Oxygenic denitrification for nitrogen removal with less greenhouse gas emissions: Microbiology and potential applications

Unveiling novel pathways and key contributors in the nitrogen cycle: Validation of enrichment and taxonomic characterization of oxygenic denitrifying microorganisms in environmental samples

Microorganisms play a crucial role in both the nitrogen cycle and greenhouse gas emissions. A recent discovery has unveiled a new denitrification pathway called oxygenic denitrification, entailing the enzymatic reduction of nitrite to nitric oxide (NO) by a putative nitric oxide dismutase (nod) enzyme. In this study, the presence of the nod gene was detected and subsequently enriched in anaerobic-activated sludge, farmland soil, and paddy soil samples. After 150 days, the enriched samples exhibited significant denitrification, and concomitant oxygen production. The removal efficiency of nitrite ranged from 64.6 % to 79.0 %, while the oxygen production rate was between 15.4 μL/min and 18.6 μL/min when exposed to a sole nitrogen source of 80 mg/L sodium nitrite. Additionally, batch experiments and kinetic analyses revealed the intricate pathways and underlying mechanisms governing the oxygenic denitrification reaction by using CARBOXY-PTIO, 18O-labelled water, and acetylene to unravel the intricacies of the reaction. The quantitative polymerase chain reaction (qPCR) results indicated a significant surge in the abundance of nod genes, escalating from 7.59 to 10.12-fold. Moreover, analysis of 16S ribosomal DNA (rDNA) amplicons revealed Proteobacteria as the dominant phylum and Thauera as the main genus, with the presumed affiliation. In this study, a new nitrogen conversion pathway, oxygenic denitrification, was discovered in environmental samples. This process provides the possibility for the control of nitrous oxide in the treatment of nitrogenous wastewater.

Parabens Increase Sulfamethoxazole-, Tetracycline- and Paraben-Resistant Bacteria and Reshape the Nitrogen/Sulfur Cycle-Associated Microbial Communities in Freshwater River Sediments

Backgrounds Parabens are pollutants of emerging concern in aquatic environments. Extensive studies regarding the occurrences, fates and behavior of parabens in aquatic environments have been reported. However, little is known about the effects of parabens on microbial communities in freshwater river sediments. This study reveals the effects of methylparaben (MP), ethylparaben (EP), propylparaben (PP) and butylparaben (BP) on antimicrobial-resistant microbiomes, nitrogen/sulfur cycle-associated microbial communities and xenobiotic degrading microbial communities in freshwater river sediments. Methods The river water and sediments collected from the Wai-shuangh-si Stream in Taipei City, Taiwan were used to construct a model system in fish tanks to test the effects of parabens in laboratory. Results Tetracycline-, sulfamethoxazole- and paraben-resistant bacteria increased in all paraben treated river sediments. The order of the overall ability to produce an increment in sulfamethoxazole-, tetracycline- and paraben-resistant bacteria was MP > EP > PP > BP. The proportions of microbial communities associated with xenobiotic degradation also increased in all paraben-treated sediments. In contrast, penicillin-resistant bacteria in both the aerobic and anaerobic culture of paraben-treated sediments decreased drastically at the early stage of the experiments. The proportions of four microbial communities associated with the nitrogen cycle (anammox, nitrogen fixation, denitrification and dissimilatory nitrate reduction) and sulfur cycle (thiosulfate oxidation) largely increased after the 11th week in all paraben-treated sediments. Moreover, methanogens and methanotrophic bacteria increased in all paraben-treated sediments. In contrast, the nitrification, assimilatory sulfate reduction and sulfate-sulfur assimilation associated to microbial communities in the sediments were decreased by the parabens. The results of this study uncover the potential effects and consequences of parabens on microbial communities in a freshwater river environment.

