Determining Multiple Responses of Pseudomonas aeruginosa PAO1 to an Antimicrobial Agent, Free Nitrous Acid

Potential of free nitrous acid (FNA) for sludge treatment and resource recovery from waste activated sludge: A review

The escalating production of waste activated sludge (WAS) presents significant challenges to wastewater treatment plants (WWTPs). Free nitrous acid (FNA), known for its biocidal effect, has gained a growing focus on sludge dewatering, sludge reduction, and resource recovery from WAS due to its eco-friendly and cost-effective properties. Nevertheless, there have been no attempts made to systematically summarize or critically analyze the application of FNA in enhancing treatment and resource utilization of sludge. In this paper, we provided an overview of the current understanding regarding the application potential and influencing factors of FNA in sludge treatment, with a specific focus on enhancing sludge dewatering efficiency and reducing volume. To foster resource development from sludge, various techniques based on FNA have recently been proposed, which were comprehensively reviewed with the corresponding mechanisms meticulously discussed. The results showed that the chemical oxidation and interaction with microorganisms of FNA played the core role in improving resource utilization. Furthermore, current challenges and future prospects of the FNA-based applications were outlined. It is expected that this review can refine the theoretical framework of FNA-based processes, providing a theoretical foundation and technical guidance for the large-scale demonstration of FNA.

Fulvic acid more facilitated the soil electron transfer than humic acid

Humus substances (HSs) participate in extracellular electron transfer (EET), which is unclear in heterogeneous soil. Here, a microbial electrochemical system (MES) was constructed to determine the effect of HSs, including humic acid, humin and fulvic acid, on soil electron transfer. The results showed that fulvic acid led to the optimal electron transfer efficiency in soil, as evidenced by the highest accumulated charges and removal of total petroleum hydrocarbons after 140 days, with increases of 161% and 30%, respectively, compared with those of the control. However, the performance of MES with the addition of humic acid and humin was comparable to that of the control. Fulvic acid amendment enhanced the carboxyl content and oxidative state of dissolved organic matter, endowing a better electron transfer capacity. Additionally, the presence of fulvic acid induced an increase in the abundance of electroactive bacteria and organic degraders, extracellular polymeric substances and functional enzymes such as cytochrome c and NADH synthesis, and the expression of m tr C gene, which is responsible for EET enhancement in soil. Overall, this study reveals the mechanism by which HSs stimulate soil electron transfer at the physicochemical and biological levels and provides basic support for the application of bioelectrochemical technology in soil.

Mechanisms of endogenous and exogenous partial denitrification in response to different carbon/nitrogen ratios: Transcript levels, nitrous oxide production, electron transport

Nitrite as an important substrate for Anammox can be provided by partial denitrification (PD). In this study, endogenous partial denitrification (EdPD) and exogenous partial denitrification (ExPD) sludge were domesticated and their nitrite transformation rate reached 74.4% and 83.4%, respectively. The impact of four carbon/nitrogen (C/N) ratios (1.5, 3.0, 5.0 and 6.0) on nitrous oxide (N2O) emission and denitrification functional genes expression in both PD systems were investigated. Results showed that elevated C/N ratios enhanced most denitrification genes expression, but in EdPD, high nitrite levels suppressed nosZ genes expression (from 9.4% to 1.4%), leading to increased N2O emission (0 to 3.4%). EdPD also exhibited lower electron transfer system activity, resulting in slower nitrogen oxide conversion efficiency and more stable nitrite accumulation compared to ExPD. These findings offer insights for optimizing PD systems under varying water quality conditions.

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

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

The co-occurrent microplastics and nano-CuO showed antagonistic inhibitory effects on bacterial denitrification: Interaction of pollutants and regulations on functional genes

The wide occurrence of microplastics (MPs) and nanoparticles resulted in their inevitable coexistence in environment. However, the joint effects of these two types of particulate emerging contaminants on denitrification have seldomly been investigated. Herein, non-biodegradable polyvinyl chloride, polypropylene, polyethylene and biodegradable polyhydroxyalkanoate (PHA) MPs were chosen to perform the co-occurrent effects with nano copper oxide (nano-CuO). Both the nano-CuO and MPs inhibited the denitrification process, and biodegradable PHA-MPs showed severer inhibition than non-biodegradable MPs. However, the presence of MPs significantly alleviated the inhibition of nano-CuO, suggesting an antagonistic effect. Other than MPs decreasing copper ion release from nano-CuO, MPs and nano-CuO formed agglomerations and induced lower levels of oxidative stress compared to individual exposure. Transcriptome analysis indicated that the co-occurrent MPs and nano-CuO induced different regulation on denitrifying genes (e. g. nar and nor) compared to individual ones. Also, the expressions of genes involved in denitrification-associated metabolic pathways, including glycolysis and NADH electron transfer, were down-regulated by nano-CuO or MPs, but exhibiting recovery under the co-occurrent conditions. This study firstly discloses the antagonistic effect of nano-CuO and MPs on environmental process, and these findings will benefit the systematic evaluation of MPs environmental behavior and co-occurrent risk with other pollutants.

The mitigation effect of free ammonia and free nitrous acid on nitrous oxide production from the full-nitrification and partial-nitritation systems

The potentials of using endogenous free ammonia (FA) and free nitrous acid (FNA) as nitrous oxide (N₂O) mitigators were investigated in treatment of both mainstream and sidestream wastewater. Although the N₂O emission factor of a sidestream partial-nitritation (PN) reactor (averaged 1.70 % ± 0.39 %, n = 30) was about 2.4 times higher than a mainstream full-nitrification (FN) reactor (averaged 0.72 % ± 0.24 %, n = 30) (P < 0.01), one-hour exposure of PN sludge to 1.5 mg HNO₂-N/L FNA could virtually abolish N₂O emission. As for FN sludge, both 45 mg NH₃-N/L FA and 0.015 mg HNO₂-N/L FNA successfully mitigated N₂O production at varying dissolved oxygen (DO) levels (50 % vs 61 %), while 1.5 mg HNO₂-N/L FNA not only reduced more N₂O (92 %) but also altered the N₂O dependency on DO. Both FNA and FA sludge treatment were effective N₂O mitigation strategies with FNA toward the end of carbon neutrality and FA being more economically appealing (2 % cost saving).

