Effect of starvation time on NO and N2O production during heterotrophic denitrification with nitrite and glucose shock loading

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.

Microbial pathways of nitrous oxide emissions and mitigation approaches in drylands

Drylands refer to water scarcity and low nutrient levels, and their plant and biocrust distribution is highly diverse, making the microbial processes that shape dryland functionality particularly unique compared to other ecosystems. Drylands are constraint for sustainable agriculture and risk for food security, and expected to increase over time. Nitrous oxide (N2O), a potent greenhouse gas with ozone reduction potential, is significantly influenced by microbial communities in drylands. However, our understanding of the biological mechanisms and processes behind N2O emissions in these areas is limited, despite the fact that they highly account for total gaseous nitrogen (N) emissions on Earth. This review aims to illustrate the important biological pathways and microbial players that regulate N2O emissions in drylands, and explores how these pathways might be influenced by global changes for example N deposition, extreme weather events, and climate warming. Additionally, we propose a theoretical framework for manipulating the dryland microbial community to effectively reduce N2O emissions using evolving techniques that offer inordinate specificity and efficacy. By combining expertise from different disciplines, these exertions will facilitate the advancement of innovative and environmentally friendly microbiome-based solutions for future climate change vindication approaches.

Optimizing waste molasses utilization to enhance electron transfer via micromagnetic carriers: Mechanisms and high-nitrate wastewater denitrification performance

The biological denitrification of high-nitrate wastewater (HNW) is primarily hindered by insufficient carbon sources and excessive nitrite accumulation. In this study, micromagnetic carriers with varying micromagnetic field (MMF) strengths (0.0, 0.3, 0.6, 0.9 mT) were employed to enhance the denitrification of HNW using waste molasses (WMs) as a carbon source. The results revealed that 0.6 mT MMF significantly improved the total nitrogen removal (TN) efficiency at 96.3%. A high nitrate (NO3--N) removal efficiency at 99.3% with a low nitrite (NO2--N) accumulation at 25.5 mg/L was achieved at 0.6 mT MMF. The application of MMF facilitated the synthesis of adenosine triphosphate (ATP) and stimulated denitrifying enzymes (e.g., nitrate reductase (NAR), nitrite reductase (NIR), and nitric oxide reductase (NOR)), which thereby promoting denitrification. Moreover, the effluent chemical oxygen demand (COD), tryptophan and fulvic-like substances exhibited their lowest levels at 0.6 mT MMF. Analysis through 16S ribosomal ribonucleic acid gene sequencing indicated a significant enrichment of denitrifying bacteria including Castellaniella Klebsiella under the influence of MMF. Besides, the proliferation of Acholeplasma, Klebsiella and Proteiniphilum at 0.6 mT MMF promoted the hydrolysis and acidification of WMs. This study offers new insights into the enhanced utilization of WMs and the denitrification of HNW through the application of MMF.

Improving nutrients removal of Anaerobic-Anoxic-Oxic process via inhibiting partial anaerobic mixture with nitrite in side-stream tanks: role of nitric oxide

A side-stream tank which was in parallel with the anoxic tank was used to improve the performance of the Anaerobic-Anoxic-Oxic process. The partial mixtures from the anaerobic tank were injected into the side-stream tank with the initial nitrite nitrogen (NO₂^--N) concentrations of 10 mg/L and 20 mg/L. When the initial NO₂^--N concentration in the tank was 20 mg/L, total nitrogen and total phosphorus removal efficiencies of the A²/O process increased from 72% and 48% to 90% and 89%, respectively. 2.23 mg/L of nitric oxide (NO) were observed in the side-stream tank. The abundance of Nitrosomonas sp. and Nitrospira sp. were varied from 0.98% and 6.13% to 2.04% and 1.13%, respectively. The abundances of Pseudomonas sp. and Acinetobacter sp. were increased from 0.81% and 0.74% to 6.69% and 5.48%, respectively. NO plays an important role for improving the nutrients removal of the A²/O process in the side-stream nitrite-enhanced strategy.

