Heterotrophic nitrification and aerobic denitrification by a novel Acinetobacter sp. TAC-1 at low temperature and high ammonia nitrogen

Molecular identification of heterotrophic nitrification and aerobic denitrification bacteria: From methods development to application demonstration

Although heterotrophic nitrification and aerobic denitrification (HN-AD) bacteria, a novel functional group involved in nitrogen conversion, have been isolated and characterized, the lack of specific molecular markers for identification severely limits the study of their role in geochemical cycling and the contribution in ecosystems. Here, a set of molecular markers was developed for the rapid identification of HN-AD bacteria, via delving into the genomics and transcriptomics of a HN-AD isolate (Pseudomonas aeruginosa SNDPR-01). Among the nine candidate genes that were significantly expressed during heterotrophic nitrification, three were involved in the conversion of hydroxylamine to nitrite, a characteristic process of HN-AD. The universality and stability of the identification methods based on the gene primer set were validated using pure HN-AD strains, mixed cultures of pure HN-AD strains, and activated sludge from laboratory-scale and real wastewater treatment plants. In all cases, the amplification outcome was positively correlated with the function and population of HN-AD bacteria, demonstrating its validity as a molecular marker. This study supports the paradigm of heterotrophic nitrification from hydroxylamine to nitrite. As an effective tool for the identification of classic HN-AD bacteria, this study lays the groundwork for research on environmental ecology and biotechnological application of HN-AD bacteria.

Bioaugmentation using HN-AD consortia for high salinity wastewater treatment: Synergistic effects of halotolerant bacteria and nitrogen removal bacteria

Bioaugmentation shows promise in enhancing nitrogen removal efficiency of high-salt wastewater, yet the impact of microbial associations on ecosystem function and community stability remains unclear. This study innovatively introduced a novel heterotrophic nitrification-aerobic denitrification bacterial consortium to improve the performance of SBR reactor for removing nitrogen from saline wastewater. The results revealed that the bioaugmented reactor (R2) exhibited superior removal performance, achieving maximum removal efficiencies of 87.8 % for COD and 97.8 % for NH4+-N. Moreover, proper salinity (2 % and 4 %) promoted the secretion of EPS and ectoine, further enhancing the resistance and stability of bacterial consortia. 16S rRNA gene sequencing and metagenomics analysis revealed the key denitrifying bacteria Pseudomonas and salt-tolerant bacteria Halomonas were successfully coexistence and the relative abundances of crucial genes (napB, nirS, norB, norC and nosZ) were increased obviously, which were benefit for the excellent nitrogen removal performance in R2. These findings elucidate microbial interactions in response to salinity in bioaugmentation, providing a valuable reference for the efficient treatment of high-saline wastewater.

Characterization and genomic insights into the nitrogen metabolism of heterotrophic nitrifying and aerobic denitrifying bacterium Pseudomonas aeruginosa WS-03

To achieve effective removal of various inorganic nitrogen in aquatic ecosystems, while expanding the applicability of existing heterotrophic nitrifying-aerobic denitrifying (HN-AD) strains and enhancing their stress tolerance, we isolated the Pseudomonas aeruginosa WS-03 from a sewage treatment plant. The results of parameter optimization indicated that the following were the most favorable conditions for nitrogen removal: using sodium citrate as the carbon source, a C/N ratio of 9, a pH of 7, a temperature of 30 °C and an NH4+-N concentrations below 300 mg/L. The maximum reduction rates of nitrogen are 8.96 mg/(L·h), 4.64 mg/(L·h) and 5.12 mg/(L·h) of NH4+-N, NO3--N and TN, respectively. The result of genome analysis and polymerase chain reaction (PCR) amplification electrophoresis revealed the presence of genes related to nitrogen metabolism, which involves nitrification, denitrification, and assimilation pathways. It also verified that absence of key nitrification genes in strain WS-03, suggesting it operates via a unique denitrification mechanism. Notably, nitrogen assimilation has been identified as the predominant pathway for nitrogen removal by the strain. The strain demonstrated an impressive efficiency of 54.28% in reducing the concentration of NH4+-N in untreated landfill leachate, highlighting its potential for application in practical wastewater treatment. This study comprehensively explored the denitrification characteristics and showed its significant role in environmental remediation.

Efficient ammonia oxidation by Pseudomonas citronellolis strain YN-21 under strongly acidic conditions: Performance and mechanism

Ammonia oxidation microorganisms generally tend to have low rates of ammonia oxidation under acidic conditions, as the protonated ammonia is not a substrate for ammonia monooxygenase. In this work, heterotrophic ammonia oxidation bacteria (HAOB) Pseudomonas citronellolis strain YN-21 showed high efficiency in removing NH4+ (12.7 mg/L/h) even at initial pH 4.5. The potential acid resistance mechanisms (H+ efflux, H+ consumption, and production of alkaline substances) maintained intracellular pH neutrality. Transcriptome analysis showed that genes involved in amino acid metabolism, carbohydrate metabolism, ABC transporter and nitrogen metabolism were significantly up-regulated, which facilitated the rapid removal of NH4+ in an acidic environment. Moreover, urea could be used as an alternative nitrogen source for YN-21 in a strongly acidic environment, and the production of NH3 from urea hydrolysis provided a substrate for ammonia oxidation. These results provide new insights into efficient ammonia oxidation in acidic environments.

A Novel Nitrite Reductase from Acinetobacter haemolyticus for Efficient Degradation of Nitrite

Nitrite reductases play a crucial role in the nitrogen cycle, demonstrating significant potential for applications in the food industry and environmental remediation, particularly for nitrite degradation and detection. In this study, we identified a novel nitrite reductase (AhNiR) from a newly isolated denitrifying bacterium, Acinetobacter haemolyticus YD01. We constructed a heterologous expression system using E. coli BL21/pET28a-AhNir, which exhibited remarkable nitrite reductase enzyme activity of 29 U/mL in the culture broth, substantially higher than that reported for other strains. Structural analysis of AhNiR revealed the presence of [Fe-S] clusters, with molecular docking studies identifying Tyr-282 and Ala-289 as key catalytic sites. The enzymatic properties of AhNiR demonstrated an optimal pH of 7.5 and an optimal catalytic temperature of 30 °C. Its kinetic parameters, Km and vmax, were 1.53 mmol/L and 10.18 mmol/min, respectively, fitting with the Michaelis-Menten equation. This study represents the first report of a nitrite reductase from a denitrifying bacterium, providing a new enzyme source for nitrite degradation applications in the food industry and environmental remediation, as well as for biosensing technologies aimed at nitrite detection.

Transcriptome analysis expands underlying mechanisms of quorum sensing mediating heterotrophic nitrification-aerobic denitrification process at low temperature

Quorum sensing (QS) could regulate the behavior of microbial communities and help them resist adverse low-temperature environments. A newly isolated heterotrophic nitrification-aerobic denitrification (HN-AD) bacterium, strain YB1107, exhibited strong tolerance to harsh cold environments, removing 93.5 % of ammonia within 36 h and achieving a maximum specific growth rate of 0.28 h-1 at 10 °C. Strain YB1107 secreted large amounts of N-butanoyl-L-homoserine or N-octanoyl-L-homoserine lactones in response to cold stimuli. The add-back experiments indicated that these two signaling molecules jointly manipulated microbial physiological behavior by improving ammonia oxidation and biofilm formation, while inhibiting aerobic denitrification. The transcriptome analysis revealed that QS systems enhanced the cold resistance of HN-AD bacteria by promoting nitrogen assimilation and reducing dissimilation through regulating related genes. This study provided new molecular insights into how QS mediated HN-AD at low temperatures and laid the foundation for the potential applications of psychrophilic HN-AD bacteria in wastewater treatment.