Improving soil fertility by driving microbial community changes in saline soils of Yellow River Delta under petroleum pollution

It is promising to use indigenous microorganisms for fertility improvement in petroleum-contaminated coastal soil. As a result, the microbial community and physicochemical property are the base for the restoration. For the detailed information, the Phragmites Communis (P), Chinese Tamarisk (C), Suaeda salsa (S), and new Bare Land (B) soil of Yellow River Delta was 90 g in 100 mL sterile bottles simulated at 25 °C with soil: petroleum = 10:1 in the incubator for four months. The samples were detected at 60 and 120 days along with untreated soil and aged Oil Sludge (O) as control. The results showed that all the samples were alkaline (pH 7.99-8.83), which the salinity and NO₃^- content of incubate soil followed the in situ samples as P (1.09-1.72‰, 8.02-8.17 mg kg^-1), C (10.61-13.79‰, 5.99-6.07 mg kg^-1), S (10.19-12.43‰, 3.64-4.22 mg kg^-1), B (31.85-32.45‰, 3.56-3.72 mg kg^-1) and O (31.61-34.30‰, 0.89-0.90 mg kg^-1). NO₃^- and organic carbon decreased after incubation, which the polluted samples (86.63-92.63 g kg^-1) still had higher organic carbon than untreated ones with more NH₄⁺ consumption. The high-throughput sequence results showed that the Gammaproteobacteria and Alphaproteobacteria were dominant in all samples, while sulfate reducting bacteria Alphaproteobacteria decreased at 120 days. Meanwhile, the electroactive Gammaproteobacteria might symbiosis with Methanosaetaceae and Methanosarcinaceae, degrading petroleum after electron receptors depletion. Nitrososphaeraceae and Nitrosopumilaceae oxidise NH₄⁺ to NO₂^- for intra-aerobic anaerobes and denitrifying bacteria producing oxygen for biodegradation in polluted Phragmites Communis soil. The halotolerant Halomicrobiaceae and Haloferacaceae predominated in saline Chinese Tamarisk, Suaeda Salsa and Bare Land, which were potential electroactive degradater. As the ageing sludge formed, the hydrogen trophic methanogens Methanothermobacteraceae (73.90-92.72%) was prevalent with the petroleum pollution. In conclusion, petroleum initiated two-phase in the sludge forming progress: electron acceptor consumption and electron transfer between degradater and methanogens. Based on the results, the domestic sewage N, P removal coupling and electron transport will be the basement for polluted soils fertility improvement.

An Innovative Process for Mature Landfill Leachate and Waste Activated Sludge Simultaneous Treatment Based on Partial Nitrification, In Situ Fermentation, and Anammox (PNFA)

An innovative partial nitrification, in situ fermentation, and Anammox (PNFA) system was developed to achieve mature landfill leachate and waste activated sludge simultaneous treatment. Three separate sequencing batch reactors (SBRs) were used for partial nitrification (PN-SBR), integrated fermentation-denitrification (IFD-SBR), and partial nitrification-Anammox (PNA-SBR). After 200 days of continuous operation, a satisfactory nitrogen removal efficiency (NRE) of 99.2 ± 0.1% was obtained, with an effluent total nitrogen (TN) of 15.2 ± 3.2 mg/L. In IFD-SBR, the volatile fatty acids generated from fermentation drove efficient denitrification, obtaining sludge and nitrogen reduction rates of 4.2 ± 0.7 and 0.61 ± 0.04 kg/m³·day, respectively. Furthermore, unwanted fermentation metabolites (134.1 mg/L NH₄⁺-N) were further treated by PNA-SBR using a combination of step-feed and intermittent aeration strategies. In PNA-SBR, Anammox significantly contributed to 82.1% nitrogen removal, and Anammox bacteria (Candidatus Brocadia, 2.3%) mutually benefited with partially denitrifying microorganisms (Thauera, 4.2%), with 66.3% of generated nitrate reduced to nitrite and then reutilized in situ by Anammox. Compared with the conventional nitrification-denitrification process, PNFA reduced oxygen energy consumption, external carbon source dosage, and CO₂ emission by 21.3, 100, and 38.9%, respectively, and obtained 50.1% external WAS reduction efficiency.

