirK/norB contributing to the emission of nitrous oxide, a potent greenhouse gas. This review provides a comprehensive examination of the presence of these four antibiotic classes across different environmental matrices and synthesizes current knowledge of their effects on the nitrogen cycle, including the underlying mechanisms. Such an overview is crucial for understanding the ecological impacts of antibiotics and for guiding future research directions. The presence of antibiotics in the environment varies widely, with significant differences in concentration and type across various settings. We conducted a comprehensive review of over 70 research articles that compare various aspects including processes, antibiotics, concentration ranges, microbial sources, experimental methods, and mechanisms of influence. Antibiotics can either inhibit, have no effect, or even stimulate nitrification, denitrification, and anammox, depending on the experimental conditions. The influence of antibiotics on the nitrogen cycle is characterized by dose-dependent responses, primarily inhibiting nitrification, denitrification, and anammox. This is achieved through alterations in microbial community composition and diversity, carbon source utilization, enzyme activities, electron transfer chain function, and the abundance of specific functional enzymes and antibiotic resistance genes. These alterations can lead to diminished removal of reactive nitrogen and heightened nitrous oxide emissions, potentially exacerbating the greenhouse effect and related environmental issues. Future research should consider diverse reaction mechanisms and expand the scope to investigate the combined effects of multiple antibiotics, as well as their interactions with heavy metals and other chemicals or organisms.

Algicidal activity synchronized with nitrogen removal by actinomycetes: Algicidal mechanism, stress response of algal cells, denitrification performance, and indigenous bacterial community co-occurrence

The harmful algal blooms (HABs) can damage the ecological equilibrium of aquatic ecosystems and threaten human health. The bio-degradation of algal by algicidal bacteria is an environmentally friendly and economical approach to control HABs. This study applied an aerobic denitrification synchronization algicidal strain Streptomyces sp. LJH-12-1 (L1) to control HABs. The cell-free filtrate of the strain L1 showed a great algolytic effect on bloom-forming cyanobacterium, Microcystis aeruginosa (M. aeruginosa). The optimal algicidal property of strain L1 was indirect light-dependent algicidal with an algicidal rate of 85.0%. The functional metabolism, light-trapping, light-transfer efficiency, the content of pigments, and inhibition of photosynthesis of M. aeruginosa decreased after the addition of the supernatant of the strain L1 due to oxidative stress. Moreover, 96.05% nitrate removal rate synchronized with algicidal activity was achieved with the strain L1. The relative abundance of N cycling functional genes significantly increased during the strain L1 effect on M. aeruginosa. The algicidal efficiency of the strain L1 in the raw water was 76.70% with nitrate removal efficiency of 81.4%. Overall, this study provides a novel route to apply bacterial strain with the property of denitrification coupled with algicidal activity in treating micro-polluted water bodies.

Sustained bacterial N2O reduction at acidic pH

Nitrous oxide (N2O) is a climate-active gas with emissions predicted to increase due to agricultural intensification. Microbial reduction of N2O to dinitrogen (N2) is the major consumption process but microbial N2O reduction under acidic conditions is considered negligible, albeit strongly acidic soils harbor nosZ genes encoding N2O reductase. Here, we study a co-culture derived from acidic tropical forest soil that reduces N2O at pH 4.5. The co-culture exhibits bimodal growth with a Serratia sp. fermenting pyruvate followed by hydrogenotrophic N2O reduction by a Desulfosporosinus sp. Integrated omics and physiological characterization revealed interspecies nutritional interactions, with the pyruvate fermenting Serratia sp. supplying amino acids as essential growth factors to the N2O-reducing Desulfosporosinus sp. Thus, we demonstrate growth-linked N2O reduction between pH 4.5 and 6, highlighting microbial N2O reduction potential in acidic soils.

Efficient nitrate and Cr(VI) removal by denitrifier: The mechanism of S. oneidensis MR-1 promoting electron production, transportation and consumption

When Cr(VI) and nitrate coexist, the efficiency of both bio-denitrification and Cr(VI) bio-reduction is poor because chromate hinders bacterial normal functions (i.e., electron production, transportation and consumption). Moreover, under anaerobic condition, the method about efficient nitrate and Cr(VI) removal remained unclear. In this paper, the addition of Shewanella oneidensis MR-1 to promote the electron production, transportation and consumption of denitrifier and cause an increase in the removal of nitrate and Cr(VI). The efficiency of nitrate and Cr(VI) removal accomplished by P. denitrificans as a used model denitrifier increased respectively from 51.3% to 96.1% and 34.3% to 99.8% after S. oneidensis MR-1 addition. The mechanism investigations revealed that P. denitrificans provided S. oneidensis MR-1 with lactate, which was utilized to secreted riboflavin and phenazine by S. oneidensis MR-1. The riboflavin served as coenzymes of cellular reductants (i.e., thioredoxin and glutathione) in P. denitrificans, which created favorable intracellular microenvironment conditions for electron generation. Meanwhile, phenazine promoted biofilm formation, which increased the adsorption of Cr(VI) on the cell surface and accelerated the Cr(VI) reduction by membrane bound chromate reductases thereby reducing damage to other enzymes respectively. Overall, this strategy reduced the negative effect of chromate, thus improved the generation, transportation, and consumption of electrons. SYNOPSIS: The presence of S. oneidensis MR-1 facilitated nitrate and Cr(VI) removal by P. denitrificans through decreasing the negative effect of chromate due to the metabolites' secretion.

Salinity stress results in ammonium and nitrite accumulation during the elemental sulfur-driven autotrophic denitrification process

In this study, we investigated the effects of salinity on elemental sulfur-driven autotrophic denitrification (SAD) efficiency, and microbial communities. The results revealed that when the salinity was ≤6 g/L, the nitrate removal efficiency in SAD increased with the increasing salinity reaching 95.53% at 6 g/L salinity. Above this salt concentration, the performance of SAD gradually decreased, and the nitrate removal efficiency decreased to 33.63% at 25 g/L salinity. Approximately 5 mg/L of the hazardous nitrite was detectable at 15 g/L salinity, but decreased at 25 g/L salinity, accompanied by the generation of ammonium. When the salinity was ≥15 g/L, the abundance of the salt-tolerant microorganisms, Thiobacillus and Sulfurimonas, increased, while that of other microbial species decreased. This study provides support for the practical application of elemental sulfur-driven autotrophic denitrification in saline nitrate wastewater.

