Heterotrophic nitrifying/aerobic denitrifying bacteria: Ammonium removal under different physical-chemical conditions and molecular characterization

Pangenomic landscapes shape performances of a synthetic genetic circuit across Stutzerimonas species

Engineering identical genetic circuits into different species typically results in large differences in performance due to the unique cellular environmental context of each host, a phenomenon known as the "chassis-effect" or "context-dependency". A better understanding of how genomic and physiological contexts underpin the chassis-effect will improve biodesign strategies across diverse microorganisms. Here, we combined a pangenomic-based gene expression analysis with quantitative measurements of performance from an engineered genetic inverter device to uncover how genome structure and function relate to the observed chassis-effect across six closely related Stutzerimonas hosts. Our results reveal that genome architecture underpins divergent responses between our chosen non-model bacterial hosts to the engineered device. Specifically, differential expression of the core genome, gene clusters shared between all hosts, was found to be the main source of significant concordance to the observed differential genetic device performance, whereas specialty genes from respective accessory genomes were not significant. A data-driven investigation revealed that genes involved in denitrification and components of trans-membrane transporter proteins were among the most differentially expressed gene clusters between hosts in response to the genetic device. Our results show that the chassis-effect can be traced along differences among the most conserved genome-encoded functions and that these differences create a unique biodesign space among closely related species.IMPORTANCEContemporary synthetic biology endeavors often default to a handful of model organisms to host their engineered systems. Model organisms such as Escherichia coli serve as attractive hosts due to their tractability but do not necessarily provide the ideal environment to optimize performance. As more novel microbes are domesticated for use as biotechnology platforms, synthetic biologists are urged to explore the chassis-design space to optimize their systems and deliver on the promises of synthetic biology. The consequences of the chassis-effect will therefore only become more relevant as the field of biodesign grows. In our work, we demonstrate that the performance of a genetic device is highly dependent on the host environment it operates within, promoting the notion that the chassis can be considered a design variable to tune circuit function. Importantly, our results unveil that the chassis-effect can be traced along similarities in genome architecture, specifically the shared core genome. Our study advocates for the exploration of the chassis-design space and is a step forward to empowering synthetic biologists with knowledge for more efficient exploration of the chassis-design space to enable the next generation of broad-host-range synthetic biology.

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.

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.

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.

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.

Isolation and characteristics of two heterotrophic nitrifying and aerobic denitrifying bacteria, Achromobacter sp. strain HNDS-1 and Enterobacter sp. strain HNDS-6

In order to solve nitrogen pollution in environmental water, two heterotrophic nitrifying and aerobic denitrifying strains isolated from acid paddy soil were identified as Achromobacter sp. strain HNDS-1 and Enterobacter sp. strain HNDS-6 respectively. Strain HNDS-1 and strain HNDS-6 exhibited amazing ability to nitrogen removal. When (NH₄)₂SO₄, KNO₃, NaNO₂ were used as nitrogen resource respectively, the NH₄⁺-N, NO₃^--N, NO₂^--N removal efficiencies of strain HNDS-1 were 93.31%, 89.47%, and 100% respectively, while those of strain HNDS-6 were 82.39%, 96.92%, and 100%. And both of them could remove mixed nitrogen effectively in low C/N (C/N = 5). Strain HNDS-1 could remove 76.86% NH₄⁺-N and 75.13% NO₃^--N. And strain HNDS-6 can remove 65.07% NH₄⁺-N and 78.21% NO₃^--N. A putative ammonia monooxygenase, nitrite reductase, nitrate reductase, assimilatory nitrate reductase, nitrate/nitrite transport protein and nitric oxide reductase of strain HNDS-1, while hydroxylamine reductase, nitrite reductase, nitrate reductase, assimilatory nitrate reductase, nitrate/nitrite transport protein, and nitric oxide reductase of strain HNDS-6 were identified by genomic analysis. DNA-SIP analysis showed that genes Nxr, narG, nirK, norB, nosZ were involved in nitrogen removal pathway, which indicates that the denitrification pathway of strain HNDS-1 and strain HNDS-6 was NO₃^-→NO₂^-→NO→N₂O→N₂ during NH₄⁺-N removal process. And the nitrification pathway of strain HNDS-1 and strain HNDS-6 was NO₂^-→NO₃^-, but the nitrification pathway of NH₄⁺→ NO₂^- needs further studies.