Electrochemical system for anaerobic oxidation of methane by DAMO microbes with nitrite as an electron acceptor

Denitrifying anaerobic methane oxidation (DAMO) is an important microbial metabolic process that simultaneously converts of methane and nitrite. In this study, electrochemical systems were investigated for DAMO with nitrite as an electron acceptor. The results showed that the auxiliary voltage enhanced anaerobic methane oxidation and nitrite reduction. The greatest methane conversion (26.61 mg L^-1 d^-1) was obtained at an auxiliary voltage of 1.6 V (EMN-1.6). Isotope tracing indicated that carbon dioxide was the oxidation product of methane, and methanol was the intermediate. The power density reached 0.60 (for EMN-0.5, the bioreactor with a voltage of 0.5 V) and 3.77 mW m^-2 (for EMN-1.6). DAMO microbes, Methylocystis sp., and Methylomonas sp. were identified as methanotrophs. Rhodococcus sp., Hyphomicrobium sp., and Thiobacillus sp. were the dominant denitrifying bacteria. The conversion pathway was speculated to be as follows: methane was oxidized to carbon dioxide and nitrite was reduced to nitrogen. The two processes were independently completed by DAMO bacteria and oxygen was simultaneously generated. For the electron transfer pathway, methanotrophs utilized the oxygen released by DAMO bacteria to convert methane into organic matter (e.g. methanol). These organic compounds were utilized by Pseudoxanthomonas sp. and Pseudomonas sp., and the generated electrons were then released to the outside of the cells and transferred to the anode. Denitrifying bacteria received electrons at the cathode, transferred them to the interior of the cell, and then converted nitrite into nitrogen. This research explored an effective consortium and a method for methane and nitrogen removal.

Improving stability of mainstream Anammox in an innovative two-stage process for advanced nitrogen removal from mature landfill leachate

This study presents an innovative mainstream Anammox based on multiple NO₂^--N supplement pathways to treat actual mature landfill leachate over 180 days. Desirable effluent quality of 11.8 mg/L total nitrogen (TN) and nitrogen removal efficiency of 98.8% were achieved despite fluctuation conditions of 1.5-fold influent substrates and 8.0-fold dissolved oxygen overload. Nitrogen mass balance confirmed Anammox was the dominant nitrogen removal pathway, contributing up to 87.9%. Functional genes of ammonia monooxygenase (amoA), hydrazine synthase (hzsB), and ratio of nitrate/nitrite reductase were highly detected. Anammox genera, Candidatus_Kuenenia (4.1%) and Candidatus_Brocadia (5.3%) were dominant in two functional systems, respectively, due to the different affinity of nitrite, oxygen, and organic carbon. As an economical and sustainable technology, the innovative process enabled a 95.1% decrease in organic carbon demand, a 61.5% reduction in aeration energy consumption, and 77.6% lower biomass production compared with traditional nitrification-denitrification process.

Denitrifying anaerobic methane oxidation (DAMO) cultures: Factors affecting their enrichment, performance and integration with anammox bacteria

The recently discovered process, denitrifying anaerobic methane oxidation (DAMO), links the carbon and nitrogen biogeochemical cycles via coupling the anaerobic oxidation of methane to denitrification. The DAMO process, in this respect, has the potential to mitigate the greenhouse effect through the assimilation of dissolved methane. Denitrification via methane oxidation rather than organic matter, provides a new perspective to performing this once thought to be well established process. The two main species responsible for this process are "Candidatus Methylomirabilis oxyfera (M. oxyfera), and "Candidatus Methanoperedens nitroreducens" (M. nitroreducens). M. oxyfera is responsible of reducing nitrite while M. nitroreducens reduces nitrate to nitrite. These two microorganisms, despite their different pathways, were found to exist together in nature through a syntrophic relationship. Their co-existence with anaerobic ammonium oxidation (Anammox) bacteria was also revealed in the last decade. Anammox bacteria are chemolithoautotrophs, converting ammonium and nitrite to N₂ and nitrate. They are responsible for the release of more than 50% of oceanic N₂, hence play an important role in the global nitrogen cycle. Factors leading to the enrichment of DAMO cultures and their cultivation with Anammox cultures are of significance for improved nitrogen removal systems with decreased greenhouse effect, and even for further full-scale applications. This study, therefore, aims to present an updated review of the DAMO process, by focusing on the factors that might have a significant role in enrichment of DAMO microorganisms and their co-existence with Anammox bacteria. Factors such as temperature, pH, inoculum and feed type, trace metals and reactor configuration are among the ones discussed in detail. Factors, which have not been investigated, are also elucidated to provide a better understanding of the process and set research goals that will aid in the development of DAMO-centered wastewater treatment alternatives.