Microbial co-culture mediated by intercellular nanotubes during DMAC degradation: Microbial interaction, communication mode, and degradation mechanism

Microbial co-culture has been proven as an effective technique for environmental remediation. In this study, co-culture mechanism of Rhodococcus ruber HJM-8 and Paracoccus communis YBH-X during N,N-dimethylacetamide (DMAC) degradation was studied. The comparison of degradation performance in monoculture and co-culture was presented; due to the efficient cooperation between the two strains via parallel and cascaded degradation, the removal efficiency of total nitrogen (TN) in co-culture could reach 90.1%, which was 1.35 and 1.21 times higher than that of HJM-8 and YBH-X, respectively. Then the communication mode of co-culture during DMAC degradation was determined as contact-independent and contact-dependent interactions between microorganisms. Meanwhile, intercellular nanotube between HJM-8 and YBH-X was found as a unique contact-dependent interaction. The cell staining experiments and RNA sequencing analyses revealed that the nanotube could be used as a bridge to exchange cytoplasmic molecules, and thus improved material transfer and enhanced cell connection in co-culture. The results of KEGG pathway showed that differentially expressed genes in co-culture have an association with cell metabolism, nanotube generation, and genetic material transfer. Furthermore, a mechanism diagram of DMAC biodegradation was proposed for co-culture, indicating that bidirectional cooperation was established between HJM-8 and YBH-X which was mediated by the conversions of acetate and nitrogen. Finally, the co-culture system was validated for treatment of an actual wastewater; results indicated that removal efficiencies of 100% and 68.2% were achieved for DMAC and TN, respectively, suggesting that co-culture had the potential for application.

Aerobic Denitrification Promoting by Actinomycetes Coculture: Investigating Performance, Carbon Source Metabolic Characteristic, and Raw Water Restoration

The coculture theory that promotes denitrification relies on effectively utilizing the resources of low-efficiency denitrification microbes. Here, the strains Streptomyces sp. PYX97 and Streptomyces sp. TSJ96 were isolated and showed lower denitrification capacity when cultured individually. However, the coculture of strains PYX97 and TSJ96 enhanced nitrogen removal (removed 96.40% of total nitrogen) and organic carbon reduction (removed 92.13% of dissolved organic carbon) under aerobic conditions. Nitrogen balance analysis indicated that coculturing enhanced the efficiency of nitrate converted into gaseous nitrogen reaching 70.42%. Meanwhile, the coculturing promoted the cell metabolism capacity and carbon source metabolic activity. The coculture strains PYX97 and TSJ96 thrived in conditions of C/N = 10, alkalescence, and 150 rpm shaking speed. The coculturing reduced total nitrogen and CODMn in the raw water treatment by 83.32 and 84.21%, respectively. During this treatment, the cell metabolic activity and cell density increased in the coculture strains PYX97 and TSJ96 reactor. Moreover, the coculture strains could utilize aromatic protein and soluble microbial products during aerobic denitrification processes in raw water treatment. This study suggests that coculturing inefficient actinomycete strains could be a promising approach for treating polluted water bodies.

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.

Extracellular electron transfer for aerobic denitrification mediated by the bioelectric catalytic system with zero-carbon source

The slow rate of electron transfer and the large consumption of carbon sources are technical bottlenecks in the biological treatment of wastewater. Here, we first proposed to domesticate aerobic denitrifying bacteria (ADB) from heterotrophic to autotrophic by electricity (0.6 V) under zero organic carbon source conditions, to accelerate electron transfer and shorten hydraulic retention time (HRT) while increasing the biodegradation rate. Then we investigated the extracellular electron transfer (EET) mechanism mediated by this process, and additionally examined the integrated nitrogen removal efficiency of this system with composite pollution. It was demonstrated that compared with the traditional membrane bioreactor (MBR), the BEC displayed higher nitrogen removal efficiency. Especially at C/N = 0, the BEC exhibited a NO3--N removal rate of 95.42 ± 2.71 % for 4 h, which was about 6.5 times higher than that of the MBR. Under the compound pollution condition, the BEC still maintained high NO3--N and tetracycline removal (94.52 ± 2.01 % and 91.50 ± 0.001 %), greatly superior to the MBR (10.64 ± 2.01 % and 12.00 ± 0.019 %). In addition, in-situ electrochemical tests showed that the nitrate in the BEC could be directly converted to N2 by reduction using electrons from the cathode, which was successfully demonstrated as a terminal electron acceptor.

Metabolite Cross-Feeding Promoting NADH Production and Electron Transfer during Efficient SMX Biodegradation by a Denitrifier and S. oneidensis MR-1 in the Presence of Nitrate

Antibiotics often coexist with other pollutants (e.g., nitrate) in an aquatic environment, and their simultaneous biological removal has attracted widespread interest. We have found that sulfamethoxazole (SMX) and nitrate can be efficiently removed by the coculture of a model denitrifier (Paracoccus denitrificans, Pd) and Shewanella oneidensis MR-1 (So), and SMX degradation is affected by NADH production and electron transfer. In this paper, the mechanism of a coculture promoting NADH production and electron transfer was investigated by proteomic analysis and intermediate experiments. The results showed that glutamine and lactate produced by Pd were captured by So to synthesize thiamine and heme, and the released thiamine was taken up by Pd as a cofactor of pyruvate and ketoglutarate dehydrogenase, which were related to NADH generation. Additionally, Pd acquired heme, which facilitated electron transfer as heme, was the important composition of complex III and cytochrome c and the iron source of iron sulfur clusters, the key component of complex I in the electron transfer chain. Further investigation revealed that lactate and glutamine generated by Pd prompted So chemotactic moving toward Pd, which helped the two bacteria effectively obtain their required substances. Obviously, metabolite cross-feeding promoted NADH production and electron transfer, resulting in efficient SMX biodegradation by Pd and So in the presence of nitrate. Its feasibility was finally verified by the coculture of an activated sludge denitrifier and So.

Insight into the enhancing mechanism of exogenous electron mediators on biological denitrification in microbial electrolytic cell

Sustained nitrate accumulation in surface water ecosystem was continuously grabbing public attention. Autotrophic denitrification by electron supplement has been applied to overcome the requirement of carbon source, thus the new problem that how to improve the efficiency of extracellular electrons transfer to denitrifiers comes to us. The addition of exogenous electron mediators has been considered as an important strategy to promote extracellular electrons transfer in reductive metabolism. To date, knowledge is lacking about the promoting effects and pathways in nitrate removal by electron mediators. Here, we fully investigated the performance of nitrogen removal as well as quantified the characteristics of biofilms with six electron mediators (riboflavin, flavin mononucleotide, AQS, AQDS, biochar and Nano-Fe3O4) treating in microbial electrolytic cell system. The six electron mediators promoted nitrate removal rate by 76.03-90.43 % with electron supplement. The growth and activity of cathodic biofilm, conductive nanowires generation and electrochemically active substance synthesis of extracellular polymeric substances were facilitated by electron mediator addition. Electrochemical analysis revealed that conductivity and redox capacity of cathodic biofilm was increased for accelerating electron transfer. Moreover, they upregulated the abundance of denitrifying communities and denitrifying genes accordingly. Their denitrification efficiency varied due to their promotion ability in the above different strategies and conductive characteristics, and the efficiency could be concluded as: Nano-Fe3O4 > riboflavin > flavin mononucleotide > AQS ≈ AQDS > biochar. This study revealed how addition of electron mediators promoted denitrification with electron supplement, and compared their promoting efficiency in several main aspects.