Distinct responses of Pseudomonas aeruginosa PAO1 exposed to different levels of polystyrene nanoplastics

Large amounts of discarded plastics in the environment can be aged into microplastics and nanoplastics, which are not easily removed, posing potential nonnegligible risks to the ecosystem and human health. Although previous studies have revealed that nanoplastics have detrimental impacts on microorganisms, the potential molecular mechanisms of nanoplastic particles' effect on microbial growth and metabolism are still lacking. Here, multiple responses of Pseudomonas aeruginosa PAO1 (PAO1) to different levels of polystyrene nanoplastics (PS NPs) exposure were investigated by physiological experiments, live/dead staining, redox status, and genome-wide RNA sequencing. The results showed that PS NPs had dual effects on PAO1, and different concentrations of PS NPs demonstrated different effects on the growth and metabolism of PAO1. All levels of PS NPs had no obvious biocidal effect on PAO1. The production and consumption of ROS were in dynamic equilibrium and could be regulated genetically to ensure that the ROS level was in the biotolerable range. 20 and 50 mg/L of PS NPs severely inhibited the nitrate reduction, while 0.1 mg/L of PS NPs promoted the denitrification and TCA cycle. Meanwhile, 20 and 50 mg/L of PS NPs resulted in intense down-regulation of genes involved in denitrification. In contrast, the expression of genes involved in respiration is promoted with generated energy to withstand stress from high-level PS NPs, coinciding with the physiological results. In addition, our results showed that PS NPs concentrations of 20 and 50 mg/L exposure substantially up-regulated the expression of genes encoding for flagellar biosynthesis and biofilm formation to tackle the stress. Our findings would provide new insights into the interactions between environmental bacteria and PS NPs at the transcriptional level, thereby enhancing our understanding of the potential risks of PS NPs to microbial ecosystems and public health.

Antidepressants promote the spread of antibiotic resistance via horizontally conjugative gene transfer

Antibiotic resistance is a global concern threatening public health. Horizontal gene transfer (HGT) between bacterial species contributes greatly to the dissemination of antibiotic resistance. Conjugation is one of the major HGT pathways responsible for the spread of antibiotic resistance genes (ARGs). Antidepressant drugs are commonly prescribed antipsychotics for major depressive disorders and are frequently detected in aquatic environments. However, little is known about how antidepressants stress bacteria and whether such effect can promote conjugation. Here, we report that commonly prescribed antidepressants, sertraline, duloxetine, fluoxetine, and bupropion, can promote the conjugative transfer of plasmid-borne multidrug resistance genes carried by environmentally and clinically relevant plasmids. Noteworthy, the transfer of plasmids across bacterial genera is significantly enhanced by antidepressants at clinically relevant concentrations. We also reveal the underlying mechanisms of enhanced conjugative transfer by employing flow cytometric analysis, genome-wide RNA sequencing and proteomic analysis. Antidepressants induce the production of reactive oxygen species and the SOS response, increase cell membrane permeability, and upregulate the expression of conjugation relevant genes. Given the contribution of HGT in the dissemination of ARGs, our findings highlight the importance of prudent prescription of antidepressants and to the potential connection between antidepressants and increasing antibiotic resistance.

Understanding the influence of free nitrous acid on microalgal-bacterial consortium in wastewater treatment: A critical review

Microalgal-bacterial consortium (MBC) constitutes a sustainable and efficient alternative to the conventional activated sludge process for wastewater treatment (WWT). Recently, integrating the MBC process with nitritation (i.e., shortcut MBC) has been proposed to achieve added benefits of reduced carbon and aeration requirements. In the shortcut MBC system, nitrite or free nitrous acid (FNA) accumulation exerts antimicrobial influences that disrupt the stable process performance. In this review, the formation and interactions that influence the performance of the MBC were firstly summarized. Then the influence of FNA on microalgal and bacterial monocultures and related mechanisms together with the knowledge gaps of FNA influence on the shortcut MBC were highlighted. Other challenges and future perspectives that impact the scale-up of the shortcut MBC for WWT were illustrated. A potential roadmap is proposed on how to maximize the stable operation of the shortcut MBC system for sustainable WWT and high-value biomass production.

Insights into the multi-targeted effects of free nitrous acid on the microalgae Chlorella sorokiniana in wastewater

Microalgal-bacterial consortium process (MBCP) proposed as an alternative to the activated sludge process contains free nitrous acid (FNA). FNA antimicrobial influences on nitrifiers have been demonstrated. However, its influence on microalgae is largely unknown, limiting the system stability of MBCP. This study revealed the multi-targeted responses of a model wastewater microalgae, Chlorella sorokiniana, to FNA exposure through physiological and transcriptomic analyses. Results showed a concentration-dependent FNA-influence as both microalgal growth and photosynthesis (Fv/Fm, rETR, Y(II), NPQ) inversely correlated with FNA doses. Increased ROS, MDA content (5.0-fold), SOD (2.7-fold), and LDH (12.0-fold) activities in the treatments revealed FNA-induced oxidative pressure. Moreover, RNA-sequencing results revealed significantly downregulated genes related to photosynthesis, respiration, nitrogen metabolism, and tricarboxylic acid cycle. Comparatively, peroxisome, chlorophyll, and carotenoid genes were upregulated. These findings elucidate the inhibitory mechanisms of FNA on microalgae and contribute towards the prospective practical application of the MBCP system for sustainable wastewater treatment.