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.

Electrochemical Removal of Nitrogen Compounds from a Simulated Saline Wastewater

In the last few years, many industrial sectors have generated and discharged large volumes of saline wastewater into the environment. In the present work, the electrochemical removal of nitrogen compounds from synthetic saline wastewater was investigated through a lab-scale experimental reactor. Experiments were carried out to examine the impacts of the operational parameters, such as electrolyte composition and concentration, applied current intensity, and initial ammoniacal nitrogen concentration, on the total nitrogen removal efficiency. Using NaCl as an electrolyte, the N_TOT removal was higher than Na₂SO₄ and NaClO₄; however, increasing the initial NaCl concentration over 250 mg·L^-1 resulted in no benefits for the N_TOT removal efficiency. A rise in the current intensity from 0.05 A to 0.15 A resulted in an improvement in N_TOT removal. Nevertheless, a further increase to 0.25 A led to basically no enhancement of the efficiency. A lower initial ammoniacal nitrogen concentration resulted in higher removal efficiency. The highest N_TOT removal (about 75%) was achieved after 90 min of treatment operating with a NaCl concentration of 250 mg·L^-1 at an applied current intensity of 0.15 A and with an initial ammoniacal nitrogen concentration of 13 mg·L^-1. The nitrogen degradation mechanism proposed assumes a series-parallel reaction system, with a first step in which NH₄⁺ is in equilibrium with NH₃. Moreover, the nitrogen molar balance showed that the main product of nitrogen oxidation was N₂, but NO₃^- was also detected. Collectively, electrochemical treatment is a promising approach for the removal of nitrogen compounds from impacted saline wastewater.

Complete genome analysis of Pseudomonas furukawaii ZS1 isolated from grass carp (Ctenopharyngodon idellus) culture water

Pseudomonas furukawaii ZS1, isolated from grass carp (Ctenopharyngodon idellus) culture water, exhibits efficient aerobic nitrate reduction without nitrite accumulation; however, the molecular pathway underlying this aerobic nitrate reduction remains unclear. In this study, we constructed a complete genome map of P. furukawaii ZS1 and performed a comparative genomic analysis with a reference strain. The results showed that P. furukawaii ZS1 genome was 6 026 050 bp in size and contained 5427 predicted protein-coding sequences. The genome contained all the necessary genes for the dissimilatory nitrate reduction to ammonia pathway but lacked those for the assimilatory nitrate reduction pathway; additionally, genes that convert ammonia to organic nitrogen were also identified. The presence of putative genes associated with the nitrogen and oxidative phosphorylation pathways implied that ZS1 can perform respiration and nitrate reduction simultaneously under aerobic conditions, so that nitrite is rapidly consumed for detoxication by denitrification. The aim of this study is to indicate the great potential of strain ZS1 for future full-scale applications in aquaculture. This work provided insights at the molecular level on the nitrogen metabolic pathways in Pseudomonas species. The understanding of nitrogen metabolic pathways also provides significant molecular information for further Pseudomonas species modification and development.

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.

Solid slow-release carbon sources improve the simultaneous nitrification and denitrification processes in low carbon resource wastewater

In this study, A Acinetobacter pittii sp. was isolated with high efficiency for heterotrophic nitrification and aerobic denitrification (HN-AD). The boundary conditions for total nitrogen (TN) removal were as follows: C/N ratios 8-14, temperature 25-35 °C, initial pH 7-9, and shaker speed 100-120 rpm. Addition of mixed carbon resources achieved 97.38 % ammonia-N and 91.50 % TN removal, which was higher than that of the group with sole carbon resources. The ammonia-N and TN removal profiles matched well with first-order kinetics in the rapid response period and zero-order kinetics in the slow reaction period. Meanwhile, enzyme activity related to nitrogen conversion would remarkably increase with mixed carbon resources. Furthermore, proposed a possible relationship between the solid carbon source, hydrolysis, soluble small molecule organic matter, microbial activity, and heterotrophic nitrification and aerobic denitrification (HN-AD). This study provides a new strategy for improving nitrogen removal in wastewater with low-carbon resources.