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.

Biochar boosts nitrate removal in constructed wetlands for secondary effluent treatment: Linking nitrate removal to the metabolic pathway of denitrification and biochar properties

Constructed wetlands (CWs) amended with biochar have attracted much attention for nitrate removal treating secondary effluent. However, little is acknowledged about the linkage among the nitrate removal performance, microbial metabolic pathway of nitrate, and biochar properties. Herein, biochars pyrolyzed under 300 °C, 500 °C, and 700 °C (BC300, BC500, and BC700, respectively) were used in CWs to reveal the relationship. Results showed that CWs amended with BC300 (59.73%), BC500 (53.27%), and BC700 (49.07%) achieved higher nitrogen removal efficiency, compared with the control (39.51%). Metagenomic analysis showed that biochars could enrich the genes, which encoded key enzymes (adenosine triphosphate production, and electrons generation, transportation, and consumption) involved in carbon and nitrate metabolism. Further, biochar pyrolyzed under lower temperature, with higher oxygen content, molar O/C ratio, and the electron donating capacity, in CWs could obtain higher nitrate removal efficiency. Overall, this research offers new understandings for the promotion of denitrification in CWs amended with biochar.

Promoted aerobic denitrification through denitrifying fungal communities: Co-occurrence patterns and treatment of low C/N micro-polluted water

Despite the growing interest in using mixed-culture aerobic denitrifying fungal flora (mixed-CADFF) for water remediation, there is limited research on their nitrogen removal performance in low C/N polluted water bodies. To address this knowledge gap, we isolated three mixed-CADFFs from overlying water in urban lakes to evaluate their removal performance. The total nitrogen (TN) removal efficiencies were 93.60 %, 94.64 %, and 95.18 %, while the dissolved organic carbon removal efficiencies were 96.64 %, 95.12 %, and 96.70 % for mixed-CADFF LN3, LN7, and LN15, respectively in the denitrification medium under aerobic conditions at 48 h cultivation. The three mixed-CADFFs could utilize diverse types of low molecular weight carbon sources to drive the aerobic denitrification processes efficiently. The optimal C/N ratio for the mixed-CADFFs were C/N = 10, and then C/N = 15, 7, 5, and 2. The high-throughput sequencing analysis of three mixed-CADFFs indicated that Eurotiomycetes, Cystobasidiomycetes, and Sordariomycetes were the dominant class in the communities at class level. The network analysis showed that the rare fungal species, such as Scedosporium dehoogii Saitozyma, and Candida intermedia presented positively co-occurred with the TN removal and organic matter reduction capacity. Immobilization mixed-CADFFs treatment raw water experiments indicated that three mixed-CADFFs could reduce nearly 62.73 % of TN in the low C/N micro-polluted raw water treatment. Moreover, the cell density and cell metabolism indexes were also increased during the raw water treatment. This study will provides new insight into resource utilization of the mixed-culture aerobic denitrifying fungal community in field of environment restoration.

Treatment of micro-polluted water with low C/N ratio by immobilized bioreactor using PVA/sintered ores@sponge cube: Performance effects and potential removal pathways

The widespread use of industrial products containing lead (Pb²⁺) and tetracycline (TC) medications led to the combined pollution of nitrate, Pb²⁺, and TC in water. A novel biomaterial containing polyvinyl alcohol (PVA) and sponge cube with sintered ores (PVA/sintered ores@sponge cube) was prepared to ensure the maximum NO₃^--N removal efficiency (96.21 %) of the bioreactor under the hydraulic retention time (HRT) of 7.0 h, pH of 6.0, and the carbon to nitrogen (C/N) of 1.5 that had the ability to remove TC and Pb²⁺ synergistically. Composite pollutants slightly decreased denitrification performance in the combined pollution system on account of the addition of sintered ores. Results of scanning electron microscopy (SEM) showed that the sintered ores in the biocarrier induced denitrification and the adsorption of bio‑iron oxides were involved in the removal of TC and Pb²⁺. The simultaneous removal of composite pollutants during denitrification was facilitated by extracellular polymeric substances (EPS) as revealed by Fourier transform infrared spectroscopy (FTIR) and fluorescence excitation-emission matrix (EEM). In addition, high-throughput sequencing results showed that Zoogloea had the highest proportion in the bioreactor.

Insights to Fe(II) on the fate of humic acid and humic acid Fe complex with biogeobattery effect in simultaneous partial nitritation, anammox and denitrification (SNAD) system

The role of Fe(II) on the humic acid (HA) transformation and the effects of humic acid Fe (HA-Fe) on simultaneous partial nitrification, anammox and denitrification (SNAD) system were investigated. After adding Fe(II), the HA content decreased and the HA inhibition on the SNAD system was released. Results showed that Fe(II) and HA formed the lower water-soluble HA-Fe, promoting the HA removal. HA-Fe with stronger electron transfer capacity constituted the interface with microorganisms to forming the biogeobattery effect. This accelerated the microbial electron transfer, as well as improved the key enzymes and ATP, indicating that HA-Fe stimulated the microbial activity of the SNAD system. Microbial community and quorum sensing analysis further demonstrated that HA-Fe enhanced the mutual symbiosis between electroactive and nitrogen removal bacteria, to ensure the stability of the SNAD system. The study provided references for efficient HA removal and revealed the biogeobattery effect of HA-Fe in the SNAD system.

Goethite and riboflavin synergistically enhance Cr(VI) reduction by Shewanella oneidensis MR-1

Bioreduction of Cr(VI) is cost-effective and environmentally friendly, however, the slow bioreduction rate limits its application. In this study, the potential synergistic enhancement of Cr(VI) bioreduction by shewanella oneidensis MR-1 (S. oneidensis) with goethite and riboflavin (RF) was investigated. The results showed that the S. oneidensis reaction system reduce 29.2% of 20 mg/L Cr(VI) after 42 h reaction, while the S. oneidensis/goethite/RF reaction system increased the Cr(VI) reduction rate to 87.74%. RF as an efficient electron shuttle and Fe(II) from goethite bioreduction were identified as the crucial components in Cr(VI) reduction. XPS analysis showed that the final precipitates of Cr(VI) reduction were Cr(CH₃C(O)CHC(O)CH₃)₃ and Cr₂O₃ and adhered to the bacterial cell surface. In this process, the microbial surface functional groups such as hydroxyl and carboxyl groups participated in the adsorption and reduction of Cr(VI). Meanwhile, an increase in cytochrome c led to an increase in electron transfer system activity (ETSA), causing a significant enhancement in extracellular electron transfer efficiency. This study provides insight into the mechanism of Cr(VI) reduction in a complex environment where microorganisms, iron minerals and RF coexist, and the synergistic treatment method of Fe(III) minerals and RF has great potential application for Cr(VI) detoxification in aqueous environment.