Effects and relevant mechanisms of non-antibiotic factors on the horizontal transfer of antibiotic resistance genes in water environments: A review

Antibiotic resistance has created obstacles in the treatment of infectious diseases with antibiotics. The horizontal transfer of antibiotic resistance genes (ARGs) can exacerbate the dissemination of antibiotic resistance in water environments. In addition to antibiotic selective pressure, multiple non-antibiotic factors can affect the horizontal transfer of ARGs. Herein, we seek to comprehensively review the effects and relevant mechanisms of non-antibiotic factors on the horizontal transfer of ARGs in water environments, especially contaminants from human activities and water treatment processes. Four pathways have been identified to accomplish horizontal gene transfer (HGT), i.e., conjugation, transformation, transduction, and vesiduction. Changes in conjugative frequencies by non-antibiotic factors are mainly related to their concentrations, which conform to hormesis. Relevant mechanisms involve the alteration in cell membrane permeability, reactive oxygen species, SOS response, pilus, and mRNA expression of relevant genes. Transformation induced by extracellular DNA may be more vulnerable to non-antibiotic factors than other pathways. Except bacteriophage infection, the effects of non-antibiotic factors on transduction exhibit many similarities with that of conjugation. Given the secretion of membrane vesicles stimulated by non-antibiotic factors, their effects on vesiduction can be inferred. Furthermore, contaminants from human activities at sub-inhibitory or environmentally relevant concentrations usually promote HGT, resulting in further dissemination of antibiotic resistance. The horizontal transfer of ARGs is difficult to be inhibited by individual water treatment processes (e.g., chlorination, UV treatment, and photocatalysis) unless they attain sufficient intensity. Accordingly, the synergistic application containing two or more water treatment processes is recommended. Overall, we believe this review can elucidate the significance for risk assessments of contaminants from human activities and provide insights into the development of environment-friendly and cost-efficient water treatment processes to inhibit the horizontal transfer of ARGs.

Sequential sulfur-based denitrification/denitritation and nanofiltration processes for drinking water treatment

Efficient and cost-effective solutions for nitrogen removal are necessary to ensure the availability of safe drinking water. This study proposes a combined treatment for nitrogen-contaminated groundwater by sequential autotrophic nitrogen removal in a sulfur-packed bed reactor (SPBR) and excess sulfate rejection via nanofiltration (NF). Autotrophic nitrogen removal in the SPBR was investigated under both denitrification and denitritation conditions under different NO₃^- and NO₂^- loading rates (LRs) and feeding strategies (NO₃^- only, NO₂^- only, or both NO₃^- and NO₂^- in the feed). Batch activity tests were carried out during SPBR operation to evaluate the effect of different feeding conditions on nitrogen removal activity by the SPBR biofilm. Bacteria responsible for nitrogen removal in the bioreactor were identified via Illumina sequencing. Dead-end filtration tests were performed with NF membranes to investigate the elimination of excess sulfate from the SPBR effluent. This study demonstrates that the combined process results in effective groundwater treatment and evidences that an adequately high nitrogen LR should be maintained to avoid the generation of excess sulfide.

Nitrate/nitrite dependent anaerobic methane oxidation coupling with anammox in membrane biotrickling filter for nitrogen removal

Combining nitrate/nitrite dependent anaerobic methane oxidation (n-DAMO) and anaerobic ammonium oxidation (Anammox) is a promising sustainable wastewater treatment technology, which simultaneously achieve nitrogen removal and methane emission mitigation. However, the practical application of n-DAMO has been greatly limited by its extremely slow growth-rate and low reaction rate. This work proposes an innovative Membrane BioTrickling Filter (MBTF), which consist of hollow fiber membrane for effective methane supplementation and polyurethane sponge as support media for the attachment and growth of biofilm coupling n-DAMO with Anammox. When steady state with a hydraulic retention time (HRT) of 6.00 h was reached, above 99.9% of nitrogen was removed from synthetic sidestream wastewater at a rate of 3.99 g N L^-1 d^-1. This system presented robust capacity to withstand unstable partial nitritation effluent, achieving complete nitrogen removal at a varied nitrite to ammonium ratio in the range of 1.10-1.40. It is confirmed that n-DAMO and Anammox microorganisms jointly dominated the microbial community by pyrosequencing technology. The complete nitrogen removal potential at high-rate and efficient biomass retention (12.4 g VSS L^-1) of MBTF offers promising alternative for sustainable wastewater treatment by the combination of n-DAMO and Anammox.

Regulation of las and rhl Quorum Sensing on Aerobic Denitrification in Pseudomonas aeruginosa PAO1

The bacterium Pseudomonas aeruginosa negatively regulates denitrification under anerobic conditions by two acyl-homoserine lactone quorum-sensing (QS) systems called las and rhl. However, it is unknown whether these systems have the same effect on denitrification in aerobic conditions. In this study, we investigated the regulation of las and rhl systems on aerobic denitrification. We showed that the removal of nitrate in P. aeruginosa PAO1 was repressed by both the las and rhl systems. The las and rhl systems had negative effects on activities of denitrifying enzymes NAP, NIR, NOR, and NOS. At the level of transcription, both QS systems inhibited the expression of target genes napA, nirS, norB, norC, and nosZ. Furthermore, the addition of an acylase, which degrades the acyl-homoserine lactone signals (AHLs), to wild type resulted in an increase in the removal of nitrate. Additionally, in aerobic denitrification process, the transcription factor DNR, which controls denitrification, was repressed by both QS systems. The results implied that modulation of QS in denitrifying bacteria, possibly through quorum quenching or QS inhibition, could help to improve the reduction of nitrate in wastewater treatment.