Enhancing nitrogen transformation and humification in cow manure composting through psychrophilic and thermophilic nitrifying bacterial consortium inoculation

Excessive nitrogen release during composting poses significant challenges to both the environment and compost quality. Biological enhancement of humification and nitrogen conservation is an environmentally friendly and cost-effective approach to composting. The aim of this study was to develop a psychrophilic and thermophilic nitrifying bacterial consortium (CNB) and investigate its role in nitrogen transformation and humification during cow manure composting. Analysis revealed that CNB inoculation promoted microbial proliferation and metabolism, significantly increased the number of nitrifying bacteria (p < 0.05), and elevated the activity of nitrite oxidoreductase and nxrA gene abundance. Compared to the control, CNB inoculation promoted the formation of NO3--N (77.87-82.35 %), while reducing NH3 (48.89 %) and N2O (20.05 %) emissions, and increased humus content (16.22 %). Mantel analysis showed that the higher abundance of nitrifying bacteria and nxrA facilitated the nitrification of NH4+-N. The improvement in nitrite oxidoreductase activity promoted NO3--N formation, leading to increased humus content and enhanced compost safety.

Unveiling cold Code: Acinetobacter calcoaceticus TY1's adaptation strategies and applications in nitrogen treatment

Overcoming low nitrogen removal efficiency at low temperatures is a challenge in biological treatment. This study investigated the cold-tolerant heterotrophic nitrification-aerobic denitrification by Acinetobacter calcoaceticus TY1. Transcriptomic and biochemical analyses indicated that strain TY1 upregulated genes for energy production, assimilation, cell motility, and antioxidant enzyme production under cold stress, maintaining functions such as energy supply, nitrogen utilization, and oxidative defense. Increasing the synthesis of extracellular polysaccharides, unsaturated fatty acids, and medium-chain fatty acids and secreting large amounts of antioxidant enzymes ensured cell membrane flexibility while enhancing the antioxidant system. Immobilization experiments showed that biofilms accelerated the removal of nitrogen pollutants and demonstrated good stability, with carriers being reusable to five times, maintaining high ammonia nitrogen (63.90 %) and total nitrogen (50.66 %) removal rates. These findings reveal the cold tolerance mechanisms of strain TY1 and its excellent practical potential as a candidate for wastewater treatment in cold regions.

Construction of an ideotype root system architecture of subsurface flow constructed wetland macrophytes by vertical spatial stress: strengthening of rhizosphere effects and determination of appropriate substrate depth

Strengthening rhizosphere effects to enhance pollutant removal is a hotspot of constructed wetlands (CWs) research in recent years, and improving the root traits and metabolic capacity of macrophytes is crucial for strengthening rhizosphere effects. In the field experiment, two types of subsurface flow (SSF) CWs (CW10 and CW20, with substrate depths of 10 and 20 cm, respectively) under the vertical spatial stress of roots (VSSR) and two types of non-VSSR SSF CWs (CW40 and CW60) were adopted with Typha orientalis as cultivated plants to investigate the variability of root development, metabolism, and pollutant removal at different substrate depths. VSSR induced substantial redundant root development, which significantly increased root-shoot ratio, fine and lateral root biomass, root porosity, and root activity, with lateral and fine root biomass of CW20 reaching 409.17 and 237.42 g/m2, respectively, which were 3.18 and 5.28 times those of CW60. The radical oxygen loss (ROL) and dissolved organic carbon (DOC) levels of CW20 single plant were 1.36 and 4.57 times higher than those of CW60, respectively, and more types of root exudates were determined (e.g., aldehydes, ketones and amides). More aerobic heterotrophs (e.g., Massilia, Planomicrobium), nitrification bacteria (e.g., Ellin6067, Nitrospira), aerobic denitrification bacteria (e.g., Bacillu, Chryseobacterium, Pseudomonas) and denitrification phosphorus accumulating organisms (e.g., Flavobacterium) were enriched in the rhizosphere of CW20. This changed the main transformation pathways of pollutants and enhanced the removal of pollutants, with the COD, TN and TP average removal rates of CW20 increasing by 9.99%, 13.28% and 8.92%, respectively, compared with CW60. The ideotype root system architecture CW (RSACW; CW20) constructed in this study, which consists of a large number of fine and lateral roots, can stimulate more efficient rhizosphere effects stably and continuously.

Efficient nitrogen removal via simultaneous ammonium assimilation and heterotrophic denitrification of Paracoccus denitrificans R-1

Although diverse microorganisms can remove ammonium and nitrate simultaneously, their metabolic mechanisms are not well understood. Paracoccus denitrificans R-1 showed the maximal NH4 + removal rate 9.94 mg L-1·h-1 and 2.91 mg L-1·h-1 under aerobic and anaerobic conditions, respectively. Analysis of the nitrogen balance calculation and isotope tracing experiment indicated that NH4 + was consumed through assimilation. The maximal NO3 - removal rate of strain R-1 was 18.05 and 19.76 mg L-1·h-1 under aerobic and anaerobic conditions, respectively. The stoichiometric consumption ratio of acetate to nitrate was 0.902 and NO3 - was reduced to N2 for strain R-1 through 15NO3 - isotopic tracing experiment, which indicated a respiratory process coupled with the oxidation of electron donors. Genomic analysis showed that strain R-1 contained genes for ammonium assimilation and denitrification, which effectively promoted each other. These findings provide insights into microbial nitrogen transformation and facilitate the simultaneous removal of NH4 + and NO3 - in a single reactor.

Effect of carbon source on carbon and nitrogen metabolism of common heterotrophic nitrification-aerobic denitrification pathway

Pseudomonas sp. ZHL02, removing nitrogen via ammonia nitrogen (NH4+) → hydroxylamine (HN2OH) → nitrite (NO2-) → nitrate (NO3-) → NO2- → nitric oxide (NO) → nitrous oxide (N2O) pathway was employed for getting in-depth information on the heterotrophic nitrification-aerobic denitrification (HNAD) pathway from carbon oxidation, nitrogen conversion, electron transport process, enzyme activity, as well as gene expression while sodium succinate, sodium citrate, and sodium acetate were utilized as the carbon sources. The nitrogen balance analysis results demonstrated that ZHL02 mainly removed NH4+-N through assimilation. The carbon source metabolism resulted in the discrepancies in electron transport chain and nitrogen removal between different HNAD bacteria. Moreover, the prokaryotic strand-specific transcriptome method showed that, amo and hao were absent in ZHL02, and unknown genes may be involved in ZHL02 during the HNAD process. As a fascinating process for removing nitrogen, the HNAD process is still puzzling, and the relationship between carbon metabolism and nitrogen metabolism among different HNAD pathways should be studied further.

Influence and mechanism of typical transition metal ions on the denitrification performance of heterotrophic nitrification-aerobic denitrification bacteria

To investigate the inhibitory effects of various transition metal ions on nitrogen removal and their underlying mechanisms, the single and combined effects of Cu2+ Ni2+ and Zn2+ on Heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria Acinetobacter sp. TAC-1 were studied in a batch experiment system. The results revealed that increasing concentrations of Cu2+ and Ni2+ had a detrimental effect on the removal of ammonium nitrogen (NH4+-N) and total nitrogen (TN). Specifically, Cu2+ concentration of 10 mg/L, the TN degradation rate was 55.09%, compared to 77.60% in the control group. Cu2+ exhibited a pronounced inhibitory effect. In contrast, Zn2+ showed no apparent inhibitory effect on NH4+-N removal and even enhanced TN removal at lower concentrations. However, when the mixed ion concentration of Zn2++Ni2+ exceeded 5 mg/L, the removal rates of NH4+-N and TN were significantly reduced. Moreover, transition metal ions did not significantly impact the removal rates of chemical oxygen demand (COD). The inhibition model fitting results indicated that the inhibition sequence was Cu2+ > Zn2+ > Ni2+. Transcriptome analysis demonstrated that metal ions influence TAC-1 activity by modulating the expression of pivotal genes, including zinc ABC transporter substrate binding protein (znuA), ribosomal protein (rpsM), and chromosome replication initiation protein (dnaA) and DNA replication of TAC-1 under metal ion stress, leading to disruptions in transcription, translation, and cell membrane structure. Finally, a conceptual model was proposed by us to summarize the inhibition mechanism and possible response strategies of TAC-1 bacteria under metal ion stress, and to address the lack of understanding regarding the influence mechanism of TAC-1 on nitrogen removal in wastewater co-polluted by metal and ammonia nitrogen. The results provided practical guidance for the management of transition metal and ammonia nitrogen co-polluted water bodies, as well as the removal of high nitrogen.