Insight into the Cold Adaptation Mechanism of an Aerobic Denitrifying Bacterium: Bacillus simplex H-b

Psychrophilic bacteria with aerobic denitrification ability have promising potential for application in nitrogen-contaminated wastewater treatment, especially under cold conditions. A better understanding of the cold adaptation mechanism during aerobic denitrification would be beneficial for the practical application of this type of functional bacterium. In this study, Bacillus simplex H-b with good denitrification performance at 5°C was used to investigate the corresponding cold tolerance mechanism. Transcriptomics and nitrogen removal characterization experiments were conducted at different temperatures (5°C, 20°C, and 30°C). At low temperatures, more nitrogen was utilized for assimilation, accompanied by the accumulation of ATP and extracellular polymeric substances (EPS), rather than transforming inorganic nitrogen in the dissimilation pathway. In addition, the proportion of unsaturated fatty acids was higher in strains cultured at low temperatures. At the molecular level, the adjustment of membrane transport, synthesis of cofactors and vitamins, and transcriptional regulators might contribute to the survival of the strain under cold conditions. Moreover, nucleotide precursor synthesis, translation, and oxidative and temperature stress response mechanisms also enhanced the resistance of strain H-b to low temperatures. The results suggest that combining multiple regulatory mechanisms and synergistic adaptation to cold stress enabled the growth and relatively high nitrogen removal rate (27.22%) of strain H-b at 5°C. By clarifying the mechanism of regulation and cold resistance of strain H-b, a theoretical foundation for enhancing the application potential of this functional bacterium for nitrogen-contaminated wastewater treatment was provided. IMPORTANCE The newly isolated aerobic denitrifying bacterium Bacillus simplex H-b removed various forms of inorganic nitrogen (nitrate, nitrite, and ammonium) from wastewater, even when the temperature was as low as 5°C. Although this environmentally functional bacterium has been suggested as a promising candidate for nitrogen-contaminated water treatment at low temperatures, understanding its cold adaptation mechanism during aerobic denitrification is limited. In this study, the cold tolerance mechanism of this strain was comprehensively explained. Furthermore, a theoretical basis for the practical application of this type of functional bacterium for nitrogen removal in cold regions is provided. The study expands our understanding of the survival strategy of psychrophilic bacteria and hence supports their further utilization in wastewater treatment applications.

Novel synergistic metabolic processes for phenanthrene biodegradation by a nitrate-reducing phenanthrene-degrading culture and an anammox culture

The synergistic metabolism by anammox cultures and nitrate-reducers for anaerobic PAH biodegradation is largely unknown, including whether anammox culture and which kind of anammox bacterium can perform nitrogen metabolism in the anaerobic PAH biodegradation processes, the inhibitory effect of PAH on anammox activity and nitrite on PAH-degrading nitrate-reducers activity, and the synergistic metabolic processes. Herein, an anammox culture that can eliminate nitrite accumulation and decrease inorganic carbon emission during anaerobic phenanthrene (a model of PAH in this study) biodegradation, the synergistic mechanism for phenanthrene biodegradation by a nitrate-reducer and such anammox culture, and the inhibition effect of phenanthrene on such anammox culture and nitrite on a phenanthrene-degrading nitrate-reducer were newly discussed. The results showed that nitrite largely accumulated during anaerobic phenanthrene biodegradation (nitrate accumulation is a common phenomenon for the biodegradation of refractory matter, including PAHs, by nitrate-reducers) by a nitrate-reducer, PheN2, which mineralizes phenanthrene to inorganic carbon, and nitrite was verified as an inhibiting factor for further biodegradation. Anaerobic phenanthrene biodegradation rates and nitrite concentrations (0-7 mM) appeared to have a negative linear correlation. The anammox culture that mainly contained Candidatus Kuenenia was newly found to efficiently reduce nitrite accumulation and inorganic carbon emissions and significantly promote biodegradation efficiency by ∼1.94-fold. Our results showed that phenanthrene absorbed in and on anammox cells had a more direct relationship with the inhibitory effect on anammox activity than phenanthrene in the environment, and 15.2 mg/gVSS phenanthrene absorbed in and on the cells (4 mM concentration in the culture) showed nearly complete inhibition of anammox culture in this study. In addition, few (less than 2% abundance) anammox bacteria were found to be enough for the removal of nitrite produced from anaerobic phenanthrene biodegradation. In an ideal world, co-pollutants of ammonia, nitrate, phenanthrene, and nitrite could be converted to nitrogen gas and biomass by the synergistic metabolism of anammox cultures and nitrate reducers. Our study reveals a new synergistic process that may exist in our environments for PAH elimination by an anammox culture and a nitrate-reducer, which provides a new strategy for the bioremediation of PAH-polluted anoxic zones.

Mechanistic insights into polyhydroxyalkanoate-enhanced denitrification capacity of microbial community: Evolution of community structure and intracellular electron transfer of nitrogen metabolism

Denitrification is the key driving force of nitrogen cycle in surface water and plays an important role in eutrophication water remediation. Compared with some other common carbon sources, such as glucose and sodium acetate, polyhydroxyalkanoates (PHAs) were found to have the distinguished advantages in screening specific denitrifying bacteria of natural surface water bodies. In this study, the large ensembles of taxa were obtained from surface water samples and then sub-cultured with PHA or glucose as the sole carbon source. The microbial community that could be screened by PHA was identified, and the environmental functions of these bacteria were analyzed. At the genus level, the main communities regulated by PHA included Pseudomonas (56.30 %), Acinetobacter (27.75 %), Flavobacterium (10.19 %) and Comamonas (3.14 %), which all had good denitrification ability. The changes in carbon source, nitrogen source and biomass (expressed by DNA) were simultaneously monitored when culturing the model strain (P. stuzeri) with PHA or glucose. Compared with the glucose group, less PHA was consumed to remove the same amount of nitrate within a shorter incubation time, and there was no significant difference in bacterial growth with PHA or glucose as the carbon source (glucose:ΔN:ΔC:ΔDNA = 1:18:0.072; PHA:ΔN:ΔC:ΔDNA = 1:11:0.063). PHA improved the denitrification efficiency by increasing the expression of NarGHI, NirB, NirK and NorB, i.e., the key enzymes in the denitrification process. In addition, PHA accelerated the assimilating rate of extracellular nitrate by bacteria through increasing the expression of NarK. Finally, PHA-regulated electron transfer during denitrification was studied by observing the changes in NADH and NAD⁺. PHA could use a large proportion of NADH to offer electrons for denitrification, which increased the rate of denitrification. Improved mechanistic insights into the PHA-enhanced denitrification capacity of the microbial community can provide novel options for the in-situ remediation of eutrophic surface water.