Dissimilatory and Cytoplasmic Antimonate Reductions in a Hydrogen-Based Membrane Biofilm Reactor

A hydrogen-based membrane biofilm reactor (H₂-MBfR) was operated to investigate the bioreduction of antimonate [Sb(V)] in terms of Sb(V) removal, the fate of Sb, and the pathways of reduction metabolism. The MBfR achieved up to 80% Sb(V) removal and an Sb(V) removal flux of 0.55 g/m²·day. Sb(V) was reduced to Sb(III), which mainly formed Sb₂O₃ precipitates in the biofilm matrix, although some Sb(III) was retained intracellularly. High Sb(V) loading caused stress that deteriorated performance that was not recovered when the high Sb(V) loading was removed. The biofilm community consisted of DSbRB (dissimilatory Sb-reduction bacteria), SbRB (Sb-resistant bacteria), and DIRB (dissimilatory iron-reducing bacteria). Dissimilatory antimonate reduction, mediated by the respiratory arsenate reductase ArrAB, was the main reduction route, but respiratory reduction coexisted with cytoplasmic Sb(V)-reduction mediated by arsenate reductase ArsC. Increasing Sb(V) loading caused stress that led to increases in the expression of arsC gene and intracellular accumulation of Sb(III). By illuminating the roles of the dissimilatory and cytoplasmic Sb(V) reduction mechanism in the biofilms of the H₂-MBfR, this study reveals that the Sb(V) loading should be controlled to avoid stress that deteriorates Sb(V) reduction.

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.

Enhanced denitrification of sewage via bio-microcapsules embedding heterotrophic nitrification-aerobic denitrification bacteria Acinetobacter pittii SY9 and corn cob

In this work, bio-microcapsules were prepared by embedding heterotrophic nitrification and aerobic denitrification (HN-AD) bacteria (Acinetobacter Pittii SY9) and corn cob. Bio-microcapsules (20 g/L of corn cob and 30% v/v suspension of strain SY9) were porous (pore size 2579.74-3725.44 nm; porosity 53.6%-79.9%). Under the appropriate conditions (C/N > 2, temperature of 20-35 ℃, rotation speed of 100-120 rpm, pH of 7-9), TN removal efficiency of bio-microcapsules reached 94.4%, and 74.0% of nitrogen was converted into N₂. The results of kinetics fitting indicated that aerobic denitrification was the limiting step during HN-AD process. Bio-microcapsules could slow the carbon release of corn cob for 120 days, which ensuring high HN-AD performance even at low C/N of 2.8. Bio-microcapsule SBR could stably run for 88 days with TN removal efficiency > 90% for synthetic sewage. Bio-microcapsules embedding strain SY9 and corn cob have prospective applications for enhancing denitrification of sewage.

Carbon source affects the resource recovery in aerobic granular sludge systems treating wastewater

This study evaluated the influence of carbon sources on alginate-like exopolymers (ALE) and tryptophan (Trp) biosynthesis in the aerobic granular sludge (AGS). With acetate, the highest biopolymers levels, per gram of volatile suspended solids (VSS) (418.7 mgALE∙g^-1 and 4.1 mgTrp∙gVSS^-1), were found likely due to biomass loss throughout the operation, which resulted in lower sludge age (4-7 days) and shorter famine period. During granulation, encouraging results on ALE production were obtained with propionate (>250 mgALE∙gVSS^-1), significantly higher than those found with glycerol, glucose, and sucrose. Regarding tryptophan production, propionate and glycerol proved to be good substrates, although the content was still lower than acetate (1.6 mgTrp∙gVSS^-1). Granules fed with glucose showed the worst results compared to the other substrates (38.5 mgALE∙VSS^-1 and 0.6 mgTrp∙gVSS^-1) due to the filamentous microorganisms' abundance found. Therefore, this study provides insights to value the production of compounds of industrial interest in AGS systems.

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.