Nitric oxide sensor NsrR is the key direct regulator of magnetosome formation and nitrogen metabolism in Magnetospirillum

Nitric oxide (NO) plays an essential role as signaling molecule in regulation of eukaryotic biomineralization, but its role in prokaryotic biomineralization is unknown. Magnetospirillum gryphiswaldense MSR-1, a model strain for studies of prokaryotic biomineralization, has the unique ability to form magnetosomes (magnetic organelles). We demonstrate here that magnetosome biomineralization in MSR-1 requires the presence of NsrRMg (an NO sensor) and a certain level of NO. MSR-1 synthesizes endogenous NO via nitrification-denitrification pathway to activate magnetosome formation. NsrRMg was identified as a global transcriptional regulator that acts as a direct activator of magnetosome gene cluster (MGC) and nitrification genes but as a repressor of denitrification genes. Specific levels of NO modulate DNA-binding ability of NsrRMg to various target promoters, leading to enhancing expression of MGC genes, derepressing denitrification genes, and repressing nitrification genes. These regulatory functions help maintain appropriate endogenous NO level. This study identifies for the first time the key transcriptional regulator of major MGC genes, clarifies the molecular mechanisms underlying NsrR-mediated NO signal transduction in magnetosome formation, and provides a basis for a proposed model of the role of NO in the evolutionary origin of prokaryotic biomineralization processes.

Genomics and metabolic characteristics of simultaneous heterotrophic nitrification aerobic denitrification and aerobic phosphorus removal by Acinetobacter indicus CZH-5

This study provides for the first time a systematic understanding of Acinetobacter indicus CZH-5 performance, metabolic pathway and genomic characteristics for aerobic nitrogen (N) and phosphorus (P) removal. Acinetobacter indicus CZH-5 showed promising performance in heterotrophic nitrification aerobic denitrification and aerobic phosphorus removal. Under optimal conditions, the maximum ammonia-N, total nitrogen and orthophosphate-P removal efficiencies were 90.17%, 86.33%, and 99.89%, respectively. The wide tolerance range suggests the strong environmental adaptability of the bacteria. The complete genome of this strain was reconstructed. Whole genome annotation was used to re-construct the N and P metabolic pathways, and related intracellular substance metabolic pathways were proposed. The transcription levels of related functional genes and enzyme activities further confirmed these metabolic mechanisms. N removal was achieved via the nitrification-denitrification pathway. Furthermore, CZH-5 exhibited significant aerobic P uptake, with phosphate diesters as the main species of intracellular P.

Isolation and characterization of a cold-tolerant heterotrophic nitrification-aerobic denitrification bacterium and evaluation of its nitrogen-removal efficiency

With a view toward addressing the poor efficiency with which nitrogen is removed from wastewater below 10 °C, in this study, we isolated a novel cold-tolerant heterotrophic nitrification-aerobic denitrification (HN-AD) bacterium from a wetland and characterized its nitrogen removal performance and nitrogen metabolic pathway. On the basis of 16S rRNA gene sequencing, this strain was identified as a species of Janthinobacterium, designated J1-1. At 8 °C, strain J1-1 showed excellent removal efficiencies of 89.18% and 68.18% for single-source NH4+-N and NO3--N, respectively, and removal efficiencies of 96.23% and 79.64% for NH4+-N and NO3--N, respectively, when supplied with mixed-source nitrogen. Whole-genome sequence analysis and successful amplification of the amoA, napA, and nirK functional genes related to nitrogen metabolism provided further evidence in support of the HN-AD capacity of strain J1-1. The deduced HN-AD metabolic pathway of the strain was NH4+-N→NH2OH→NO2--N→NO3--N→NO2--N→NO→N2O. In addition, assessments of NH4+-N removal under different conditions revealed the following conditions to be optimal for efficient removal: a temperature of 20 °C, pH of 7, shaking speed of 150 rpm, sodium succinate as a carbon source, and a C/N mass ratio of 16. Given its efficient nitrogen removal capacity at 8 °C, the J1-1 strain characterized in this study has considerable application potential in the treatment of low-temperature wastewater.

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.

Multi-omics analysis to reveal key pathways involved in low C/N ratio stress response in Pseudomonas sp. LW60 with superior nitrogen removal efficiency

In practical engineering, nitrogen removal at low temperatures or low C/N ratios is difficult. Although strains can remove nitrogen well at low temperatures, there is no research on the performance and deep mechanism of strains under low C/N ratio stress. In this study, Pseudomonas sp. LW60 with superior nitrogen removal efficiency under low C/N ratio stress was isolated at 4 °C. With a C/N ratio of 2-10, the NH4+-N removal efficiency was 40.02 %-100 % at 4 °C. Furthermore, the resistance mechanism of Pseudomonas sp. LW60 to low C/N ratio stress was deeply investigated by multi-omics. The results of transcriptome, proteome, and metabolome revealed that the resistance of strain LW60 to low C/N ratio stress was attributed to enhanced central carbon metabolism, amino acid metabolism, and ABC transporters, rather than nitrogen removal pathways. This study isolated a strain with low C/N ratio tolerance and deeply explored its tolerance mechanism by multi-omics.

Performance evaluation of H2S and NH3 removal by biological trickling filter reactors with various fillers under heterotrophic conditions

A pilot-scale biological trickling filter (BTF) reactor (13.5 L) packed with different fillers (Pine bark, Cinder, Straw, and MBBR (mobile bed biofilm reactor) filler was employed to evaluate their removal performance of H2S and NH3 after heterotrophic bacterium addition, and some parameters, including different packing heights, empty bed residence time (EBRT), inlet titers, loading ratios, and restart trial, were investigated in this study. According to the experimental results, BTF filled with pine bark exhibited better removal efficiency than other reactors under a variety of conditions. The removal efficiency of H2S and NH3 reached to as high as 81.31 % and 91.72 %, respectively, with the loading range of 3.29-67.70 g/m3·h. Moreover, due to the addition of heterotrophic bacterium, the removal efficiency was enhanced and capable to eliminate majority of H2S and NH3 even though the packing height was reduced to 400 mm. After 15 days of idle, the BTF reactor was able to resume rapidly and execute deodorization with high efficiency. The degradation mechanism was further explored by a thorough examination of microbial species which degraded contaminants, as well as by functional prediction and correlation analyses. In a word, these results laid a foundation for the application of heterotrophic microorganisms in BTF, which could improve the removal efficiency of biological deodorization.