Nitric oxide and Nitrous oxide accumulation, oxygen production during nitrite denitrification in an anaerobic/anoxic sequencing batch reactor: exploring characteristics and mechanism

Nitrite denitrification has received increasing attention due to its high efficiency, low energy consumption, and sludge yield. However, the nitric oxide (NO) and nitrous oxide (N₂O) which are harmful to the environment, microorganisms, and humans are produced in this process. In order to mitigate NO and N₂O production, the biological mechanisms of NO and N₂O accumulation, as well as NO detoxification during nitrite denitrification in a sequencing batch reactor were studied. Results showed that the peak of NO accumulation increased from 0.29 [Formula: see text] 0.01 to 3.12 [Formula: see text] 0.34 mg L^-1 with the increase of carbon to nitrogen ratio (COD/N), which is caused by the sufficient electron donor supply for NO₂^--N reduction process at high COD/N. Furthermore, the result suggested that NO accumulation with no pH adjustment was 12 times higher than that with pH adjustment. It is related to the inhibition on NO reductase caused by the high free nitrous acid (FNA) and NO concentration with no pH adjustment. The pathways of NO detoxification included NO emission, reduction, and dismutation, and the more NO produced, the high proportion of NO dismutation pathway. Result showed that the maximum of oxygen production during NO dismutation reached to 1.39 mg L^-1. N₂O accumulation was mainly associated with FNA and NO inhibition, COD/N. The peak of N₂O accumulation presented a completely opposite trend at pH adjustment and no pH adjustment, it is because that the higher FNA and NO concentration at high COD/N without pH adjustment will inhibit the N₂O reductase activity, resulting in the N₂O reduction was hindered during nitrite denitrification.

Significant production of nitric oxide by aerobic nitrite reduction at acidic pH

The acidic (i.e., pH ∼5) activated sludge process is attracting attention because it enables stable nitrite accumulation and enhances sludge reduction and stabilization, compared to the conventional process at neutral pH. Here, this study examined the production and potential pathways of nitric oxide (NO) and nitrous oxide (N₂O) during acidic sludge digestion. With continuous operation of a laboratory-scale aerobic digester at high dissolved oxygen concentration (DO>4 mg O₂ L^-1) and low pH (4.7±0.6), a significant amount of total nitrogen (TN) loss (i.e., 18.6±1.5% of TN in feed sludge) was detected. Notably, ∼40% of the removed TN was emitted as NO, with ∼8% as N₂O. A series of batch assays were then designed to explain the observed TN loss under aerobic conditions. All assays were conducted with a low concentration of volatile solids (VS), i.e., VS<4.5 g L^-1. This VS concentration is commensurate with the values commonly found in the aeration tanks of full-scale wastewater treatment systems, and thus no significant nitrogen loss should be expected when DO is controlled above 4 mg O₂ L^-1. However, nitrite disappeared at a significant rate (with the chemical decomposition of nitrite excluded), leading to NO production in the batch assays at pH 5. The nitrite reduction could be associated with endogenous microbial activities, e.g., nitrite detoxification. The significant NO production illustrates the importance of aerobic nitrite reduction during acidic aerobic sludge digestion, suggesting this process cannot be neglected in developing acidic activated sludge technology.

Towards efficient heterotrophic recovery of N2O via Fe(II)EDTA-NO: A modeling study

Efficient recovery of nitrous oxide (N₂O) through heterotrophic denitrification with the help of Fe(II)EDTA-NO as a chelating agent has been regarded as an ideal technology to treat nitric oxide (NO)-rich flue gas. In this study, an integrated NO-based biological denitrification model was developed to describe the sequential reduction of the NO fixed in Fe(II)EDTA-NO with organic carbon as the electron donor. With the inclusion of only the key pathways contributing to nitrogen transformation, the model was firstly developed and then calibrated/validated and evaluated using the data of batch tests mediated by the identified functional heterotrophic bacteria at various substrates concentrations and then used to explore the possibility of enhancing N₂O recovery by altering the substrates condition and reactor setup. The results demonstrated that the optimal COD/N ratio decreased consistently from 1.5 g-COD/g-N at the initial NO concentration of 40 g-N/m³ to 1.0 g-COD/g-N at the initial NO concentration of 420 g-N/m³. Furthermore, sufficiently increasing the headspace volume of the reactor was considered an ideal strategy to obtain ideal N₂O production of 86.6 % under the studied conditions. The production of high-purity N₂O (98 %) confirmed the practical application potential of this integrated treatment technology to recover a valuable energy resource from NO-rich flue gas.