Accelerating denitrification and mitigating nitrite accumulation by multiple electron transfer pathways between Shewanella oneidensis MR-1 and denitrifying microbial community

The bio-denitrification was usually retarded by the unbalance of electron generation and consumption. In this study, mixing S. oneidensis MR-1 with denitrifying microbial community increased the nitrogen removal rate by 74.74 % via the interspecies electron transfer (IET), and reduced the accumulated nitrite from 9.90 ± 0.81 to 0.02 ± 0.03 mg/L. Enhanced denitrification still appeared but relatively decreased, when S. oneidensis MR-1 was separated by a dialysis bag (MW < 3000), indicating mediated interspecies electron transfer (MIET) counted in IET. The results of electron transfer activity and sludge conductivity suggested DIET and MIET jointly transfer electrons from MR-1 to electroactive denitrifying bacteria (EDB), improving denitrifying reductase activities. Electron distribution among denitrifying reductases was found to be associated with the IET rate. Microbial insights showed the total abundance of EDB was increased, and denitrifying genes were correspondingly enriched. Pseudomonas was found to cooperate with exoelectrogens in a complicated microbial community.

Characterization of non-taste & odor produced aerobic denitrification actinomycetes strains Streptomyces spp. isolated from reservoir ecosystem: Denitrification performance and carbon source metabolism

The aerobic denitrification performance of actinomycetes was investigated. Two strains of actinomycetes were isolated and identified as Streptomyces sp. LJH-12-1 and Streptomyces diastatochromogenes LJH-12-2. Strain LJH-12-1 could remove 94% of organic carbon and 91% of total nitrogen. Meanwhile, strain LJH-12-2 could reduce 96% of organic carbon and 93% of total nitrogen. Two strains of actinomycetes revealed excellent carbon source metabolism activity. Moreover, the total nitrogen removal efficiencies were 69%, and 54%, respectively for strains LJH-12-1, and LJH-12-2 during the micro-polluted landscape raw water treatment. Futhermore, strains LJH-12-1 and LJH-12-2 could utilize aromatic proteins, soluble microbial products, and humic acid to drive aerobic denitrification processes in the landscape water bodies. These results will provide a new insight into applying aerobic denitrification actinomycetes to treat micro-polluted water bodies.

Bioaugmentation performance for moving bed biofilm reactor (MBBR) treating mariculture wastewater by an isolated novel halophilic heterotrophic nitrification aerobic denitrification (HNAD) strain (Zobellella B307)

Moving bed biofilm reactor (MBBR) demonstrates weak nitrogen removal for mariculture wastewater treatment under high salinity environment. An isolated novel halophilic heterotrophic nitrification aerobic denitrification (HNAD) strain (Zobellella B307) was applied in MBBR process to enhance nitrogen removal. Results showed that strain Zobellella B307 could remove 90.9% ammonia nitrogen (NH₄⁺-N) and 97.1% nitrate nitrogen (NO₃^--N) after 10 h cultivation, and strong resistance to salinity variation (high growth and nitrogen removal efficiency with salinity of 65‰) was observed. Besides, the chemical oxygen demand (COD), NH₄⁺-N and NO₃^--N removal reached 95.6%, 94.4% and 85.7% with the strain added into MBBR process. In addition, microbial community structure analysis reflected that the strain Zobellella B307 successfully proliferated (the relative abundance increased to 2.33%). The HNAD bacteria abundance increased and dominated during the nitrogen removal process with the strain inoculation. A microbial functional analysis revealed that the main dominant functional categories (carbohydrate metabolism and amino acid metabolism) increased with the bioaugmentation of strain Zobellella B307, thus improving the nitrogen removal.

Effects of sponge iron dosage on nitrogen removal performance and microbial community structure in sequencing batch reactors

The application of sponge iron (SI) carriers can improve the biochemical treatment performance of sequencing batch reactors (SBR) during wastewater treatment. This study used SBR reactors to explore the effects of SI dosage on the nitrogen removal performance and reactor stability and microbial community structure under low temperature and ultra-low load. In contrast to conventional SBR, the average removal rate of total nitrogen (TN) in the biological sponge iron system (BSIS) was increased by 5.38 % for 45 g/L, 18.93 % for 90 g/L, and 13.52 % for 135 g/L, respectively. The nitrogen removal performance and reactor stability showed the best performance under the SI dosage of 90 g/L. The addition of SI formed the anaerobic-anoxic-aerobic microenvironments, which facilitate the propagation of denitrifying bacteria (Saccharimonadales, Hydrogenophaga) and iron bacteria (Rhodoferax and Acinetobacter) in the BSIS. This study provides a new insight on the application of SI in the wastewater treatment.

Propionibacterium freudenreichii-Assisted Approach Reduces N2O Emission and Improves Denitrification via Promoting Substrate Uptake and Metabolism

N₂O emission is often encountered during biodenitrification. In this paper, a new approach of using microorganisms to promote substrate uptake and metabolism to reduce denitrification intermediate accumulation was reported. With the introduction of Propionibacterium freudenreichii to a biodenitrification system, N₂O and nitrite accumulation was, respectively, decreased by 74 and 60% and the denitrification efficiency was increased by 150% at the time of 24 h with P. freudenreichii/groundwater denitrifier of 1/5 (OD₆₀₀). Propionate, produced by P. freudenreichii, only accelerated nitrate removal and was not the main reason for the decreased intermediate accumulation. The proteomic and enzyme analyses revealed that P. freudenreichii stimulated biofilm formation by upregulating proteins involved in porin forming, putrescine biosynthesis, spermidine/putrescine transport, and quorum sensing and upregulated transport proteins, which facilitated the uptake of the carbon source, nitrate, and Fe and Mo (the required catalytic sites of denitrification enzymes). Further investigation revealed that P. freudenreichii activated the methylmalonyl-CoA pathway in the denitrifier and promoted it to synthesize heme/heme d1, the groups of denitrification enzymes and electron transfer proteins, which upregulated the expression of denitrifying enzyme proteins and enhanced the ratio of NosZ to NorB, resulting in the increase of generation, transfer, and consumption of electrons in biodenitrification. Therefore, a significant reduction in the denitrification intermediate accumulation and an improvement in the denitrification efficiency were observed.