Phylogenetic diversity, distribution, and gene structure of the pyruvic oxime dioxygenase involved in heterotrophic nitrification

Some heterotrophic microorganisms carry out nitrification to produce nitrite and nitrate from pyruvic oxime. Pyruvic oxime dioxygenase (POD) is an enzyme that catalyzes the degradation of pyruvic oxime to pyruvate and nitrite from the heterotrophic nitrifying bacterium Alcaligenes faecalis. Sequence similarity searches revealed the presence of genes encoding proteins homologous to A. faecalis POD in bacteria of the phyla Proteobacteria and Actinobacteria and in fungi of the phylum Ascomycota, and their gene products were confirmed to have POD activity in recombinant experiments. Phylogenetic analysis further classified these POD homologs into three groups. Group 1 POD is mainly found in heterotrophic nitrifying Betaproteobacteria and fungi, and is assumed to be involved in heterotrophic nitrification. It is not clear whether group 2 POD, found mainly in species of the Gammaproteobacteria and Actinobacteria, and group 3 POD, found simultaneously with group 1 POD, are involved in heterotrophic nitrification. The genes of bacterial group 1 POD comprised a single transcription unit with the genes related to the metabolism of aromatic compounds, and many of the genes group 2 POD consisted of a single transcription unit with the gene encoding the protein homologous to 4-hydroxy-tetrahydrodipicolinate synthase (DapA). LysR- or Cro/CI-type regulatory genes were present adjacent to or in the vicinity of these POD gene clusters. POD may be involved not only in nitrification, but also in certain metabolic processes whose functions are currently unknown, in coordination with members of gene clusters.

A novel hypothermic strain, Pseudomonas reactans WL20-3 with high nitrate removal from actual sewage, and its synergistic resistance mechanism for efficient nitrate removal at 4 °C

Nitrate can be well removed by bacteria at 25-30 °C. However, nitrate removal almost ceases at temperatures lower than 5 °C. In this study, a novel hypothermic strain, Pseudomonas reactans WL20-3 exhibited an excellent aerobic nitrate removal ability at 4 °C. It had high capability for the removal of nitrate, total dissolved nitrogen (TDN), and dissolved organic carbon (DOC) at 4 °C, achieving removal efficiencies of 100%, 87.91%, and 97.48%, respectively. The transcriptome analysis revealed all genes involved in the nitrate removal pathway were significantly up-regulated. Additionally, the up-regulation of ABC transporter genes and down-regulation of respiratory chain genes cooperated with the nitrate metabolism pathway to resist low-temperature stress. In actual sewage, inoculated with WL20-3, the nitrate removal efficiency was found to be 70.70%. Overall, these findings demonstrated the impressive capacity of the novel strain WL20-3 to remove nitrate and provided novel insights into the synergistic resistance mechanism of WL20-3 at low temperature.

Comparative genomic analysis of two Arctic Pseudomonas strains reveals insights into the aerobic denitrification in cold environments

BACKGROUND

Biological denitrification has been commonly adopted for the removal of nitrogen from sewage effluents. However, due to the low temperature during winter, microorganisms in the wastewater biological treatment unit usually encounter problems such as slow cell growth and low enzymatic efficiency. Hence, the isolation and screening of cold-tolerant aerobic denitrifying bacteria (ADB) have recently drawn attention. In our previous study, two Pseudomonas strains PMCC200344 and PMCC200367 isolated from Arctic soil demonstrated strong denitrification ability at low temperatures. The two Arctic strains show potential for biological nitrogen removal from sewage in cold environments. However, the genome sequences of these two organisms have not been reported thus far.

RESULTS

Here, the basic characteristics and genetic diversity of strains PMCC200344 and PMCC200367 were described, together with the complete genomes and comparative genomic results. The genome of Pseudomonas sp. PMCC200344 was composed of a circular chromosome of 6,478,166 bp with a G + C content of 58.60% and contained a total of 5,853 genes. The genome of Pseudomonas sp. PMCC200367 was composed of a circular chromosome of 6,360,061 bp with a G + C content of 58.68% and contained 5,801 genes. Not only prophages but also genomic islands were identified in the two Pseudomonas strains. No plasmids were observed. All genes of a complete set of denitrification pathways as well as various putative cold adaptation and heavy metal resistance genes in the genomes were identified and analyzed. These genes were usually detected on genomic islands in bacterial genomes.

CONCLUSIONS

These analytical results provide insights into the genomic basis of microbial denitrification in cold environments, indicating the potential of Arctic Pseudomonas strains in nitrogen removal from sewage effluents at low temperatures.

Efficient nitrogen removal by a novel extreme strain, Pseudomonas reactans WL20-3 under dual stresses of low temperature and high alkalinity: Characterization, mechanism, and application

Although many studies report the resistance of heterotrophic nitrification-aerobic denitrification (HN-AD) strains to single environmental stress, there is no research on its resistance to dual stresses of low temperature and high alkalinity. A novel bacterium Pseudomonas reactants WL20-3 isolated in this study showed removal efficiencies of 100%, 100%, and 97.76% for ammonium, nitrate, and nitrite, respectively, at 4 °C and pH 11.0. Transcriptome analysis revealed that the resistance of strain WL20-3 to dual stresses was attributed not only to the regulation of genes in the nitrogen metabolic pathway, but also to genes in other pathways such as the ribosome, oxidative phosphorylation, amino acid metabolism, and ABC transporters. Additionally, WL20-3 removed 83.98% of ammonium from actual wastewater at 4 °C and pH 11.0. This study isolated a novel strain WL20-3 with superior nitrogen removal under dual stresses and provided a molecular understanding of its tolerance mechanism to low temperature and high alkalinity.

Efficient nitrogen removal of a novel Pseudomonas chengduensis strain BF6 mainly through assimilation in the recirculating aquaculture systems

Biological nitrogen removal has received increasing attention in wastewater treatment. A bacterium with excellent nitrogen removal performance was isolated from biofilters of recirculating aquaculture systems (RAS) and identified as Pseudomonas chengduensis BF6. It was indicated that inorganic nitrogen is transformed into gaseous and biological nitrogen by the metabolic pathways of denitrification, anammox, and assimilation, which is the main nitrogen removal pathway of strain BF6. The strain BF6 could effectively remove nitrogen within 24 h under the conditions of ammonia, nitrate, nitrite, and mixed nitrogen sources with maximum total nitrogen removal efficiencies reaching 97.00 %, 61.40 %, 79.10 %, and 84.98 %, respectively. The strain BF6 exhibited total nitrogen removal efficiency of 91.14 %, altered the microbial diversity and enhanced the relative abundance of Pseudomonas in the RAS biofilter. These findings demonstrate that Pseudomonas sp. BF6 is a highly efficient nitrogen-removing bacterium with great potential for application in aquaculture wastewater remediation.

Pangenome-driven insights into nitrogen metabolic characteristics of Citrobacter portucalensis strain AAK_AS5 associated with wastewater nitrogen removal

Nitrogen metabolism in the genus Citrobacter is very poorly studied despite its several implications in wastewater treatment. In the current study, Citrobacter portucalensis strain AAK_AS5 was assessed for remediation of simulated wastewater supplemented with different inorganic nitrogen sources. Combination of (NH₄)₂SO₄ with KNO₃ was the most preferred for achieving high growth density followed by (NH₄)₂SO₄ and KNO₃ alone. This was in agreement with highest ammonical nitrogen removal of 92.9% in the presence of combined nitrogen sources and the corresponding nitrate nitrogen removal of 93% in the presence of KNO₃. Furthermore, these removal capacities were validated by investigating the uniqueness and the spread of metabolic features through pan-genomic approach that revealed the largest number of unique genes (2097) and accessory genes (705) in strain AAK_AS5. Of the total 44 different types of nitrogen metabolism-related genes, 39 genes were associated with the core genome, while 5 genes such as gltI, nasA, nasR, nrtA, and ntrC uniquely belonged to the accessory genome. Strain AAK_AS5 possessed three major nitrate removal pathways viz., assimilatory and dissimilatory nitrate reduction to ammonia (ANRA & DNRA), and denitrification; however, the absence of nitrification was compensated by ammonia assimilation catalyzed by gene products of the GDH and GS-GOGAT pathways. narGHIJ encoding the respiratory nitrate reductase was commonly identified in all the studied genomes, while genes such as nirK, norB, and nosZ were uniquely present in the strain AAK_AS5 only. A markedly different genetic content and metabolic diversity between the strains reflected their adaptive evolution in the environment thus highlighting the significance of C. portucalensis AAK_AS5 for potential application in nitrogen removal from wastewater.