Structural Changes in Cell-Wall and Cell-Membrane Organic Materials Following Exposure to Free Nitrous Acid

Previous studies demonstrate that free nitrous acid (FNA, i.e., HNO₂) is biocidal for a range of microorganisms. The biocidal mechanisms of FNA are largely unknown. In this work, it is hypothesized that FNA will break bonds in molecules found in the cell envelope, thus causing cell lysis. Selected molecules representing components found in the cell envelope were treated with FNA at 6.09 mg N/L (NO₂^- = 250 mg N/L, pH 5.0) for 24 h (conditions typically used in applications) to evaluate the hypothesized chemical interactions. Molecular changes were observed using analytical techniques including proton (¹H) nuclear magnetic resonance spectroscopy (NMR) and electrospray ionization mass spectrometry (ESI-MS). It was found that FNA broke down a range of cell envelope molecules. The spectral data demonstrated that the FNA reactions proceeded via two general pathways. One consisted of electrophilic substitution, whereby the nitrosonium ion (NO⁺) was the reactive electrophile. The other was via oxidative reactions involving nitrogen radicals (e.g., •NO₂ and •NO) formed from the decomposition of FNA. We further revealed that it was HNO₂ that caused the breakdown, rather than the exclusive action of the acid (H⁺) or nitrite (NO₂^-) counterparts. The fragmentation of these representative cell envelope molecules provides insight into the biocidal effects of FNA on microorganisms.

Transcriptional and metabolic response against hydroxyethane-(1,1-bisphosphonic acid) on bacterial denitrification by a halophilic Pannonibacter sp. strain DN

Biological denitrification is an environmentally sound pathway for the elimination of nitrogen pollution in wastewater treatment. Extreme environmental conditions, such as the co-existence of toxic organic pollutants, can affect biological denitrification. However, the potential underlying mechanism remains largely unexplored. Herein, the effect of a model pollutant, hydroxyethane-(1,1-bisphosphonic acid) (HEDP), a widely applied and consumed bisphosphonate, on microbial denitrification was investigated by exploring the metabolic and transcriptional responses of an isolated denitrifier, Pannonibacter sp. strain DN. Results showed that nitrate removal efficiency decreased from 85% to 50% with an increase in HEDP concentration from 0 to 3.5 mM, leading to nitrite accumulation of 204 mg L^-1 in 3.5 mM HEDP. This result was due to the lower bacterial population count and reduction in the live cell percentage. Further investigation revealed that HEDP caused a decrease in membrane potential from 0.080 ± 0.005 to 0.020 ± 0.002 with the increase in HEDP from 0 to 3.5 mM. This hindered electron transfer, which is required for nitrate transformation into nitrogen gas. Moreover, transcriptional profiling indicated that HEDP enhanced the genes involved in ROS (O₂^-) scavenging, thus protecting cells against oxidative stress damage. However, the suppression of genes responsible for the production of NADH/FADH₂ in tricarboxylic acid cycle (TCA), NADH catalyzation (NADH dehydrogenase) in (electron transport chain) ETC system and denitrifying genes, especially nor and nir, in response to 2.5 mM HEDP were identified as the key factor inhibiting transfer of electron from TCA cycle to denitrifying enzymes through ETC system.

Enhanced shortcut nitrogen removal and metagenomic analysis of functional microbial communities in a double sludge system treating ammonium-rich wastewater

Biological nitrogen removal processes based on partial nitrification are promising for ammonium-rich wastewater treatment. In this study, a partial nitrification-denitrification double sludge system was applied to treat synthetic ammonium-rich wastewater. Metagenomic analysis of functional genes and metabolic pathways was conducted, also with the evaluation of system performance and nitrous oxide (N₂O) emission. In the nitrifying sequencing batch reactor (SBR_PN), the removal percentage of ammonium nitrogen reached to 99.98% with a high nitritation efficiency of 93.24%, and the N₂O emission factor was 0.88%. In the denitrifying sequencing batch reactor (SBR_DN), there was almost no nitrate nitrogen and nitrite nitrogen in the effluent, and the maximum N₂O emission was 0.078 mg N/L. The dominant ammonia oxidizing bacteria was Nitrosomonas in SBR_PN (13.6%), and the main potential denitrifiers in SBR_DN were Thauera (14.6%), an uncultured genus in the Comamonadaceae family (4.0%), an uncultured genus in Rhodocyclaceae family (2.4%) and Comamonas (1.1%). Metagenomic analysis revealed that amo mainly distributed in Nitrosomonas eutropha (38.3%), Nitrosomonas europaea (27.1%), Nitrosomonas sp. GH22 (20.5%) and Nitrosomonas sp. TK794 (15.0%), and Bacteroidetes had the N₂O reduction potential in SBR_PN.

Mechanisms of nitrous oxide emission during photoelectrotrophic denitrification by self-photosensitized Thiobacillus denitrificans

Photoelectrotrophic denitrification (PEDeN) using bio-hybrids has the potential to remove nitrate (NO₃^-) from wastewater in an economical and sustainable way. As a gas of global concern, the mechanisms of nitrous oxide (N₂O) emissions during this novel process remain unclear. Herein, a self-photosensitized bio-hybrid, i. e., Thiobacillus denitrificans-cadmium sulfide, was constructed and the factors affecting N₂O emissions during PEDeN by the bio-hybrids were investigated. The system was sensitive to the input NO₃^--N and NO₂^--N, resulting in changes in the N₂O/(N₂+N₂O) ratio from 1% to 95%. In addition to free nitrous acid (FNA), reactive oxidative species (ROS) were a unique factor affecting N₂O emission during PEDeN. Importantly, the N₂O reduction step exhibited greater susceptibility to the ROS than nitrate reduction step. The contributions of hydrogen peroxide (H₂O₂), superoxides (O₂^-•), hydroxyl radicals (•OH) and FNA to the inhibition of N₂O reduction were >15.0%, >5.4%, 1.3%, and <70.2%, respectively for a reduction of 13.5 mg/L NO₃^--N. A significant down-regulation of the relative transcription of the gene nosZ demonstrated that the inhibition of N₂O reductase occurred at the gene level. This finding has important implications not only for mitigating N₂O emissions during the PEDeN process but also for encouraging a reexamination process of N₂O emissions in nature, particularly in systems in which ROS are present during the denitrification process.