Different responses of representative denitrifying bacterial strains to gatifloxacin exposure in simulated groundwater denitrification environment

The impact of antibiotics on denitrification in the ecological environment has attracted widespread attention. However, the concentration threshold and inhibitory effect of the same antibiotic on denitrification mediated by mixed denitrifying microbes were conflicting in some studies. In this study, Paracoccus denitrificans, Acidovorax sp., and Pseudomonas aeruginosa were selected as representative denitrifying bacterial strains to explore the response of a single strain to gatifloxacin (GAT) exposure in groundwater denitrification. The results showed that the nitrate and nitrite removal efficiencies of Pseudomonas aeruginosa decreased by 34.87-36.25 % and 18.27-23.31 %, respectively, with exposure to 10 μg/L GAT, accompanied by a significant decline in denitrifying enzyme activity and gene expression. In contrast, the elevated denitrifying enzyme activity and gene expression of Paracoccus denitrificans promoted its nitrate and nitrite reduction by 2.09-10.00 % and 0-8.44 %, respectively. Additionally, there were no obvious effects on the removal of nitrate and nitrite by Acidovorax sp. in the presence of 10 μg/L GAT, which was consistent with the variation in denitrifying enzyme activity and total gene expression levels. The fit results of the Monod equation and its modification further elucidated the nitrate degradation characteristics from the perspective of denitrification kinetics. Furthermore, antibiotic resistance gene (ARG) analysis showed that the addition of 10 μg/L GAT (approximately 30 days) did not observably increase the relative abundance of ARGs. This study provides some preliminary understanding of the response differences of representative denitrifying bacterial strains to antibiotic exposure in groundwater denitrification.

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.

Influences of carbon sources on N2O production during denitrification in freshwaters: Activity, isotopes and functional microbes

Denitrification is one of the major sources of N₂O in freshwaters. Diverse forms of organic compounds act as the electron donors for microbial denitrification. However, the influences of carbon sources on N₂O production, N₂O reduction, isotope fractionation and functional microbes during denitrification were largely unknown. In this study, five forms of carbon sources (i.e. acetate, citrate, glucose, cellobiose and leucine) were used to enrich denitrifiers in freshwater sediments. N₂O conversion in the enrichments was investigated by a combination of inhibition technique, natural stable isotope method and metagenomics. Acetylene was effective in inhibiting N₂O reduction without influencing the isotopic characteristics during N₂O production. Glucose led to the least N₂O production and reduction, in accordance with the lowest abundance of both NO and N₂O reductases in this enrichment. δ¹⁸O and site preference value (SP, =δ¹⁵N^α-δ¹⁵N^β) of N₂O were sensitive to discriminate the five carbon sources, except when comparing acetate and leucine. Isotopic values of N₂O were not significantly different in these two enrichments due to the similarity of NO reductases - Pseudomonas-type cNorB. Specifically, the enrichment with cellobiose produced N₂O with the lowest δ¹⁸O values (39.4‰±1.1‰), due to Alicycliphilus with both cNorB and qNorB. The enrichment with glucose led to the highest SP values (8.9‰±8.6‰), caused by Thiobacillus-type cNorB. Our results demonstrated the link between carbon sources, N₂O production and reduction, isotopic signatures, microbial populations and enzymes during denitrification in freshwaters.

Versatile mechanisms and enhanced strategies of pollutants removal mediated by Shewanella oneidensis: A review

The removal of environmental pollutants is important for a sustainable ecosystem and human health. Shewanella oneidensis (S. oneidensis) has diverse electron transfer pathways and can use a variety of contaminants as electron acceptors or electron donors. This paper reviews S. oneidensis's function in removing environmental pollutants, including heavy metals, inorganic non-metallic ions (INMIs), and toxic organic pollutants. S. oneidensis can mineralize o-xylene (OX), phenanthrene (PHE), and pyridine (Py) as electron donors, and also reduce azo dyes, nitro aromatic compounds (NACs), heavy metals, and iodate by extracellular electron transfer (EET). For azo dyes, NACs, Cr(VI), nitrite, nitrate, thiosulfate, and sulfite that can cross the membrane, S. oneidensis transfers electrons to intracellular reductases to catalyze their reduction. However, most organic pollutants cannot be directly degraded by S. oneidensis, but S. oneidensis can remove these pollutants by self-synthesizing catalysts or photocatalysts, constructing bio-photocatalytic systems, driving Fenton reactions, forming microbial consortia, and genetic engineering. However, the industrial-scale application of S. oneidensis is insufficient. Future research on the metabolism of S. oneidensis and interfacial reactions with other materials needs to be deepened, and large-scale reactors should be developed that can be used for practical engineering applications.

Interactive effects of aquatic nitrogen and plant biomass on nitrous oxide emission from constructed wetlands

Understanding of mechanisms in nitrous oxide (N₂O) emission from constructed wetland (CW) is particularly important for the establishment of related strategies to reduce greenhouse gas (GHG) production during its wastewater treatment. However, plant biomass accumulation, microbial communities and nitrogen transformation genes distribution and their effects on N₂O emission from CW as affected by different nitrogen forms in aquatic environment have not been reported. This study investigated the interactive effects of aquatic nitrogen and plant biomass on N₂O emission from subsurface CW with NH₄⁺-N (CW-A) or NO₃^--N (CW-B) wastewater. The experimental results show that NH₄⁺-N and NO₃^--N removal efficiencies from CW mesocosms were 49.4% and 87.6%, which indirectly lead to N₂O emission fluxes of CW-A and CW-B maintained at 213 ± 67 and 462 ± 71 μg-N/(m²·h), respectively. Correlation analysis of nitrogen conversion dynamic indicated that NO₂^--N accumulation closely related to N₂O emission from CW. Aquatic NH₄⁺-N could up-regulate plant biomass accumulation by intensifying citric acid cycle, glycine-serine-threonine metabolism etc., resulting in more nitrogen uptake and lower N₂O emission/total nitrogen (TN) removal ratio of CW-A compared to CW-B. Although the abundance of denitrifying bacteria and N₂O reductase nosZ in CW-B were significantly higher than that of CW-A, after fed with mixed NH₄⁺-N and NO₃^--N influent, N₂O fluxes and N₂O emission/TN removal ratio in CW-A were extremely close to that of CW-B, suggesting that nitrogen form rather than nitrogen transformation microbial communities and N₂O reductase nosZ determines N₂O emission from CW. Hence, the selection of nitrate-loving plants will play an important role in inhibiting N₂O emission from CW.