Domestication and pilot-scale culture of mixed bacteria HY-1 capable of heterotrophic nitrification-aerobic denitrification

To further investigate the potential of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria for practical applications, the HN-AD mixed bacteria HY-1 were enriched and domesticated in this study. After five generations of domestication, the mixture was able to remove 98% of ammonia nitrogen (400 mg/L) and 81.9% of mixed nitrogen source (nitrate, nitrite). Changes in community structure in the domestication process of mixed microorganisms were studied using 16S rDNA-seq. The results indicated an increase in the abundance of Acinetobacter from 16.9% to 80%. The conditions for the expanded culture of the HY-1 were also optimized. Moreover, A pilot-scale expanded reactor with a capacity of 1000L was constructed, and the HY-1 was successfully expanded from 0.1L to 800L. The community structures of the HY-1 remained stable after the expanded culture, with Acinetobacter as the dominant species. Moreover, the HY-1 demonstrated adaptability to actual high ammonia nitrogen wastewater and showed potential for practical application.

Root vertical spatial stress: A method for enhancing rhizosphere effect of plants in subsurface flow constructed wetland

The depth of the substrate of subsurface flow (SSF) constructed wetlands (CWs) is closely related to their cost and operation stability. To explore the physiological regulation mechanism of wetland plants and pollutant removal potential of SSF CWs under "vertical spatial stress of roots" (by greatly reducing the depth of the substrate in SSF CWs to limit the vertical growth space of roots, VSSR), the physiological response and wetland purification effect of a 0.1 m Canna indica L. CW under VSSR were studied compared with conventional SSF CWs (0.6 m, 1.2 m). The results demonstrated that VSSR significantly enhanced the dissolved oxygen (DO) concentration (p < 0.05) within the SSF CWs, with the DO in 0.1 m CW remaining stable at over 3 mg/L. Under the same hydraulic retention time (HRT), VSSR significantly improved the removal effect of pollutants (p < 0.05). The removal rates of COD, NH₄⁺-N, and total phosphorus (TP) remained above 87%, and the mean removal rates of total nitrogen (TN) reached 91.71%. VSSR promoted the morphological adaptation mechanisms of plants, such as significantly increased root-shoot ratio (p < 0.05), changed biomass allocation. Plants could maintain the stability of the photosynthetic mechanism by changing the distribution of light energy. The results of microbial community function prediction demonstrated that aerobic denitrification was the main mechanism of N transformation in the 0.1 m CW under VSSR. VSSR could induce the high root activity of plants, augment the concentration of root exudates, enhance the redox environment of the plant rhizosphere, further foster the enrichment of aerobic denitrifying bacteria, and strengthen the absorption efficiency of wetland plants and substrate, thus achieving an efficient pollutant removal capacity. Studies showed that VSSR was an effective means to enhance the rhizosphere effect of plants and pollutant removal in SSF CWs.

Genomics and nitrogen metabolic characteristics of a novel heterotrophic nitrifying-aerobic denitrifying bacterium Acinetobacter oleivorans AHP123

A novel aerobic strain of Acinetobacter oleivorans AHP123 was isolated from activated sludge, which could conduct heterotrophic nitrification and denitrification simultaneously. This strain has excellent NH₄⁺-N removal ability, with 97.93% removal rate at 24-hour. To identify the metabolic pathways of this novel strain, genes of gam, glnA, gdhA, gltB, nirB, nasA, nar, nor, glnK and amt were detected by genome analysis. Through RT-qPCR, it was found that the expression of key genes confirmed two possible ways of nitrogen removal in strain AHP123: nitrogen assimilation and heterotrophic nitrification aerobic denitrification (HNAD). However, the absence of some common HNAD genes (amo, nap and nos) suggested that strain AHP123 might have a different HNAD pathway from other HNAD bacteria. Nitrogen balance analysis revealed that strain AHP123 assimilated most of the external nitrogen sources into intracellular nitrogen.

Low C/N promotes stable partial nitrification by enhancing the cooperation of functional microorganisms in treating high-strength ammonium landfill leachate

Partial nitrification is an effective process for treating high-strength ammonium landfill leachate with low C/N ratio, for the cooperation with denitrification can save almost 40% carbon addition in biological nitrogen removal. However, high ammonia loading often causes the instability of partial nitrification process. Less carbon addition can promote the stability of partial nitrification and increase the nitrite accumulation ratio (NAR). Nevertheless, the microbial mechanisms within remain further elusive. In this study, two laboratory-scale sequencing batch reactors were constructed and operated for 125 days, which were fed with ammonia synthetic wastewater with C/N of 0.6 (CN system) and C/N of 0.0 as the control (N system). CN system performed more stably and had the highest NAR of 100%. Extracellular polymeric substances (EPS) generated from carbon source provided spatial and nutrient niches to tighten the cooperation of functional microorganisms, thus, enhanced the stability and efficiency of partial nitrification. Thauera was the dominant denitrifier in CN system. Nitrosomonas was one of the most important autotrophic ammonia oxidizing bacteria, while Paracoccus and Flavobacterium were the main heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria in CN system. The enrichment of HN-AD bacteria outcompeted nitrite oxidizing bacteria (NOB), therefore leaded to higher nitrite accumulation in CN system. The findings of this study may be conducive to increasing the understanding of the microbial collaboration mechanisms of partial nitrification, thereby provides theoretical support for the improvement of biological nitrogen removal technology.

Simultaneous nitrogen removal via heterotrophic nitrification and aerobic denitrification by a novel Lysinibacillus fusiformis B301

Simultaneous nitrogen removal via heterotrophic nitrification and aerobic denitrification (HN-AD) has received widespread attention in biological treatment of wastewater. This study reported a novel Lysinibacillus fusiformis B301 strain, which effectively removed nitrogenous pollutants via HN-AD in one aerobic reactor with no nitrite accumulated. It exhibited the optimal nitrogen removal efficiency under 30°C, citrate as the carbon source and C/N ratio of 15. The maximum nitrogen removal rates were up to 2.11 mgNH₄ ⁺ -N/(L·h), 1.62 mgNO₃ ^- -N/(L·h), and 1.41 mgNO₂ ^- -N/(L·h), respectively, when ammonium, nitrate, and nitrite were employed as the only nitrogen source under aerobic conditions. Ammonium nitrogen was preferentially consumed via HN-AD in the coexistence of three nitrogen species, and the removal efficiencies of total nitrogen were up to 94.26%. Nitrogen balance analysis suggested that 83.25% of ammonium was converted to gaseous nitrogen. The HD-AD pathway catalyzed by L. fusiformis B301 followed NH 4 + → N H 2 OH → NO 2 - → NO 3 - → NO 2 - → N 2 , supported by the results of key denitrifying enzymatic activities. PRACTITIONER POINTS: The novel Lysinibacillus fusiformis B301 exhibited the outstanding HN-AD ability. The novel Lysinibacillus fusiformis B301 simultaneously removed multiple nitrogen species. No nitrite accumulated during the HN-AD process. Five key denitrifying enzymes were involved in the HN-AD process. Ammonium nitrogen (83.25%) was converted to gaseous nitrogen by the novel strain.

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.