Reactive Transport of NH4+ in the Hyporheic Zone from the Ground Water to the Surface Water

Nowadays, ammonia nitrogen (NH4+) pollution gets more and more attention in drinking water sources. This study investigated the main behavior of biogeochemical NH4+ from groundwater to surface water in a hyporheic zone (HZ) sediment from a reservoir. The experiments were conducted using synthetic groundwater to investigate ammonium transformation. The results indicated that ammonium concentration decreased, apparently resulting from the influence of microbial oxidation and ion exchange with Ca2+, Mg2+, K+, and Na+. However, all the ammonium in the sediment was oxidized, then the adsorbed NH4+ became bioavailable by being released back when NH4+ concentration decreased in the aqueous phase. The results showed NH4+ behavior in a HZ where the aerobic and anaerobic environments frequently exchange, with different hydrological conditions controlled by a strong coupling between microbial activities, geochemistry, hydrology, and ion exchange.

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.

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

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

A review of research progress of heterotrophic nitrification and aerobic denitrification microorganisms (HNADMs)

Traditional nitrogen removal relies on the autotrophic nitrification and anaerobic denitrification process. In the system, autotrophic microorganisms achieve nitrification under aerobic condition and heterotrophic microorganisms complete the denitrification in anaerobic condition. As the two types of microorganisms have different tolerance on oxygen concentration, nitrification and denitrification are normally set in two compartments for high nitrogen removal. Therefore, large land occupying is required. In fact, there is a special type of microorganism called heterotrophic nitrification & aerobic denitrification microorganisms (HNADMs) which can oxidize ammonium nitrogen, and perform denitrification in the presence of oxygen. HNADMs have been reported in many environments. It was found that HNADMs could simultaneously achieve nitrification and denitrification. In addition, some HNADMs not only have the ability to remove nitrogen, but also have the ability to remove phosphorus. It suggests that HNADMs have great potential for pollution removal from wastewater. So far, individual work on single strain was carried out. Comprehensive summary of the HNADMs would provide a better picture for understanding and directing its application. In this paper, the studies related on HNADMs were reviewed. The nitrogen metabolism pathway of HNADMs was summarized. The impact of pH, DO, carbon source, and C/N on HNADMs growth and metabolism were discussed. In addition, the extracellular polymeric substance (EPS) production, quorum sensing (QS) secretion and P removal by HNADMs were displayed.

Enhanced nitrogen removal from low-temperature wastewater by an iterative screening of cold-tolerant denitrifying bacteria

The biological process to remove nitrogen in winter effluent is often seriously compromised due to the effect of low temperatures (< 13 °C) on the metabolic activity of microorganisms. In this study, a novel heterotrophic nitrifying-aerobic denitrifying bacterium with cold tolerance was isolated by iterative domestication and named Moraxella sp. LT-01. The LT-01 maintained almost 60% of its maximal growth activity at 10 °C. Under initial concentrations of 100 mg/L, the removal efficiencies of ammonium, nitrate, nitrite by LT-01 were 70.3%, 65.4%, 61.7% respectively for 72 h incubation at 10 °C. Nitrogen balance analysis showed that about 46% of TN was released as gases and 16% of TN was assimilated for cell growth. The biomarker genes involved in nitrification and denitrification pathways were identified by gene-specific PCR and revealed that the LT-01 has nitrite reductase (NirS) but not hydroxylamine reductase (HAO), which implies the involvement of other genes in the process. The study indicates that LT-01 has the potential for use in low-temperature regions for efficient sewage treatment.

Characteristics and mechanism of heterotrophic nitrification/aerobic denitrification in a novel Halomonas piezotolerans strain

A strain was isolated from an activated sludge system and identified as Halomonas piezotolerans HN2 in this study, which is the first strain in H. piezotolerans with the capability of heterotrophic nitrification and aerobic denitrification. Strain HN2 showed the maximum nitrogen removal rate of 9.10 mg/L/h by utilizing ammonium at the salinity of 3.0%. Under saline environment, HN2 could remove nitrogen efficiently in neutral and slightly alkaline environments, with the carbon sources of sodium succinate and sodium citrate and the C/N ratio of 15-20, and the maximum removal efficiencies of ammonium, nitrite, and nitrate were 100%, 96.35%, and 99.7%, respectively. The genomic information revealed the presence of amoA, napA, and nosZ genes in strain HN2, and the target bands of nirS were obtained via a polymerase chain reaction. Therefore, we inferred that ammonium was mainly utilized for the growth of strain HN2 through assimilation, and another part of the initial ammonium was converted into nitrate through nitrification, and then into gaseous nitrogen through denitrification. This report indicated the potential application of strain HN2 and other nitrifying and denitrifying Halomonas strains in the removal of nitrogen pollution in marine-related environments and also implies the important role of Halomonas in the nitrogen cycle process of the ocean.