Improving wastewater management using free nitrous acid (FNA)

Free nitrous acid (FNA), the protonated form of nitrite, has historically been an unwanted substance in wastewater systems due to its inhibition on a wide range of microorganisms. However, in recent years, advanced understanding of FNA inhibitory and biocidal effects on microorganisms has led to the development of a series of FNA-based applications that improve wastewater management practices. FNA has been used in sewer systems to control sewer corrosion and odor; in wastewater treatment to achieve carbon and energy efficient nitrogen removal; in sludge management to improve the sludge reduction and energy recovery; in membrane systems to address membrane fouling; and in wastewater algae systems to facilitate algae harvesting. This paper aims to comprehensively and critically review the current status of FNA-based applications in improving wastewater management. The underlying mechanisms of FNA inhibitory and biocidal effects are also reviewed and discussed. Knowledge gaps and current limitations of the FNA-based applications are identified; and perspectives on the development of FNA-based applications are discussed. We conclude that the FNA-based technologies have great potential for enhancing the performance of wastewater systems; however, further development and demonstration at larger scales are still required for their wider applications.

Efficient nitrous oxide recovery from incineration leachate by a nosZ-deficient strain of Pseudomonas aeruginosa

In this study, nitrous oxide was recovered from a lab-scale moving-bed biofilm reactor (MBBR) treating partial nitrification-treated leachate supplemented with a nosZ-deficient strain of Pseudomonas aeruginosa. Batch culture tests with the nosZ-deficient strain determined that the threshold for free nitrous acid (FNA) inhibition was 0.016 mg/L and that FNA concentrations above this threshold severely inhibited denitrification and transcription of genes from the dissimilatory nitrate reduction pathway (narG, nirS, and norB). High nitrite removal and N₂O conversion efficiencies (>95%) were achieved with long-term operation of this MBBR. N₂O accounted for the majority of biogas (80%) produced when the MBBR was fed partial nitrification-treated leachate with high nitrite concentrations and the drainage ratio was adjusted to 30%. Bacterial community analysis revealed that the nosZ-deficient Pseudomonas strain remained metabolically active and was primarily responsible for denitrification processes in the reactor. This study presents a promising method for N₂O recovery from incineration leachate.

Granular Sludge Coupling Nitrate/Nitrite Dependent Anaerobic Methane Oxidation with Anammox: from Proof-of-Concept to High Rate Nitrogen Removal

This work developed a novel Membrane Granular Sludge Reactor (MGSR) equipped with a gas permeable membrane module for efficient methane delivery to cultivate nitrate/nitrite dependent anaerobic methane oxidation (n-DAMO) microorganisms in granular sludge. As proof of concept, the MGSR was fed with synthetic wastewater containing nitrate and ammonium to facilitate the growth of n-DAMO microorganisms. The granular sludge of n-DAMO and Anammox was gradually developed and achieved a nitrogen removal rate of 1.08 g NO₃^--N L^-1 d^-1 and 0.81 g NH₄⁺-N L^-1 d^-1. Finally, enriched granular sludge was successfully applied for nitrogen removal from the synthetic partial nitritation effluent. The combined dominance of n-DAMO archaea, Anammox bacteria, and n-DAMO bacteria in the microbial community was confirmed by 16S rRNA amplicon sequencing. Fluorescence in situ hybridization revealed that a layered structure was formed in the granular sludge with Anammox bacteria in the outer layer and n-DAMO microorganisms in the inner layer when granules were fed with nitrite and ammonium. The high performance of nitrogen removal (16.53 kg N m^-3 d^-1) with satisfactory effluent quality (∼8 mg N L^-1) and excellent biomass retention capacity (43 g VSS L^-1) make the MGSR promising for the practical application of n-DAMO and Anammox in wastewater treatment.

Non-invasive, ratiometric determination of intracellular pH in Pseudomonas species using a novel genetically encoded indicator

The ability of Pseudomonas species to thrive in all major natural environments (i.e. terrestrial, freshwater and marine) is based on its exceptional capability to adapt to physicochemical changes. Thus, environmental bacteria have to tightly control the maintenance of numerous physiological traits across different conditions. The intracellular pH (pHi ) homoeostasis is a particularly important feature, since the pHi influences a large portion of the biochemical processes in the cell. Despite its importance, relatively few reliable, easy-to-implement tools have been designed for quantifying in vivo pHi changes in Gram-negative bacteria with minimal manipulations. Here we describe a convenient, non-invasive protocol for the quantification of the pHi in bacteria, which is based on the ratiometric fluorescent indicator protein PHP (pH indicator for Pseudomonas). The DNA sequence encoding PHP was thoroughly adapted to guarantee optimal transcription and translation of the indicator in Pseudomonas species. Our PHP-based quantification method demonstrated that pHi is tightly regulated over a narrow range of pH values not only in Pseudomonas, but also in other Gram-negative bacterial species such as Escherichia coli. The maintenance of the cytoplasmic pH homoeostasis in vivo could also be observed upon internal (e.g. redirection of glucose consumption pathways in P. putida) and external (e.g. antibiotic exposure in P. aeruginosa) perturbations, and the PHP indicator was also used to follow dynamic changes in the pHi upon external pH shifts. In summary, our work describes a reliable method for measuring pHi in Pseudomonas, allowing for the detailed investigation of bacterial pHi homoeostasis and its regulation.

Low-level free nitrous acid efficiently inhibits the conjugative transfer of antibiotic resistance by altering intracellular ions and disabling transfer apparatus

Recently, the dissemination of antibiotic resistance genes (ARGs) via plasmid-mediated conjugation has been reported to be facilitated by a series of contaminants. This has highlighted potential challenges to the effective control of this principal mode of horizontal transfer. In the present study, we found that low levels (<0.02 mgN/L) of free nitrous acid (FNA) remarkably inhibited (over 90%) the conjugative transfer of plasmid RP4, a model broad-host-range plasmid, between Escherichia coli. The antimicrobial role of FNA at the applied dosages was firstly ruled out, since no dramatic reductions in viabilities of donor or recipient were observed. Instead, FNA appeared to reduce the available intracellular free Mg²⁺, which was confirmed to be triggered by the liberation of intracellular Fe²⁺. These alterations in intracellular Mg²⁺ and Fe²⁺ concentrations were found to significantly limit the available energy for conjugative transfer through suppression of glycolysis by decreasing the activities of glycogen phosphorylase and glyceraldehyde-3-phosphate dehydrogenase and also by diverting the glycolytic flux into the pentose phosphate pathway via activation of glucose-6-phosphate dehydrogenase towards the generation of NADPH rather than ATP. Moreover, RP4-encoding genes responsible for DNA transfer and replication (traI, traJ and trfAp), coupling (traG) and mating pair formation (traF and trbBp) were all significantly down-regulated after FNA treatment, indicating that the transfer apparatus required for plasmid processing and delivery was deactivated. By validating the inhibitory effects of FNA on conjugation in real wastewater, this study highlights a promising method for controlling the dissemination of ARGs in systems such as wastewater treatment plants.