Increasing N,N-dimethylacetamide degradation and mineralization efficiency by co-culture of Rhodococcus ruber HJM-8 and Paracoccus communis YBH-X

In this work, Rhodococcus ruber HJM-8 and Paracoccus communis YBH-X were isolated and used to enhance N,N-dimethylacetamide (DMAC) degradation and mineralization efficiencies. The monoculture and co-culture of the two strains for DMAC degradation were compared; results indicated that, a degradation efficiency of 97.62% was obtained in co-culture, which was much higher than that of monocultures of HJM-8 (57.34%) and YBH-X (34.02%). The degradation mechanism showed that co-culture could efficiently improve extracellular polymeric substances production, electron transfer, and microbial activity. Meanwhile, the mineralization mechanism suggested that acetate was the dominant intermediate which had an inhibitory effect on HJM-8, and co-culture was conducive to mineralization due to the high performance of acetate conversion and Na⁺ K⁺-ATPase vitality. Besides, a pathway of DMAC biodegradation was proposed for co-culture: DMAC was degraded into acetate by HJM-8, then the accumulated acetate was mineralized by YBH-X. Additionally, the co-culture system was further optimized by Box-Behnken design.

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.

Shewanella sp. T2.3D-1.1 a Novel Microorganism Sustaining the Iron Cycle in the Deep Subsurface of the Iberian Pyrite Belt

The Iberian Pyrite Belt (IPB) is one of the largest deposits of sulphidic minerals on Earth. Río Tinto raises from its core, presenting low a pH and high metal concentration. Several drilling cores were extracted from the IPB’s subsurface, and strain T2.3D-1.1 was isolated from a core at 121.8 m depth. We aimed to characterize this subterranean microorganism, revealing its phylogenomic affiliation (Average Nucleotide Identity, digital DNA-DNA Hybridization) and inferring its physiology through genome annotation, backed with physiological experiments to explore its relationship with the Fe biogeochemical cycle. Results determined that the isolate belongs to the Shewanella putrefaciens (with ANI 99.25 with S. putrefaciens CN-32). Its genome harbours the necessary genes, including omcA mtrCAB, to perform the Extracellular Electron Transfer (EET) and reduce acceptors such as Fe³⁺, napAB to reduce NO₃⁻ to NO₂⁻, hydAB to produce H₂ and genes sirA, phsABC and ttrABC to reduce SO₃²⁻, S₂O₃²⁻ and S₄O₆²⁻, respectively. A full CRISPR-Cas 1F type system was found as well. S. putrefaciens T2.3D-1.1 can reduce Fe³⁺ and promote the oxidation of Fe²⁺ in the presence of NO₃⁻ under anaerobic conditions. Production of H₂ has been observed under anaerobic conditions with lactate or pyruvate as the electron donor and fumarate as the electron acceptor. Besides Fe³⁺ and NO₃⁻, the isolate also grows with Dimethyl Sulfoxide and Trimethyl N-oxide, S₄O₆²⁻ and S₂O₃²⁻ as electron acceptors. It tolerates different concentrations of heavy metals such as 7.5 mM of Pb, 5 mM of Cr and Cu and 1 mM of Cd, Co, Ni and Zn. This array of traits suggests that S. putrefaciens T2.3D-1.1 could have an important role within the Iberian Pyrite Belt subsurface participating in the iron cycle, through the dissolution of iron minerals and therefore contributing to generate the extreme conditions detected in the Río Tinto basin.

Cooperation triggers nitrogen removal and algal inhibition by actinomycetes during landscape water treatment: Performance and metabolic activity

The actinomycetes strain Streptomyces sp. XD-11-9-3 and Streptomyces sp. 5 were isolated and presented poor denitrification performance. Co-culture of actinomycetes triggers nitrogen removal capacity under aerobic conditions (reduced 96% of total nitrogen). Nitrogen balance analysis presented that 71% of initial nitrogen converted as gaseous nitrogen. Moreover, co-culture increased the concentrations of adenosine triphosphate (>2.1 folds) and electron-transmission system activity (>1.5 folds) significantly. The co-culture presented excellent carbon source metabolism activity (especially amines and carboxylic acids) compared with monoculture. The removal efficiency of total nitrogen in the micro-polluted landscape water water reached 61% in the co-culture system, and the algal survival could be inhibited significantly. However, the dominant niche of the co-culture system restrained the diversity of the indigenous nirS-type denitrifying bacterial community. This study provided a novel pathway to the research of co-culture inefficiency aerobic denitrifier and further application in the restoration of polluted water.

Biochar fungal pellet based biological immobilization reactor efficiently removed nitrate and cadmium

To efficiently and simultaneously remove nitrate (NO₃^--N) and Cd(II) from aqueous solution, a novel type of biochar fungal pellet (BFP) immobilized denitrification bacteria (Cupriavidus sp. H29) composite was used in a bioreactor. The removal performance of the bioreactor R1 for the initial concentration of 27.7 mg L^-1 nitrate and 10.0 mg L^-1 Cd(II) reached 98.1 and 93.9% respectively, and the inoculation of strain H29 in bioreactor R1 significantly enhanced the removal efficiency of contaminants. The 3D-EEM spectra analysis showed that the activity of microorganisms in the bioreactor was higher at a lower concentration of Cd(II). FTIR indicated the effect of functional groups in BFP in bioadsorption of Cd(II). In addition, high-throughput analysis of species composition of the microbial community in the bioreactors at different levels demonstrated that strain H29 played a significant part in the bioreactor. This research provided a perspective for simultaneous restoration of nitrate and heavy metals in wastewater, and also enriched the application of fungal pellet (FP) in reactors.

Inducing high nitrite accumulation via modulating nitrate reduction power and carbon flux with Thauera spp. selection

Partial-denitrification (PD, NO₃^--N → NO₂^--N) is emerging as a promising approach for application of anaerobic ammonium oxidation (anammox) process. In this study, stable PD with high nitrite (NO₂^--N) accumulation was achieved by modulating nitrate (NO₃^--N) reduction activity and carbon metabolism. With the influent NO₃^--N increasing from 30 to 200 mg/L, specific NO₃^--N reduction rates (r_no3) were significantly improved, corresponding to the nitrate-to-nitrite transforming ratio (NTR) increasing rapidly to 80.0% within just 70 days. The required COD/NO₃^--N decreased from 4.5 to 2.0 and the carbon flux was more shared in NO₃^--N reduction to NO₂^--N. Notably, Thauera spp. as core denitrifying bacteria was highly enriched with the relative abundance of 70.5%∼82.1% despite different inoculations. This study provided a new insight into inducing high NO₂^--N accumulation and promoting practical application of anammox technology.