Advances in Studies on Microbiota Involved in Nitrogen Removal Processes and Their Applications in Wastewater Treatment

The discharge of excess nitrogenous pollutants in rivers or other water bodies often leads to serious ecological problems and results in the collapse of aquatic ecosystems. Nitrogenous pollutants are often derived from the inefficient treatment of industrial wastewater. The biological treatment of industrial wastewater for the removal of nitrogen pollution is a green and efficient strategy. In the initial stage of the nitrogen removal process, the nitrogenous pollutants are converted to ammonia. Traditionally, nitrification and denitrification processes have been used for nitrogen removal in industrial wastewater; while currently, more efficient processes, such as simultaneous nitrification-denitrification, partial nitrification-anammox, and partial denitrification-anammox processes, are used. The microorganisms participating in nitrogen pollutant removal processes are diverse, but information about them is limited. In this review, we summarize the microbiota participating in nitrogen removal processes, their pathways, and associated functional genes. We have also discussed the design of efficient industrial wastewater treatment processes for the removal of nitrogenous pollutants and the application of microbiome engineering technology and synthetic biology strategies in the modulation of the nitrogen removal process. This review thus provides insights that would help in improving the efficiency of nitrogen pollutant removal from industrial wastewater.

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

A novel strain was isolated from swinewastewater and identified as Acinetobacter sp. TAC-1 based on its phylogenetic and phenotypic characteristics. The strain TAC-1 was found to have a high ability to metabolize ammonium-N under low temperature condition. The strain TAC-1 could remove approximately 94.6% of ammonium-N (400 mg/L), 93.3% of nitrate-N (400 mg/L) and 42.4% of nitrite-N (400 mg/L) at 5 °C. The functional genes nitrate reductase gene (narG) and nitrite reductase gene (nirK, nirS) were successfully amplified by qPCR, further evidencing the heterotrophic nitrification and aerobic denitrification capability of Acinetobacter sp. TAC-1. The transcriptome data confirmed that the membrane transport protein and unsaturated fatty acid dehydrogenase-related genes of the strain TAC-1 were significantly up-regulated at 5 °C, enabling it to survive low temperatures. The high nitrogen removal ability at 5 °C makes this strain have a good application prospect.

Performance of single-stage partial nitritation and anammox reactor treating low-phenol/ammonia ratio wastewater and analysis of microbial community structure

Phenol and ammonia are common pollutants in many industrial wastewaters. The partial nitritation and anammox process is a very promising technology for treating phenol-ammonia wastewater. This study was the first time to rapidly achieve the start-up and operation of the single-stage partial nitritation /anammox reactor treating phenol-ammonia wastewater. The optimized ratio of phenol and nitrogen (phenol/NH₄ ⁺ -N=0.3) was set to start-up the reactor. After 60 days of operation, the total nitrogen and COD removal efficiencies were around 73.0% and 79.5%, respectively. The activity of ammonium-oxidizing bacteria was291.1 ± 3.0 mg NH₄ ⁺ -N g^-1 MLVSS d^-1 and the specific anammox activity was 20.9 ± 1.0 mg NH₄ ⁺ -N g^-1 MLVSS d^-1 . The results indicated that the anammox bacteria had adapted to phenol condition and remained stable activity after the 60 days' operation in the reactor. The sequence analysis of 16SrRNA showed that the microbial community structure evolved to a balanced distribution that the removals of phenol and ammonia could be achieved simultaneously. PRACTITIONER POINTS: Phenol/N ratio of 0.3 was set to start up the single-stage partial nitritation/anammox reactor. The single-stage partial nitritation /anammox reactor was rapidly started up when treating the phenol-ammonia wastewater. Total nitrogen removal rate and COD removal efficiencies could achieve to 73.0% and 79.5%, respectively. Microbial community structure evolved to stable distribution of which AOB, anammox bacteria, denitrification bacteria and heterotrophic nitrification bacteria coexisted.