A comparative proteomic analysis of Desulfovibrio vulgaris Hildenborough in response to the antimicrobial agent free nitrous acid

Sulfate reducing bacteria (SRB) can contribute to facilitating serious concrete corrosion through the production of hydrogen sulfide in sewers. Recently, free nitrous acid (FNA) was discovered as a promising antimicrobial agent to inhibit SRB activities thereby limiting hydrogen sulfide production in sewers. However, knowledge of the bacterial response to increasing levels of the antimicrobial agent is unknown. Here we report the proteomic response of Desulfovibrio vulgaris Hildenborough and reveal that the antimicrobial effect of FNA is multi-targeted and dependent on the FNA levels. This was achieved using a sequential window acquisition of all theoretical mass spectrometry analysis to determine protein abundance variations in D. vulgaris during exposure to different FNA concentrations. When exposed to 1.0 μg N/L FNA, nitrite reduction (nitrite reductase) related proteins and nitrosative stress related proteins, including the hybrid cluster protein, showed distinct increased abundances. When exposed to 4.0 and 8.0 μg N/L FNA, increased abundance was detected for proteins putatively involved in nitrite reduction. Abundance of proteins involved in the sulfate reduction pathway (from adenylylphophosulfate to sulfite) and lactate oxidation pathway (from pyruvate to acetate) were initially inhibited in response to FNA at 8 h incubation, and then recovered at 12 h incubation. Lowered ribosomal protein abundance in D. vulgaris was detected, however, total cellular protein levels were mostly constant in the presence or absence of FNA. In addition, this study indicates that proteins coded by genes DVU2543, DVU0772, and DVU3212 potentially participate in resisting oxidative stress with FNA exposure. These findings share new insights for understanding the dynamic responses of D. vulgaris to FNA and could be useful to guide and improve the practical applications of FNA-based technologies for control of sewer corrosion.

Physiological and transcriptomic analyses reveal CuO nanoparticle inhibition of anabolic and catabolic activities of sulfate-reducing bacterium

The widespread use of CuO nanoparticles (NPs) results in their continuous release into the environment, which could pose risks to public health and to microbial ecosystems. Following consumption, NPs will initially enter into sewer systems and interact with and potentially influence sewer microbial communities. An understanding of the response of microbes in sewers, particularly sulfate-reducing bacteria (SRB), to the CuO NPs induced stress is important as hydrogen sulfide produced by SRB can cause sewer corrosion and odour emissions. In this study, we elucidated how the anabolic and catabolic processes of a model SRB, Desulfovibrio vulgaris Hidenborough (D. vulgaris), respond to CuO NPs. Physiological analyses indicated that the exposure of the culture to CuO NPs at elevated concentrations (>50 mg/L) inhibited both its anabolic and catabolic activities, as revealed by lowered cell proliferation and sulfate reduction rate. The antibacterial effects of CuO NPs were mainly attributed to the overproduction of reactive oxygen species. Transcriptomic analysis indicated that genes encoding for flagellar assembly and some genes involved in electron transfer and respiration were down-regulated, while genes for the ferric uptake regulator (Fur) were up-regulated. Moreover, the CuO NPs exposure significantly up-regulated genes involved in protein synthesis and ATP synthesis. These results suggest that CuO NPs inhibited energy conversion, cell mobility, and iron starvation to D. vulgaris. Meanwhile, D. vulgaris attempted to respond to the stress of CuO NPs by increasing protein and ATP synthesis. These findings offer new insights into the bacterial-nanoparticles interaction at the transcriptional level, and advance our understanding of impacts of CuO NPs on SRB in the environment.

Inhibition by free nitrous acid (FNA) and the electron competition of nitrite in nitrous oxide (N2O) reduction during hydrogenotrophic denitrification

Hydrogenotrophic denitrification is a promising technology for nitrate removal from organic-deficient wastewater or groundwater, and the attention of nitrous oxide (N₂O) emission during this process is required. Both nitrite and free nitrous acid (FNA or HNO₂) were reported to exert significant effects on N₂O reduction in heterotrophic denitrification, whereas, little knowledge has been obtained in hydrogenotrophic denitrification. In this study, we conducted a series of batch tests to comprehensively investigate the effects of nitrite, pH and FNA on N₂O production and reduction in a hydrogenotrophic denitrification process. The results showed that N₂O reduction rate decreased under both conditions of low pH and presence of nitrite, which would exert synergetic inhibition on N₂O reduction. The potential mechanisms that give rise to the results included electron competition and FNA inhibition. Electron competition between nitrite and N₂O reductases occurred when both nitrite and N₂O were added, which might contribute to the decrease in the N₂O reduction rate. The electron supply, which was obtained from the uptake of molecular hydrogen, declined with increasing FNA concentration according to a logarithmic model (R² = 0.9240). Additionally, the electron consumption rate of N₂O reductase to nitrite reductase ratio was initially stable and then decreased with increasing FNA concentration. The inhibition of N₂O reduction by FNA was determined to be reversible. The study suggested that both of the electron supply and N₂O reduction in hydrogenotrophic denitrification could be inhibited by FNA.