N-Acyl-Homoserine Lactone (AHL)-Mediated Microalgal-Bacterial Communication Driving Chlorella-Activated Sludge Bacterial Biofloc Formation

N-acyl-homoserine lactones (AHLs) as autoinducers of Gram-negative bacteria for quorum sensing regulation have shown positive effects on the production of aromatic proteins in extracellular polymeric substances (EPSs) during bioflocculation. To investigate the role of AHLs in aromatic protein production, a Chlorella-bacteria system with great bioflocculation was established via fed-batch cultivation. Tryptophan and aromatic proteins as the main compounds in the EPS of bioflocs showed an increasing trend during fed-batch cultivation. The Chlorella cells only secreted tryptophan rather than aromatic proteins during axenic cultivation. N-dodecanoyl-l-homoserine lactone (C12-HSL) was correlated with the flocculation activity and extracellular protein content of bioflocs during fed-batch cultivation. The addition of exogenous C12-HSL enhanced the flocculation activity of the Chlorella-bacteria system and aromatic protein production in the EPS. Chlorella cells sensed exogenous C12-HSL and significantly upregulated the aromatic protein synthesis pathway during axenic cultivation. In addition, vanillin as a quorum-sensing inhibitor suppressed the positive effect of C12-HSL on flocculation activity and aromatic protein production and synthesis. This result indicated that vanillin intercepts the response of Chlorella cells to C12-HSL. Overall, C12-HSL is supposed to be an important signal molecule to achieve communication between Chlorella and Gram-negative bacteria and subsequently induce Chlorella cells to produce aromatic proteins for biofloc formation.

An attempt to stimulate aniline degrading bioreactor by exogenous auto-inducer: Decontamination performance, sludge characteristics, and microbial community structure response

To break the contradiction between aniline and nitrogen metabolism in activated sludge reactor by influencing microbial interspecific communication, Auto-inducer C6-HSL and 3-oxo-C8-HSL were selected in this study to interfere with aniline degradation system. The two Auto-inducers enhanced the aniline degradation rate and ammonia removal efficiency of the systems, especially C6-HSL. Meanwhile, the main ammonia removal way was assimilation. Exogenous Auto-inducer effectively stabilized the sludge structure and activity from the destruction of aniline, and promoted EPS secretion. Microbial diversity analysis showed that most of functional microflora of seed sludge gradually deactivated with the operation of the reactor, while Rhodococcus, Leucobacter, g_norank_f_Saprospiraceae proliferated wildly under the action of Auto-inducer. Additionally, the interspecific relationship also demonstrated a different trend. Exogenous Auto-inducer was proved to exert positive effects on aniline degradation system to a certain extent, providing new insights in the field of aniline wastewater bio-degradation.

Integrating Microbial Protein Production and Harvest Systems into Pilot-Scale Recirculating Aquaculture Systems for Sustainable Resource Recovery: Linking Nitrogen Recovery to Microbial Communities