Recent advancements in the biological treatment of high strength ammonia wastewater

The estimated global population growth of 81 million people per year, combined with increased rates of urbanization and associated industrial processes, result in volumes of high strength ammonia wastewater that cannot be treated in a cost-effective or sustainable manner using the floc-based conventional activated sludge approach of nitrification and denitrification. Biofilm and aerobic granular sludge technologies have shown promise to significantly improve the performance of biological nitrogen removal systems treating high strength wastewater. This is partly due to enhanced biomass retention and their ability to sustain diverse microbial populations with juxtaposing growth requirements. Recent research has also demonstrated the value of hybrid systems with heterogeneous bioaggregates to mitigate biofilm and granule instability during long-term operation. In the context of high strength ammonia wastewater treatment, conventional nitrification-denitrification is hampered by high energy costs and greenhouse gas emissions. Anammox-based processes such as partial nitritation-anammox and partial denitrification-anammox represent more cost-effective and sustainable methods of removing reactive nitrogen from wastewater. There is also growing interest in the use of photosynthetic bacteria for ammonia recovery from high strength waste streams, such that nitrogen can be captured and concentrated in its reactive form and recycled into high value products. The purpose of this review is to explore recent advancements and emerging approaches related to high strength ammonia wastewater treatment.

Role of heterotrophic nitrifiers and aerobic denitrifiers in simultaneous nitrification and denitrification process: A non-conventional nitrogen removal pathway in wastewater treatment

Bacterial species capable of performing both nitrification and denitrification in a single vessel under similar conditions have gained significance in the wastewater treatment scenario considering their unique character of performing the above reactions under heterotrophic and aerobic conditions respectively. Such a novel strategy often referred to as simultaneous nitrification and denitrification (SND) has a tremendous potential in dealing with various wastewaters having low C:N content, considering that the process needs very little or no external carbon source and oxygen supply thus adding to its cost-effective and environmentally friendly nature. Though like other microorganisms, heterotrophic nitrifiers and aerobic denitrifiers convert inorganic or organic nitrogen-containing substances into harmless dinitrogen gas in the wastewater, their ecophysiological role in the global nitrogen cycle is still not yet fully understood. Attempts to highlight the role played by the heterotrophic nitrifiers and aerobic denitrifiers in dealing with nitrogen pollution under various environmental operating conditions will help in developing a mechanistic understanding of the SND process to address the issues faced by the traditional methods of aerobic autotrophic nitrification-anaerobic heterotrophic denitrification.

Impacts of ammonia loading and biofilm age on the prevalence of nitrogen-cycling microorganisms in a full-scale submerged attached-growth reactor

This study reports the impacts of seasonal ammonia load changes and biofilm age on the quantity of biomass and on the prevalence of ammonia- and nitrite-metabolizing organisms within a submerged attached-growth reactor (SAGR™) following lagoon treatment. Ammonia (NH₃ ) loadings (0.12-3.17 kg/d) in the primary SAGR were measured over 223 days from May to December in 2017. Adjustment of the wastewater flow path on September 1 successfully increased NH₃ loading to the primary SAGR, which subsequently caused reactor biomass to increase. The NH₃ removal rate in October (0.5 kg/d) was greater than rates in June and July (0.3 and 0.2 kg/d) despite a water temperature decrease from >24 to 15.6°C. This elevated removal rate in October, and the sustained removal rate in December (0.4 kg/d, 5.3°C) were associated with a measured increase in microbial biomass. The relative abundance of the anammox organism C. Brocadia was 5 times greater in the mature biofilm after 686 days of growth, and the genus Pseudomonas increased sevenfold. The presence of Pseudomonas, which contains denitrifying species, and anammox suggests a high potential for removal of total nitrogen in SAGRs. PRACTITIONER POINTS: Pseudomonas prevalence and the presence of anammox suggest a high potential for total nitrogen removal in mature SAGR biofilms. The abundance of the anammox microorganism C. Brocadia was greater after 686 days of biofilm growth compared with 33 days. Simple operational changes can increase biomass in the SAGR to maintain, or even increase, NH₃ transformation rates during cold weather.