Te(IV) bioreduction in the sulfur autotrophic reactor: Performance, kinetics and synergistic mechanism

A laboratory-scale sulfur autotrophic reactor (SAR) was first constructed for treating tellurite [Te(IV)] wastewater. The SAR had excellent Te(IV) bioreduction efficiency (90-96%) at 5-30 mg/L and conformed to the First-order kinetic model. The Te(IV) bioreduction was elaborated deeply from extracellular polymeric substances (EPS) functions, microbial metabolic activity, key enzyme activity, microbial community succession and quorum sensing. Te(IV) stimulated the increase of redox substances in EPS and the improved cell membrane permeability led to the increase of electron transport system activity. Catalase and reduced nicotinamide adenine dinucleotide (NADH) alleviated the oxidative stress caused by Te(IV) toxicity to maintain metabolic activity. The increase of sulfur dioxygenase activity (SDO) suggested that more ATP produced by sulfur oxidation might provide energy for various physiological activities. Meanwhile, nitrate reductase (NAR), nitrite reductase (NIR) and sulfide: quinone oxidoreductase (SQR) played an active role in sulfur oxidation and Te(IV) bioreduction. Combined with the above results and dynamic succession of three functional microbial communities, a synergistic mechanism was proposed to explain the excellent performance of SAR. This work provided a promising strategy for Te(IV) wastewater treatment process and Te(IV) bioreduction mechanism.

Biological removal of sulfamethoxazole enhanced by S. oneidensis MR-1 via promoting NADH generation and electron transfer and consumption

The bio-removal efficiency of sulfamethoxazole (SMX) from wastewater is usually very poor. In this paper a new efficient method to biodegrade SMX was reported. The SMX biodegradation efficiency by Paracoccus denitrificans was observed to be remarkably enhanced from 48.9% to 94.2% after Shewanella oneidensis MR-1 addition. The mechanisms investigation revealed that P. denitrificans was the dominant microbe for SMX biodegradation. Although SMX biodegradation by S. oneidensis MR-1 alone was negligible, its presence advanced NADH generation. The proteomics assay revealed that the expression of key proteins relevant with complex I and III and cytochrome c in electron transfer chain were increased due to P. denitrificans acquiring iron from periplasm to cytoplasm being improved. In addition, the extracellular electron transfer capability was enhanced as S. oneidensis MR-1 not only produced flavin, but caused P. denitrificans to secret more extracellular polymeric substances. Further investigation indicated that the expression of key enzymes related to electron consumption in SMX biodegradation was up-regulated. Based on these findings, the pathways of S. oneidensis MR-1 promoting SMX biodegradation were proposed. As all nitrate could be removed with almost no nitrite accumulation, this study would also provide an attractive way for simultaneous bio-removal of multiple pollutants from wastewater.

Enhancement of denitrification in biofilters by immobilized biochar under low-temperature stress

Efficient removal of nitrate under low temperature is challenging because of the reduction of the microbial activity. This study successfully explored the promotion on the performance of denitrification utilizing the immobilized biochar in biofilters under low temperature (6 ± 2 °C). The results showed that the immobilized biochar increased the denitrification rate by 76.8% and decreased the nitrous oxide emissions by 82.5%. Mechanistic studies revealed that the immobilized biochar increased the activities of the denitrifying enzymes and three enzymes involved in glycolysis. Furthermore, the immobilized biochar elevated the activity of the electron transport system by 31.8%. Finally, structural equation model explained that the increase of nitrate reductase activity was a crucial factor to enhance the total nitrogen removal efficiency in biofilters with immobilized biochar. Overall, the use of immobilized biochar can be a novel strategy to enhance nitrogen removal and reduce greenhouse gas emissions in biofilters under low temperature.

Nitrate removal by alkali-resistant Pseudomonas sp. XS-18 under aerobic conditions: Performance and mechanism

To improve poor nitrate removal by microorganisms under strong alkaline conditions, a new type of aerobic nitrification-reducing bacteria was isolated in this study. Using nitrogen balance and genome information, the capacity of Pseudomonas XS-18 to remove nitrate and the mechanism of alkali tolerance were analyzed. At pH 11.0, XS-18 could remove 12.17 mg N/(L·h) nitrate. At C/N ratios of 13.0 and 25 °C, nitrite and ammonia nitrogen were barely enriched. XS-18 could reduce nitrate through dissimilation and assimilation, and 21.74% and 77.39% of nitrate was converted into cellular components and organic nitrogen, respectively. Meanwhile, functional genes (nirBD, nasAB, gdhA, glnA, and gltBD) associated with nitrogen metabolism were determined. In addition, Na⁺/H⁺ antiporters (MnhACDEFG, PhaACDEFG, NhaCD and TrkAH) and a cell surface protein (SlpA) from the XS-18 genome, as well as compatible solutes that help stabilize intracellular pH, were also characterized. XS-18 possessed significant potential in alkaline wastewater treatment.

Enhanced thermophilic denitrification performance and potential microbial mechanism in denitrifying granular sludge system

Thermophilic biological nitrogen removal will provide low-cost strategies for the treatment of high-temperature nitrogenous wastewater (greater than 45 ℃). In this study, a thermophilic denitrifying granular sludge system was established at 50 ℃ and compared with mesophilic systems (30 ℃ and 40 ℃). The results showed a significant increase in COD and nitrate removal rate with the elevating temperature. Besides, the microbial community analysis indicated an obvious succession of key functional bacteria at different temperatures. Enriched thermophiles including Truepera, Azoarcus, and Elioraea were the dominant denitrifiers in the thermophilic denitrifying granular sludge system, which ensured the high nitrate removal at 50 ℃. Moreover, the functional gene prediction also denoted an enrichment of nitrate reduction genes and carbon metabolism pathways at 50 ℃, which could explain the enhancement of thermophilic denitrification. These findings could provide new insight into the application of denitrifying granular sludge in thermophilic wastewater treatment.

Performances and mechanisms of simultaneous nitrate and phosphate removal in sponge iron biofilter

Sponge iron is a potential material for nitrogen and phosphate removal. To explore the performances and mechanisms of nitrogen and phosphate removal by sponge iron, a sponge iron biofilter was established. The results showed that nitrate was completely removed at HRT of 48 h without external carbon source and at HRT of 3 h with C/N ratio of 5. Furthermore, it was easy to achieve partial denitrification at HRT of 1 h with C/N ratio of 3. The mechanisms of nitrate removal were chemical reduction and nitrate dependent ferrous oxidation without external carbon source and heterotrophic denitrification with external carbon source. XPS result indicated that phosphate was removed by the formation of ferric phosphate precipitation. High throughput sequencing showed that iron-oxidizing bacteria Gallionellaceae was highly enriched in biofilter, accounting for 17.83%, which indicated that it was feasible to achieve autotrophic nitrate removal by sponge iron biofilter.