In aquaculture, it is important to raise the nitrogen recovery efficiency (NRE) to improve sustainability. To achieve this, recovery of microbial protein (RMP), instead of nitrification/denitrification in conventional wastewater treatment, is a promising approach whose microbiological mechanisms must be characterized. Here, periodic RMP was conducted in an in situ biofloc-based aquaculture system (IBAS) and a separating assimilation reactor-based recirculating aquaculture system (SRAS). Kinetic analysis indicated that a microbial biomass level of 3 g L^-1 was optimal for inorganic N removal, and excess biomass was harvested to improve the NRE. Unlike the IBAS, the SRAS eliminated the fluctuation in water quality caused by the RMP. Periodic RMP significantly increased the NRE to 44-57% by promoting the filamentous bacterium Herpetosiphon and suppressing anaerobic denitrifiers. Aerobic chemoheterotrophy was the main microbial metabolic process for energy. After RMP, nitrate reductase-encoded functional genes (napA and narG) significantly decreased, while nitrite reductase-encoded functional genes, especially nirK, significantly increased. Co-occurrence networks analysis indicated that the cooperation and competition among organic matter degraders, filamentous bacteria, nitrifiers, and denitrifiers determined the microbial protein yield. These results provide fundamental insights into the influence of the RMP on microbial communities and functions, which is important for realizing sustainable aquaculture.

Effective restoration of partial nitritation and anammox biofilm process by short-term hydroxylamine dosing: Mechanism and microbial interaction

The one-stage partial nitritation and anammox (PN-A) process frequently experiences deterioration from ammonium accumulation and nitrate build-up. In this study, hydroxylamine was dosed to restore the process from deterioration in a continuously aerated PN-A sequencing biofilm batch reactor, and the impact of hydroxylamine on the metabolism of PN-A process was studied. PN-A process was totally restored in 5 days via 10 mg N·L^-1 hydroxylamine dosing, reducing nitrate-produced/ammonium-removed ratio from 28.5% to less than 11.0%. hydroxylamine dosing promoted biological production of nitric oxide and nitrous oxide and reduced the production of nitrate in the PN-A process. This study advanced the understanding of the metabolism versatility of hydroxylamine and nitric oxide as well as their function in interaction between aerobic ammonium oxidation bacteria and anaerobic ammonium oxidation bacteria, and proposed the potential application of hydroxylamine dosing in ammonium-contained wastewater treatment.

Solid-phase denitrification in high salinity and low-temperature wastewater treatment

Nitrogen removal from wastewater is often deteriorated under high salinity and low temperature. Solid-phase denitrification (SPD) might improve total nitrogen removal efficiency (TNRE) by stably supplying carbon resources under adverse conditions. In this study, an SPD biofilm reactor was successfully established by inoculating halophilic sludge and filling poly (butanediol succinate) (PBS) granules, and achieved over 96% TNRE at low temperature. More extracellular polysaccharides were produced at low temperature. Microbial network analysis evidenced dominant heterotrophic denitrifiers (Marinicella, Fusibacter, Saccharicrinis and Vitellibacter) at 25 °C were replaced by genera Melioribacter, Marinobacter, Desulfatitalea and Thiomicrospira at 15 °C. At low temperature, genes nirS and narG might be mainly responsible for denitrification. Fluorescence spectrum coupled with fluorescence regional integration and parallel factor analysis revealed low temperature increased the proportion of proteins of soluble microbial products. This study provides guidance for the practical application of SPD in the treatment of high salinity and low-temperature wastewater.

Fast start-up and enhancement of partial nitritation and anammox process for treating synthetic wastewater in a sequencing bath biofilm reactor: Strategy and function of nitric oxide

In this study, the partial nitritation and anammox (PN-A) process was initiated within 30 days in a sequencing batch biofilm reactor (SBBR) by employing pre non-aeration and post non-aeration with fixed aeration rates. The average ammonia removal efficiency (ARE), total nitrogen removal efficiency (TNRE) of 98.5 ± 1.5% and 89.5 ± 1.6% were achieved. By doubling aeration rate and agitation rate and adopting pre non-aeration, the TNRR was promoted from 0.135 ± 0.013 kg N·m^-3·d^-1 to 0.285 ± 0.015 kg N·m^-3·d^-1, obtaining an average ARE and TNRE of 97.5 ± 1.5% and 85.5 ± 2.6%. Nitric oxide might induce anaerobic ammonia oxidation bacteria (AnAOB) during the start-up stage, and could be an indicator for synergetic state between ammonia oxidation bacteria (AOB) and AnAOB. Lower nitrous oxide emission factor of 0.51% was obtained. The abundance of AOB, AnAOB and nitrite oxidation bacteria (NOB) accounted for 1.6%, 19.3% and 0.3%, respectively.