Culturable nitrogen-transforming bacteria from sequential sedimentation biofiltration systems and their potential for nutrient removal in urban polluted rivers

Novel heterotrophic bacterial strains—Bzr02 and Str21, effective in nitrogen transformation, were isolated from sequential sedimentation-biofiltration systems (SSBSs). Bzr02, identified as Citrobacter freundii, removed up to 99.0% of N–NH₄ and 70.2% of N–NO₃, while Str21, identified as Pseudomonas mandelii, removed up to 98.9% of N–NH₄ and 87.7% of N–NO₃. The key functional genes napA/narG and hao were detected for Bzr02, confirming its ability to reduce nitrate to nitrite and remove hydroxylamine. Str21 was detected with the genes narG, nirS, norB and nosZ, confirming its potential for complete denitrification process. Nitrogen total balance experiments determined that Bzr02 and Str21 incorporated nitrogen into cell biomass (up to 94.7% and 74.7%, respectively), suggesting that nitrogen assimilation was also an important process occurring simultaneously with denitrification. Based on these results, both strains are suitable candidates for improving nutrient removal efficiencies in nature-based solutions such as SSBSs.

Biological nitrogen removal capability and pathways analysis of a novel low C/N ratio heterotrophic nitrifying and aerobic denitrifying bacterium (Bacillus thuringiensis strain WXN-23)

A novel heterotrophic nitrification and aerobic denitrification (HNAD) bacteria, identified as Bacillus thuringiensis strain WXN-23, was isolated from husk feed filtrate of a pig farm. It was the first report of Bacillus thuringiensis with the capability for HNAD and could adapt to the condition of low Carbon/Nitrogen (C/N) ratio. Nitrogen could be efficiently removed by the strain WXN-23 in simulated wastewater, be it in single or mixed form nitrogen sources. The nitrogen balance revealed that 63.5% of the initial nitrogen (5.32 mg) was lost in the form of N₂. The conditions for maximum total nitrogen (TN) removal efficiency (95.996%) were shaking speed of 126.89 r/min, a carbon C/N ratio of 5.91, the temperature of 32.81 °C, and a pH value of 8.17. The nitrification-denitrification metabolic pathway (NH₄⁺-N→NH₂OH→NO₂^--N→NO₃^--N→NO₂^--N→NO→N₂O→N₂) under aerobic conditions was determined on the basic of characteristic of N removal, N balance analysis, enzyme assay and functional genes amplification results. Strain WXN-23 was effective at wastewater treatment, with TN, NH₄⁺-N, NO₃^--N and NO₂^--N removal efficiencies of 82.12%, 86.74%, 90.74% and 100%, respectively.

Heterotrophic nitrification-aerobic denitrification characteristics and antibiotic resistance of two bacterial consortia from Marinomonas and Halomonas with effective nitrogen removal in mariculture wastewater

Heterotrophic nitrification-aerobic denitrification (HNAD) characteristics and antibiotic resistance of two bacterial consortia, Marinomonas communis & Halomonas titanicae (MCH) and Marinomonas aquimarina & Halomonas titanicae (MAH), and their single isolates (MC, MA, and H) were determinated in this study. When cultured in sole and mixed N-source media (NH₄⁺-N and/or NO₂^--N of 10 mg/L), MCH and MAH exhibited greater efficiency and stability of inorganic-N removal than single isolates, and these strains preferred to remove NH₄⁺-N by simultaneous HNAD in mixed N-source media. Meanwhile, 45%-70% of NH₄⁺-N and/or NO₂^--N was mainly converted to organic nitrogen (15%-25%) and gaseous nitrogen (30%-40%) by these strains, and more inorganic-N was transformed to intracellular-N by MCH and MAH via assimilation instead of gaseous-N production by denitrification. Both isolates and their consortia had the maximal NH₄⁺-N or NO₂^--N removal efficiency above 95% under the optimum conditions including temperature of 20-30 °C, C/N ratios of 15-20, and sucrose as carbon source. Interestingly, bacterial consortia performed greater nitrogen removal than single isolates under the low temperature of 10 °C or C/N ratios of 2-5. In real mariculture wastewater, MCH and MAH also showed higher NH₄⁺-N removal efficiency (65%-68%) and more stable cell quantity (4.2-5.2 × 10⁸ CFU/mL) than single strains, due to the interspecific coexistence detected by bacterial quantitation with indirect immunoassay. Additionally, these isolates and consortia had stronger resistances to polypeptides, tetracyclines, sulfonamides, furanes, and macrolides than other antibiotics. These findings will be conducive to the applications of HNAD bacteria of Marinomonas and Halomonas on reducing nitrogen pollution in mariculture or other saline environments.