Heterotrophic nitrification and aerobic denitrification characteristics of the psychrotolerant Pseudomonas peli NR-5 at low temperatures

The nitrogen removal efficiency of heterotrophic nitrification and aerobic denitrification (HN-AD) bacteria can be seriously inhibited at low temperatures (< 15 °C). A novel psychrotolerant bacterium, Pseudomonas peli NR-5 (P. peli NR-5), with efficient HN-AD capability was isolated and screened from river sediments in cold areas. When P. peli NR-5 was aerobically cultivated for 60 h at 10 °C with NH₄⁺-N, NO₃^--N, and NO₂^--N as the sole nitrogen sources (N 105 mg/L), the nitrogen removal efficiencies were 97.3, 95.3, and 87.8%, respectively, without nitrite accumulation, and the corresponding average nitrogen removal rates were 1.71, 1.67, and 1.55 mg/L/h, respectively. Meanwhile, P. peli NR-5 exhibited excellent simultaneous nitrification and denitrification capabilities at 10 °C. Sodium succinate was the most favorable carbon substrate for bacterial growth and ammonia removal by strain NR-5. The optimal culture conditions determined by the response surface methodology model were a carbon to nitrogen ratio of 5.9, temperature of 11.5 °C, pH of 7.0, and shaking speed of 144 rpm. Under these conditions, 99.1% of the total nitrogen was removed in the verification experiments, which was not significantly different from the predicted maximum removal in the model (99.6%). Six functional genes participating in the HN-AD process were successfully obtained by polymerase chain reaction amplification, which further confirmed the HN-AD capability of P. peli NR-5 and proposed the metabolic pathway of HN-AD. The above results provide a theoretical background of psychrotolerant HN-AD bacteria in wastewater purification under low-temperature conditions.

The optimum particle size of anaerobic ammonia oxidation granular sludge under different substrate concentrations

The nitrogen removal performance of anaerobic ammonia oxidation granular sludge (AnGS) varies widely among particle sizes. In this paper, the nitrogen removal performance, extracellular polymeric substances (EPS) secretion level and microbial community of AnGS with different particle sizes were investigated to select the optimal particle size for different substrate concentrations. The results showed that the optimal particle size migrated from 0.6-1.6 mm to 1.6-2.5 mm and then to 2.5-3.2 mm as the substrate concentration increased. When the influent concentration of NH₄⁺-N was 110 mg/L, granular sludge with particle size of 1.6-2.5 mm showed excellent nitrogen removal performance with the highest EPS secretion, while the highest EPS secretion gradually migrated to smaller particles as the substrate concentration decreased. The nitrogen removal performance of AnGS with different particle sizes depends on different proportions of anaerobic ammonium-oxidizing (anammox) bacteria (Candidates_Jettenia, Candidates_Kuenenia, Candidatus_Brocadia), heterotrophic nitrification aerobic denitrifying bacteria (Acinetobacter) and denitrifying bacteria (Denitratisoma). The optimum particle size range for AnGS has been clarified for different influent nitrogen concentrations, which can provide some new understanding for the application of anammox reactors.

Seasonal succession of microbial community co-occurrence patterns and community assembly mechanism in coal mining subsidence lakes

Coal mining subsidence lakes are classic hydrologic characteristics created by underground coal mining and represent severe anthropogenic disturbances and environmental challenges. However, the assembly mechanisms and diversity of microbial communities shaped by such environments are poorly understood yet. In this study, we explored aquatic bacterial community diversity and ecological assembly processes in subsidence lakes during winter and summer using 16S rRNA gene sequencing. We observed that clear bacterial community structure was driven by seasonality more than by habitat, and the α-diversity and functional diversity of the bacterial community in summer were significantly higher than in winter (p < 0.001). Canonical correspondence analysis indicated that temperature and chlorophyll-a were the most crucial contributing factors influencing the community season variations in subsidence lakes. Specifically, temperature and chlorophyll-a explained 18.26 and 14.69% of the community season variation, respectively. The bacterial community variation was driven by deterministic processes in winter but dominated by stochastic processes in summer. Compared to winter, the network of bacterial communities in summer exhibited a higher average degree, modularity, and keystone taxa (hubs and connectors in a network), thereby forming a highly complex and stable community structure. These results illustrate the clear season heterogeneity of bacterial communities in subsidence lakes and provide new insights into revealing the effects of seasonal succession on microbial assembly processes in coal mining subsidence lake ecosystems.

Metagenomic and extracellular polymeric substances analysis reveals the mechanism of exogenous N-hexanoyl-L-homoserine lactone in alleviating the inhibition of perfluorooctanoic acid on anammox process

To alleviate the negative effects of perfluorooctanoic acid (PFOA) on nitrogen removal via anaerobic ammonia oxidation (anammox), an exogenous signaling factor (N-hexanoyl-L-homoserine lactone, C₆-HSL) was introduced into an anammox reactor. Results showed that 2 μmol/L C₆-HSL promoted the nitrogen removal efficiency of the anammox reactor under PFOA stress, with the removal efficiencies of ammonia and nitrite increasing from 79.7 ± 4.8 % and 80.8 ± 3.8 %, to 94.4 ± 4.3 % and 97.1 ± 3.8 %. Exogenous C₆-HSL enhanced the compactness of the extracellular proteins, and improved the sludge hydrophobicity. Meanwhile, C₆-HSL resulted in a microbial shift, with the relative abundance of Planctomycetes increasing from 30.2 % to 49.5 %. Candidatus Kuenenia stuttgartiensis replaced Candidatus Brocadia sp. BL1 as the dominant species, while the available space for other nitrogen-removing bacteria was reduced. Exogenous C₆-HSL promoted the expression of anammox-related genes, such as hzsB and hdh, while denitrifying genes were down-regulated. In addition, the relative abundance of HdtS, which synthesizes AHLs, increased by 0.02446%.

Effect of dissolved oxygen on high C/N wastewater treatment in moving bed biofilm reactors based on heterotrophic nitrification and aerobic denitrification: Nitrogen removal performance and potential mechanisms

In this study, it was investigated the nitrogen removal (NR) performance and potential mechanism for high C/N wastewater treatment under different dissolved oxygen (DO) concentrations. The results showed that DO concentration significantly affected the removal efficiency of total nitrogen (TN). When the initial DO increased from 0.5 to 1.5 mg/L, TN removal efficiency significantly increased from 65 % to 85 %. However, a further DO increase did not promote TN removal, and the NR was only 80 % with an initial DO concentration of 3.5 mg/L. The effect of DO concentration on NR was influenced by the combined action of functional bacteria and electron flow. Excessive DO concentration did not positively affect NR efficiency but promoted electron utilization and respiratory proliferation. When the DO concentration was 1.5 mg/L, more electrons generated by sodium acetate metabolism were transferred to the aerobic denitrification process, compared to when the DO concentration was 3.5 mg/L.

A mechanistic review on aerobic denitrification for nitrogen removal in water treatment

The traditional biological nitrogen removal technology consists of two steps: nitrification by autotrophs in aerobic circumstances and denitrification by heterotrophs in anaerobic situations; however, this technology requires a huge area and stringent environmental conditions. Researchers reached the conclusion that the denitrification process could also be carried out in aerobic circumstances with the discovery of aerobic denitrification. The aerobic denitrification process is carried out by aerobic denitrifying bacteria (ADB), most of which are heterotrophic bacteria that can metabolize various forms of nitrogen compounds under aerobic conditions and directly convert ammonia nitrogen to N₂ for discharge from the system. Despite the fact that there is no universal agreement on the mechanism of aerobic denitrification, this article reviewed four current explanations for the denitrification mechanism of ADB, including the microenvironment theory, theory of enzyme, electron transport bottlenecks theory, and omics study, and summarized the parameters affecting the denitrification efficiency of ADB in terms of carbon source, temperature, dissolved oxygen (DO), and pH. It also discussed the current status of the application of aerobic denitrification in practical processes. Following the review, the difficulties of present aerobic denitrification technology are outlined and future research options are highlighted. This review may help to improve the design of current wastewater treatment facilities by utilizing ADB for effective nitrogen removal and provide the engineers with relevant references.