Anaerobic nitrate reduction divergently governs population expansion of the enteropathogen Vibrio cholerae

To survive and proliferate in the absence of oxygen, many enteric pathogens can undergo anaerobic respiration within the host by using nitrate (NO₃^-) as electron acceptor ^1,2. In these bacteria, NO₃^- is typically reduced by a nitrate reductase to nitrite (NO₂^-), a toxic intermediate that is further reduced by a nitrite reductase³. However, Vibrio cholerae, the intestinal pathogen that causes cholera, lacks a nitrite reductase, leading to NO₂^- accumulation during nitrate reduction⁴. Thus, V. cholerae is thought to be unable to undergo NO₃^--dependent anaerobic respiration⁴. Here, we show that during hypoxic growth, NO₃^- reduction in V. cholerae divergently impacts bacterial fitness in a manner dependent on environmental pH. Remarkably, in alkaline conditions, V. cholerae can reduce NO₃^- to support population growth. Conversely, in acidic conditions, accumulation of NO₂^- from NO₃^- reduction simultaneously limits population expansion and preserves cell viability by lowering fermentative acid production. Interestingly, other bacterial species such as Salmonella typhimurium, enterohemorrhagic Escherichia coli (EHEC) and Citrobacter rodentium also reproduced this pH-dependent response suggesting that this mechanism might be conserved within enteric pathogens. Our findings explain how a bacterial pathogen can use a single redox reaction to divergently regulate population expansion depending on fluctuating environmental pH.

Effect of CO2 on NADH production of denitrifying microbes via inhibiting carbon source transport and its metabolism

The potential effect of CO₂ on environmental microbes has drawn much attention recently. As an important section of the nitrogen cycle, biological denitrification requires electron donor to reduce nitrogen oxide. Nicotinamide adenine dinucleotide (NADH), which is formed during carbon source metabolism, is a widely reported electron donor for denitrification. Here we studied the effect of CO₂ on NADH production and carbon source utilization in the denitrifying microbe Paracoccus denitrificans. We observed that NADH level was decreased by 45.5% with the increase of CO₂ concentration from 0 to 30,000ppm, which was attributed to the significantly decreased utilization of carbon source (i.e., acetate). Further study showed that CO₂ inhibited carbon source utilization because of multiple negative influences: (1) suppressing the growth and viability of denitrifier cells, (2) weakening the driving force for carbon source transport by decreasing bacterial membrane potential, and (3) downregulating the expression of genes encoding key enzymes involved in intracellular carbon metabolism, such as citrate synthase, aconitate hydratase, isocitrate dehydrogenase, succinate dehydrogenase, and fumarate reductase. This study suggests that the inhibitory effect of CO₂ on NADH production in denitrifiers might deteriorate the denitrification performance in an elevated CO₂ climate scenario.

Mechanisms of Persistence of the Ammonia-Oxidizing Bacteria Nitrosomonas to the Biocide Free Nitrous Acid

Free nitrous acid (FNA) exerts a broad range of antimicrobial effects on bacteria, although susceptibility varies considerably among microorganisms. Among nitrifiers found in activated sludge of wastewater treatment processes (WWTPs), nitrite-oxidizing bacteria (NOB) are more susceptible to FNA compared to ammonia-oxidizing bacteria (AOB). This selective inhibition of NOB over AOB in WWTPs bypasses nitrate production and improves the efficiency and costs of the nitrogen removal process in both the activated sludge and anaerobic ammonium oxidation (Anammox) system. However, the molecular mechanisms governing this atypical tolerance of AOB to FNA have yet to be understood. Herein we investigate the varying effects of the antimicrobial FNA on activated sludge containing AOB and NOB using an integrated metagenomics and label-free quantitative sequential windowed acquisition of all theoretical fragment ion mass spectra (SWATH-MS) metaproteomic approach. The Nitrosomonas genus of AOB, on exposure to FNA, maintains internal homeostasis by upregulating a number of known oxidative stress enzymes, such as pteridine reductase and dihydrolipoyl dehydrogenase. Denitrifying enzymes were upregulated on exposure to FNA, suggesting the detoxification of nitrite to nitric oxide. Interestingly, proteins involved in stress response mechanisms, such as DNA and protein repair enzymes, phage prevention proteins, and iron transport proteins, were upregulated on exposure to FNA. In addition enzymes involved in energy generation were also upregulated on exposure to FNA. The total proteins specifically derived from the NOB genus Nitrobacter was low and, as such, did not allow for the elucidation of the response mechanism to FNA exposure. These findings give us an understanding of the adaptive mechanisms of tolerance within the AOB Nitrosomonas to the biocidal agent FNA.

Silver nanoparticles stimulate the proliferation of sulfate reducing bacterium Desulfovibrio vulgaris

The intensive use of silver nanoparticles (AgNPs) in cosmetics and textiles causes their release into sewer networks of urban water systems. Although a few studies have investigated antimicrobial activities of nanoparticles against environmental bacteria, little is known about potential impacts of the released AgNPs on sulfate reducing bacteria in sewers. Here, we investigated the effect of AgNPs on Desulfovibrio vulgaris Hidenborough (D. vulgaris), a typical sulfate-reducing bacterium (SRB) in sewer systems. We found AgNPs stimulated the proliferation of D. vulgaris, rather than exerting inhibitory or biocidal effects. Based on flow cytometer detections, both the cell growth rate and the viable cell ratio of D. vulgaris increased during exposure to AgNPs at concentrations of up to 100 mg/L. The growth stimulation was dependent on the AgNP concentration. These results imply that the presence of AgNPs in sewage may affect SRB abundance in sewer networks. Our findings also shed new lights on the interactions of nanoparticles and bacteria.