Nitrogen removal bacterial strains, MSNA-1 and MSD4, with wide ranges of salinity and pH resistances

Nitrogenous wastewater is difficult to treat using conventional microorganisms in high salinity and acidic/alkaline environments. Two halotolerant bacteria, heterotrophic nitrifying Stenotrophomonas sp. MSNA-1 and aerobic denitrifying Pseudomonas sp. MSD4, were isolated, and the amplification of functional genes provided the evidences of nitrogen removal performance. The results regarding salinity and pH resistance showed that strain MSNA-1 is robust at salinities of 0-15% and pH of 3-10. It can remove 51.2% of NH₄⁺-N (180 mg/L) at salinity of 10% (pH: 7) and 49.2% of NH₄⁺-N under pH 4 (salinity: 3%). For strain MSD4, it is robust at salinities of 0-10% and pH of 5-11. It can remove 62.4% of TN (100 mg/L) at salinity of 7% (pH: 7) and 72.2% of TN under pH 9 (salinity: 3%). Their excellent salinity and pH resistances make them promising candidates for treating nitrogenous wastewaters under extreme conditions with low operational cost.

Ammonium Removal by a Newly Isolated Heterotrophic Nitrification-Aerobic Denitrification Bacteria Pseudomonas Stutzeri SDU10 and Its Potential in Treatment of Piggery Wastewater

A strain SDU10 was isolated from swine manure compost and identified as Pseudomonas stutzeri SDU10. It demonstrated excellent capability in NH₄⁺-N removal. Optimal conditions of NH₄⁺-N removal were determined, which were sodium acetate as the optimal carbon source, carbon to nitrogen (C/N) ratio of 10, temperature of 30 °C, pH of 7.0. Especially, P. stutzeri SDU10 could remove high concentration NH₄⁺-N of 1500.0 and 2000.0 mg/l in 120 h with the NH₄⁺-N removal rates of 91.1% and 61.6%, respectively. In batch experiments, the highest NH₄⁺-N removal rate of 97.6% and chemical oxygen demand (COD) removal rate of 94.2% were obtained at initial C/N ratio 10 during piggery wastewater treatment using P. stutzeri SDU10. Results showed that P. stutzeri SDU10 had the potential for treatment of wastewater of high NH₄⁺-N concentration.

Isolation and characterization of a salt-tolerant denitrifying bacterium Alishewanella sp. F2 from seawall muddy water

A salt-tolerant denitrifying bacterium strain F2 was isolated from seawall muddy water in Dalian City, Liaoning Province, China. Strain F2 was identified by morphological observations, physiological and biochemical characteristics and 16 S rDNA identification. The salt tolerance of strain F2 was verified and the factors affecting the removal ability of strain F2 to nitrous nitrogen (NO2-N) and nitrate nitrogen (NO3-N) in saline conditions were investigated. Strain F2 was identified as Alishewanella sp., named Alishewanella sp. F2. Strain F2 can tolerate NaCl concentrations up to 70 g/L, and its most efficient denitrification capacity was observed at NaCl concentrations of 0-30 g/L. In the medium with NaCl concentrations of 0-30 g/L, strain F2 exhibited high removal efficiencies of NO2-N and NO3-N, with the removal percentages for both NO2-N and NO3-N of approximately 99%. In saline conditions with 30 g/L NaCl, the optimum culture pH, NaNO2 initial concentrations and inoculation sizes of strain F2 were 8-10, 0.4-0.8 g/L and 5-7%, respectively. Strain F2 was highly effective in removing NO2-N and NO3-N in saline conditions, and it has a good application potential in saline wastewater treatment.