Effect mechanism of individual and combined salinity on the nitrogen removal yield of heterotrophic nitrification-aerobic denitrification bacteria

One of the biggest challenges of applying heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria to treat high salt organic wastewater lies in the inhibitory effect exerted by salinity. To study the inhibition effect and underlying mechanism induced by different ion types and ion composition, the individual and combined effects of NaCl, KCl and Na₂SO₄ on HN-AD bacteria Acinetobacter sp. TAC-1 were systematically investigated by batch experiments. Results indicated that the ammonia nitrogen removal yield and TAC-1 activity decreased with increased salt concentration. NaCl, KCl and Na₂SO₄ exerted different degrees of inhibition on TAC-1, with half concentration inhibition constant values of 0.205, 0.238 and 0.110 M, respectively. A synergistic effect on TAC-1 was found with the combinations of NaCl + KCl, NaCl + Na₂SO₄ and NaCl + KCl + Na₂SO₄. The whole RNA resequencing suggested that transcripts of denitrification genes (nirB and nasA) were significantly downregulated with increased Na₂SO₄ concentration. Simultaneously, Na₂SO₄ stress disrupted cell respiration, DNA replication, transcription, translation, and induced oxidative stress. Finally, we proposed a conceptual model to summarize the inhibition mechanisms and possible response strategies of TAC-1 bacteria under Na₂SO₄ stress.

Efficient nitrate removal of immobilized mixed aerobic denitrifying bacteria and community dynamics response to temperature and low carbon/nitrogen polluted water

The denitrification performance of immobilized mixed aerobic denitrifying bacteria (IMADB) was investigated. IMADB displayed strong temperature adaptability under low Carbon/Nitrogen conditions. At 5, 15, and 25 °C, the nitrate removal efficiencies of volcanic rock and polyester fiber sponge immobilized system reached 83.95%-98.25% and 89.71%-98.14%, respectively. The nitrate content removed by the carrier accounted for 41.18%-82.47% of the nitrate content removed by the immobilized system at different temperature, and played a major role in nitrate removal. The lower the temperature, the greater the role of the carrier. At the same temperature, carrier had a relatively higher richness, diversity, and evenness. Network analysis revealed that carrier species, which were positively correlated with nitrate removal efficiency, had the largest OTUs and abundance. Meanwhile, carrier had the widest niche. The total nitrogen removal efficiency of IMADB reached 56.10%-62.31% in the natural water system, highlighting a promising application prospect.

Biological nitrogen removal and metabolic characteristics of a novel cold-resistant heterotrophic nitrification and aerobic denitrification Rhizobium sp. WS7

For improving the poor de-nitrogen efficiency and effluent quality faced by wastewater treatment plants in winter, a novel cold-resistant strain, Rhizobium sp. WS7 was isolated. Strain WS7 presented dramatic de-nitrogen efficiencies including 98.73 % of NH₄⁺-N, 99.98 % of NO₃^--N, 100 % of NO₂^--N and approximately 100 % of mixed nitrogen (NH₄⁺-N and NO₃^--N) at 15 °C. Optimum parameters of WS7 for aerobic denitrification were determined. Additionally, functional genes (amoA, napA, nirK, norB, and nosZ) and key enzymes (nitrate reductase and nitrite reductase) activities were determined. Nitrogen balance analysis suggested that assimilation played a dominant role in de-nitrogen by WS7, the NH₄⁺-N metabolic pathway was deduced as NH₄⁺-N → NH₂OH → NO → N₂O → N₂, and the NO₃^--N/NO₂^--N metabolic pathway was deduced as NO₃^--N → NO₂^--N → NO → N₂O → N₂. The cold-resistant Rhizobium sp. WS7 has great application feasibility in cold sewage treatment.

Bioaugmentation with Acinetobacter sp. TAC-1 to enhance nitrogen removal in swine wastewater by moving bed biofilm reactor inoculated with bacteria

To enhance the performance of moving bed biofilm reactor (MBBR) inoculated with heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria, bioaugmentation with Acinetobacter sp. TAC-1 was firstly employed and then the treatment performance for real swine wastewater was presented in this study. Results indicated that NH₄⁺-N and TN removal rates of bioaugmented reactor were significantly improved from 16.53 mg/L/h and 16.15 mg/L/h to 24.58 mg/L/h and 24.45 mg/L/h, respectively. The efficient removal performance (NH₄⁺-N 95.01%, TN 86.40%) for real swine wastewater was achieved within 24 h. Microbial analysis indicated that the composition of functional bacteria varied with the introduction of Acinetobacter sp. TAC-1, especially the abundance of Acinetobacter, Paracoccus and Rhodococcus related to the nitrogen removal. Furthermore, bioaugmentation with Acinetobacter sp. TAC-1 increased abundance of enzymes and functional genes (nirS, nirK and norZ) corresponding to denitrification that may be responsible for the enhanced nitrogen removal performance.

A divided flow aerobic-anoxic baffled reactor for simultaneous nitrification-denitrification of domestic wastewater

A novel aerobic-anoxic baffled reactor is designed for the effective denitrification of real domestic wastewater without an external carbon source. The flow is divided between two inlets at the beginning of each zone to provide a carbon source for the denitrifying bacteria. The effects of operating parameters such as the ratio of chemical oxygen demand to nitrogen (COD/N), flow division ratio, and hydraulic retention time (HRT) on the nitrogen removal were investigated. The optimum values of COD/N and HRT were estimated using response surface methodology (RSM) coupled with a central composite experimental design. The addition of porous biomass support media considerably improved the denitrification and removal of COD. Furthermore, the aerobic-anoxic system showed high stability against sudden HRT and COD/N ratio changes. The microbial analysis showed that Alcaligenes, Achromobacter, and Bordetella were the dominant denitrifying bacteria in the anoxic zone, whereas other species coexisted in the aerobic zone.

Characterization of a novel salt-tolerant strain Sphingopyxis sp. CY-10 capable of heterotrophic nitrification and aerobic denitrification

A novel heterotrophic nitrification and aerobic denitrification (HN-AD) strain CY-10 was isolated and identified as Sphingopyxis sp. When ammonium, nitrate or nitrite was used as the sole nitrogen source (300 mg/L), the maximum nitrogen removal efficiency of strain CY-10 were 100%, 91.1% and 68.5%, respectively. The optimal salinity for ammonia nitrogen removal by strain CY-10 was in the range of 0-5%. At the salinity of 5%, a maximum nitrogen removal rate of 6.25 mg/(L·h) was realized. Metabonomics data showed that the metabolic levels of sucrose and D-tagatose increased significantly at 5% salinity condition, enabling the strain to regulate osmotic pressure and survive in high-salt environments. Functional genes were successfully amplified by quantitative PCR, and HN-AD pathway of strain CY-10 followed NH₄⁺-N → NH₂OH → NO₂^--N → NO → N₂O → N₂. These findings show that strain CY-10 has great potential in nitrogen removal treatment of saline wastewater.

The Growth of Vallisneria natans and Its Epiphytic Biofilm in Simulated Nutrient-Rich Flowing Water

This paper investigates the effects of water flow on the growth and physiological indicators of the submerged macrophyte, Vallisneria natans, and the bacteria and algae community composition on its epiphytic biofilm-covered leaves. The authors set up a simulated flowing water laboratory experiment testing high nitrogen (N) and phosphorus (P) concentrations. Total chlorophyll and dissolved oxygen (DO) was significantly enhanced, and turbidity was reduced, thereby accelerating the growth of V. natans. These experiments were compared to another set of observations on a static group. The accumulation of malonaldehyde (MDA) in the dynamic groups was significantly higher than that in the static group. As an antioxidant stress response, the total superoxide dismutase (T-SOD) was also induced in plants exposed to nutrient-rich flowing water. The results of 16S rRNA high-throughput sequencing analyses showed that the water flow increased the bacteria community diversity of biofilm-producing bacteria with N and P removing bacteria, carbon cycle bacteria, and plant growth-promoting rhizobacteria on the epiphytic biofilm. This research determined that water flow alleviates the adverse effects of eutrophication when V. natans grows in water containing high N and P concentrations. Water flow also inhibits the growth of cyanobacteria (also referred to as blue-green algae) in epiphytic biofilm. The ecological factor of water flow, such as water disturbance and aeration measures, could alleviate the adverse effect of eutrophic water by providing a new way to restore submerged macrophytes, such as V. natans, in eutrophic water.