Engineering biological nitrogen removal in wastewater treatment via the control of nitrite oxidising bacteria using free nitrous acid

Nitrogen compounds need to be removed or captured from wastewater streams before disposal to protect our aquatic environments from eutrophication. Particular bacteria facilitating the biological removal of nitrogen during wastewater treatment include ammonia oxidising bacteria (AOB), nitrite oxidising bacteria (NOB), denitrifiers, as well as anaerobic ammonium oxidising (Anammox) bacteria. Manipulating these microbial communities can improve efficiency in nitrogen removal. Bypassing nitrate production by selectively inhibiting NOB reduces the need for oxygen and the addition of external carbon for the nitrogen removal. Various approaches to selectively inhibit NOB in the nitrification process are available. Here we present an approach using the biocide, free nitrous acid (FNA) to selectively suppress NOB growth thereby improving the efficiency of the nitrogen removal process.

Modulation of Nitrous Oxide (N2O) Accumulation by Primary Metabolites in Denitrifying Cultures Adapting to Changes in Environmental C and N

Metabolomics provides insights into the actual physiology of cells rather than their mere "potential", as provided by genomic and transcriptomic analysis. We investigate the modulation of nitrous oxide (N₂O) accumulation by intracellular metabolites in denitrifying bacteria using metabolomics and genome-based metabolic network modeling. Profiles of metabolites and their rates of production/consumption were obtained for denitrifying batch cultures under four conditions: initial COD:N ratios of 11:1 and 4:1 with and without nitrite spiking (28 mg-N L^-1). Only the nitrite-spiked cultures accumulated N₂O. The NO₂^- spiked cultures with an initial COD:N = 11:1 accumulated 3.3 ± 0.57% of the total nitrogen added as N₂O and large pools of tricarboxylic acid cycle intermediates and amino acids. In comparison, the NO₂^- spiked cultures with COD:N = 4:1 showed significantly higher (p = 0.028) N₂O accumulation (8.5.3 ± 0.9% of the total nitrogen added), which was linked to the depletion of C11-C20 fatty acids. Metabolic modeling analysis shows that at COD:N of 4:1 the denitrifying cells slowly generate electron equivalents as FADH₂ through β-oxidation of saturated fatty acids, while COD:N of 11:1 do it through the TCA cycle. When combined with NO₂^- shock, this prolonged the duration over which insufficient electron equivalents were available to completely reduce NOx to N₂, resulting in increased N₂O accumulation. Results extend the understanding of how organic carbon and nitrite loads modulate N₂O accumulation in denitrification, which may contribute to further design strategies to control greenhouse gas emissions from agricultural soils or wastewater treatment systems.

Aerobic condition enhances bacteriostatic effects of silver nanoparticles in aquatic environment: an antimicrobial study on Pseudomonas aeruginosa

The intensive applications of silver nanoparticles (AgNPs) inevitably cause continuous release of such materials into environments, as a consequence posing potential risks to microbial communities in engineered or natural ecosystems. However, the magnitude of antibacterial capacity of nanoparticles is still inconclusive, owing to influential factors such as the size of nanoparticle, microbial species, or environmental conditions. To reveal whether the presence of air would alter AgNPs ecotoxicity, Pseudomonas aeruginosa PAO1, a facultative denitrifying bacterium and an opportunity pathogen, was used to study antibacterial assays under both anaerobic and aerobic conditions. The results indicate that the respiration status of P. aeruginosa affect the ecotoxicity of AgNPs. P. aeruginosa cultured under aerobic condition were more susceptible to AgNPs than that under anaerobic condition. Aerobic condition greatly enhanced bacteriostatic effects of AgNPs but not their bactericidal effects, as the ratio of viable but nonculturable (VBNC) bacteria remained above 90% when 5 mg L⁻¹ AgNPs applied. Our findings offer further understanding for the degree of toxicity of nanoparticles on microbial ecosystems and underscore the importance of exposure condition (e.g. oxygen) in the mode of action of AgNPs.

Antimicrobial Effects of Free Nitrous Acid on Desulfovibrio vulgaris: Implications for Sulfide-Induced Corrosion of Concrete

Hydrogen sulfide produced by sulfate-reducing bacteria (SRB) in sewers causes odor problems and asset deterioration due to the sulfide-induced concrete corrosion. Free nitrous acid (FNA) was recently demonstrated as a promising antimicrobial agent to alleviate hydrogen sulfide production in sewers. However, details of the antimicrobial mechanisms of FNA are largely unknown. Here, we report the multiple-targeted antimicrobial effects of FNA on the SRB Desulfovibrio vulgaris Hildenborough by determining the growth, physiological, and gene expression responses to FNA exposure. The activities of growth, respiration, and ATP generation were inhibited when exposed to FNA. These changes were reflected in the transcript levels detected during exposure. The removal of FNA was evident by nitrite reduction that likely involved nitrite reductase and the poorly characterized hybrid cluster protein, and the genes coding for these proteins were highly expressed. During FNA exposure, lowered ribosome activity and protein production were detected. Additionally, conditions within the cells were more oxidizing, and there was evidence of oxidative stress. Based on an interpretation of the measured responses, we present a model depicting the antimicrobial effects of FNA on D. vulgaris These findings provide new insight for understanding the responses of D. vulgaris to FNA and will provide a foundation for optimal application of this antimicrobial agent for improved control of sewer corrosion and odor management.IMPORTANCE Hydrogen sulfide produced by SRB in sewers causes odor problems and results in serious deterioration of sewer assets that requires very costly and demanding rehabilitation. Currently, there is successful application of the antimicrobial agent free nitrous acid (FNA), the protonated form of nitrite, for the control of sulfide levels in sewers (G. Jiang et al., Water Res 47:4331-4339, 2013, http://dx.doi.org/10.1016/j.watres.2013.05.024). However, the details of the antimicrobial mechanisms of FNA are largely unknown. In this study, we identified the key responses (decreased anaerobic respiration, reducing FNA, combating oxidative stress, and shutting down protein synthesis) of Desulfovibrio vulgaris Hildenborough, a model sewer corrosion bacterium, to FNA exposure by examining the growth, physiological, and gene expression changes. These findings provide new insight and underpinning knowledge for understanding the responses of D. vulgaris to FNA exposure, thereby benefiting the practical application of FNA for improved control of sewer corrosion and odor.