Nitric oxide and nitrous oxide production in anaerobic/anoxic nitrite-denitrifying phosphorus removal process: effect of phosphorus concentration

Nitric oxide (NO) and nitrous oxide (N₂O) production in biological nutrient removal has been studied widely due to the strong negative effects on the environment. Nitrite-denitrifying phosphorus removal (N-DPR), as a significant source of NO and N₂O production, has received great attention. However, the mechanism of NO and N₂O production at different phosphorus concentrations is not well understood. Therefore, this study was conducted to investigate the effect of phosphorus concentration on pollutant removal, as well as NO and N₂O production during the N-DPR process. The results showed that the phosphorus removal efficiency was improved with the increase of phosphorus concentration, which is caused by the enrichment of denitrifying phosphorus accumulating organisms (DPAOs) at high phosphorus concentration. High NO production was observed at phosphorus concentration of 0.5 mg L^-1, which is mainly attributed to the slow recovery of reductase activity and low abundance of DPAOs. The maximal N₂O accumulation of 31.45 mg L^-1 was also achieved at phosphorus concentration of 0.5 mg L^-1. The possible reason is that fewer poly-β-hydroxyalkanoates (PHAs) were synthesized by glycogen accumulating organisms (GAOs) at low phosphorus concentration, which could intensify the electron competition among different reductases. In addition, free nitrous acid (FNA) inhibition was another significant reason for high N₂O production.

Nitric oxide reduction by microbial fuel cell with carbon based gas diffusion cathode for power generation and gas purification

Nitric oxide (NO) from anthropogenic emission is one of the main air contaminants and induces many environmental problems. Microbial fuel cells (MFCs) with gas diffusion cathode provide an alternative technology for NO reduction. In this work, pure NO as the sole electron acceptor of MFCs with gas diffusion cathode (NO-MFCs) was verified. The NO-MFCs obtained a maximum power density of 489 ± 50 mW/m². Compared with MFCs using O₂ in air as electron acceptor (Air-MFCs), the columbic efficiency increased from 23.2% ± 4.3% (Air-MFCs) to 55.7% ± 4.6% (NO-MFCs). The NO removal rate was 12.33 ± 0.14 mg/L/h and N₂ was the main reduction product. Cathode reduction was the dominant pathway of NO conversion in NO-MFCs, including abiotic electrochemical reduction and microbial denitrification process. The predominant genera in anodic microbial community changed from exoelectrogenic bacteria in Air-MFCs to denitrifying bacteria in NO-MFCs and effected the power generation.

Bacterial community structure and gene function prediction in response to long-term running of dual graphene modified bioelectrode bioelectrochemical systems

This work studied bacterial community structure and gene function prediction in long-term running of dual graphene modified bioelectrode bioelectrochemical systems (LT D-GM-BE BES, 2 year). The maximum power density of LT D-GM-BE BES was 99.03 ± 3.64 mW/m², which was 3.66 times of dual control BES (D-C-BE BES), and the transfer resistance of LT GM-BE was just approximately 1/4 of control bioelectrode (C-BE). Proteobacteria and Firmicutes were dominant bacteria in long-term modified bioanode (LT GM-BA, 30.03% and 45.64%), and in long-term modified biocathode (LT GM-BC) was Armatimonadetes (47.14%) in phylum level. The dominant bacteria in LT GM-BA was Clostridium (30.56%), in GM-BC was Chthonomonas (47.14%) in genus level. Gene function related with substrate, energy metabolism and environmental adaptation were enriched. LT GM-BE was tended to enrich dominant bacteria and enrich gene to adapt to micro-environmental changes. This study would provide metagenomics information for long-term running of BES in future.