Nitrogen removal by a novel heterotrophic nitrification and aerobic denitrification bacterium Acinetobacter calcoaceticus TY1 under low temperatures

A new bacterial strain, Acinetobacter calcoaceticus TY1, was identified in activated sludge. This strain efficiently metabolized nitrogen from ammonium at low temperatures, utilizing NH₄⁺-N, NO₃^--N, and NO₂^--N as nitrogen sources. Of these, NH₄⁺-N was superior in terms of both assimilation and heterotrophic nitrification at 8 °C. The nitrogen metabolism-associated genes amoA, nirK, and nosZ were identified in TY1. Optimal requirements for growth and nitrogen removal were pH 7, shaking speed of 90 rpm, a C/N ratio of 10, and sodium citrate for the carbon supply. The ability to denitrify at low temperature suggests TY1's potential for wastewater management.

Aeration strategy based on numerical modelling and the response mechanism of microbial communities under various operating conditions

Aeration system is the main energy consumer in a wastewater treatment process. In this paper, the Naive Bayes classification (NBC) algorithm and response surface method (RSM) were firstly used to establish a methodology to improve the aeration efficiency and estimate effluent quality. Lab-scale experiments were conducted to verify the model. The errors between experimental values and predicted values were 3.36, -0.67 and -3.78% at operating temperatures of 20, 30 and 35 °C, indicating the applicability. To further elucidate the biological mechanisms of the experimental results, the microbial community composition was investigated under various operating conditions, the results shows that aerobic heterotrophic bacteria (HET) activity and COD removal efficiency were promoted at 30 °C. AOB and NOB activity and NH₄⁺-N removal efficiency were promoted at 30-35 °C. These findings together suggest that operating temperature is crucial for activated sludge treatment, which should be considered when regulating DO content or aeration rate in practical application.

Nitrogen Removal Characteristics of a Cold-Tolerant Aerobic Denitrification Bacterium, Pseudomonas sp. 41

Nitrogen pollution of surface water is the main cause of water eutrophication, and is considered a worldwide challenge in surface water treatment. Currently, the total nitrogen (TN) content in the effluent of wastewater treatment plants (WWTPs) is still high at low winter temperatures, mainly as a result of the incomplete removal of nitrate (NO3−-N). In this research, a novel aerobic denitrifier identified as Pseudomonas sp. 41 was isolated from municipal activated sludge; this strain could rapidly degrade a high concentration of NO3−-N at low temperature. Strain 41 completely converted 100 mg/L NO3−-N in 48 h at 15 °C, and the maximum removal rate reached 4.0 mg/L/h. The functional genes napA, nirS, norB and nosZ were successfully amplified, which provided a theoretical support for the aerobic denitrification capacity of strain 41. In particular, the results of denitrification experiments showed that strain 41 could perform aerobic denitrification under the catalysis of NAP. Nitrogen balance analysis revealed that strain 41 degraded NO3−-N mainly through assimilation (52.35%) and aerobic denitrification (44.02%), and combined with the gene amplification results, the nitrate metabolism pathway of strain 41 was proposed. Single-factor experiments confirmed that strain 41 possessed the best nitrogen removal performance under the conditions of sodium citrate as carbon source, C/N ratio 10, pH 8, temperature 15–30 °C and rotation speed 120 rpm. Meanwhile, the bioaugmentation test manifested that the immobilized strain 41 remarkably improved the denitrification efficiency and shortened the reaction time in the treatment of synthetic wastewater.

New insight into the nitrogen removal capacity and mechanism of Streptomyces mediolani EM-B2

The utilization of actinomycetes as the bioresources for heterotrophic nitrification and aerobic denitrification is rarely reported due to the lack of work to explore their nitrogen biodegradation capabilities. Streptomyces mediolani EM-B2 belonging to actinomycetes could effectively remove high concentration of multiple nitrogen forms, and the maximum removal rates of ammonium, nitrate and nitrite reached 3.46 mg/(L·h), 1.71 mg/(L·h) and 1.73 mg/(L·h), respectively. Nitrite was preferentially consumed from the simultaneous nitrification and denitrification reaction system. Nitrogen balance analysis uncovered that more than 37% of the initial total nitrogen was converted to nitrogenous gas by aerobic denitrification. Experiments with specific inhibitors of nitrification and denitrification revealed that strain EM-B2 contained ammonia monooxygenase, hydroxylamine oxidoreductase, nitrate reductase and nitrite oxidoreductase, which were successfully expressed and detected as 0.43, 0.59, 0.12 and 0.005 U/mg proteins, respectively. These findings may provide new insights into the actinomycetes for bioremediation of nitrogen pollution wastewater.

Heterotrophic nitrification and aerobic denitrification process: Promising but a long way to go in the wastewater treatment

The traditional biological nitrogen removal (BNR) follows the conventional scheme of sequential nitrification and denitrification. In recent years, novel processes such as anaerobic ammonia oxidation (anammox), complete oxidation of ammonia to nitrate in one organism (comammox), heterotrophic nitrification and aerobic denitrification (HN-AD), and dissimilatory nitrate reduction to ammonium (DNRA) are gaining tremendous attention after the discovery of metabolically versatile bacteria. Among them, HN-AD offers several advantages because individual bacteria could achieve one-stage nitrogen removal under aerobic conditions in the presence of organic carbon. In this review, besides classical BNR processes, we summarized the existing literature on HN-AD bacteria which have been isolated from diverse habitats. A particular focus was given on the diversity and physiology of HN-AD bacteria, influences of physiological and biochemical factors on their growth, nitrogen removal performances, as well as limitations and strategies in unraveling HN-AD metabolic pathways. We also presented case studies of HN-AD application in wastewater treatment facilities, pointed out forthcoming challenges of HN-AD in these systems, and presented modulation strategies for HN-AD application in engineering. This review may help improve the existing design of wastewater treatment plants by harnessing HN-AD bacteria for effective nitrogen removal.

Microstructure and Mechanical Properties of Carbide Reinforced TiC-Based Ultra-High Temperature Ceramics: A Review

TiC ceramics have become one of the most potential ultra-high temperature structural materials, because of its high melting point, low density, and low price. However, the poor mechanical properties seriously limit its development and application. In this work, this review follows PRISMA standards, the mechanism of the second phase (particles, whiskers, and carbon nanotubes) reinforced TiC ceramics was reviewed. In addition, the effects of the second phase on the microstructure, phase composition and mechanical properties of TiC ceramics were systematically studied. The addition of carbon black effectively eliminates the residual TiO2 in the matrix, and the bending strength of the matrix is effectively improved by the strengthening bond formed between TiC; SiC particles effectively inhibit the grain growth through pinning, the obvious crack deflection phenomenon is found in the micrograph; The smaller grain size of WC plays a dispersion strengthening role in the matrix and makes the matrix uniformly refined, and the addition of WC forms (Ti, W) C solid solution, WC has a solid solution strengthening effect on the matrix; SiC whiskers effectively improve the fracture toughness of the matrix through bridging and pulling out, the microscopic diagram and mechanism diagram of SiC whisker action process are shown in this paper. The effect of new material carbon nanotubes on the matrix is also discussed; the bridging effect of CNTs can effectively improve the strength of the matrix, during sintering, some CNTs were partially expanded into GNR, in the process of crack bridging and propagation, more fracture energy is consumed by flake GNR. Finally, the existing problems of TiC-based composites are pointed out, and the future development direction is prospected.