Pseudomonas stutzeri GF2 augmented the denitrification of low carbon to nitrogen ratio: Possibility for sewage wastewater treatment

A denitrifying strain with high efficiency at low carbon to nitrogen (C/N) ratio of 2.0 was isolated and characterized. It belongs to the genus Pseudomonas. Scanning electron microscopy (SEM) showed that GF2 was rod-shaped. The nitrate removal efficiency reached up to 92.41% (1.85 mg L^-1 h^-1) with the C/N ratio of 2.0 and the nitrite accumulation eventually decreased to 0.88 mg L^-1. By response surface method (RSM) method, three reaction conditions of strain GF2 were optimized, including pH, C/N ratio, and nitrate concentration. Nitrogen balance and gas detection revealed that 88.03% of nitrogen was removed in gaseous form (included 98.80% nitrogen gas), which confirmed its efficient denitrification ability and pathway. 3D fluorescence spectrum (3D-EEM) manifested that in the absence of organic matter, strain GF2 can utilize extracellular polymeric substance (EPS) as carbon source for efficient denitrification. This research strived to provide new research ideas for low C/N ratio sewage treatment.

Methanotrophic denitrification in wastewater treatment: microbial aspects and engineering strategies

Anaerobic technologies are consolidated for sewage treatment and are the core processes for mining marketable products from waste streams. However, anaerobic effluents are supersaturated with methane, which represents a liability regarding greenhouse gas emissions. Meanwhile, anaerobic technologies are not capable of nitrogen removal, which is required to ensure environmental protection. Methane oxidation and denitrification processes can be combined to address both issues concurrently. Aerobic methane oxidizers can release intermediate organic compounds that can be used by conventional denitrifiers as electron donors. Alternatively, anoxic methanotrophic species combine methane oxidation with either nitrate or nitrite reduction in the same metabolism. Engineered systems need to overcome the long doubling times and low NO_x consumption rates of anoxic methanotrophic microorganisms. Another commonly reported bottleneck of methanotrophic denitrification relates to gas-liquid mass transfer limitations. Although anaerobic effluents are supersaturated with methane, experimental setups usually rely on methane supply in a gaseous mode. Hence, possibilities for the application of methane-oxidation coupled to denitrification in full scale might be overlooked. Moreover, syntrophic relationships among methane oxidizers, denitrifiers, nitrifiers, and other microorganisms (such as anammox) are not well understood. Integrating mixed populations with various metabolic abilities could allow for more robust methane-driven wastewater denitrification systems. This review presents an overview of the metabolic capabilities of methane oxidation and denitrification and discusses technological aspects that allow for the application of methanotrophic denitrification at larger scales.

Nitric oxide-dependent biodegradation of phenanthrene and fluoranthene: The co-occurrence of anaerobic and intra-aerobic pathways

Polycyclic aromatic hydrocarbons (PAHs) pollution as well as the emissions of nitric oxide (NO) and greenhouse gas nitrous oxide (N₂O) in denitrification processes are currently two environmental issues of great concern. Although bioremediation of PAHs under denitrification is considered a promising approach, denitrification was an important contributor to N₂O and NO emissions. This long-term study confirmed for the first time that microorganisms could utilize NO to efficiently degrade phenanthrene and fluoranthene. When the two systems of NO-dependent phenanthrene and fluoranthene degradation were stable, the first-order rate constants of phenanthrene and fluoranthene in the two systems (0.1940 and 0.0825 day^-1, respectively) were close to those values (0.2290 and 0.1085 day^-1, respectively) observed at nitrate-reducing conditions. Further analysis of functional genes revealed that phenanthrene and fluoranthene might be degraded under the combined action of the anaerobic pathway mediated by NO reduction and intra-aerobic pathway mediated by NO dismutation. The genomic analysis showed that Nod genes had high diversity and most of them were similar to aquifer cluster group in the two systems. Microbial community structure analysis indicated that Pseudomonas and Ochrobactrum might be key participants in NO-dependent phenanthrene degradation system, and Azoarcus, Alicycliphilus and Moheibacter might play vital roles in NO-dependent fluoranthene degradation system. This study provides new perspective for anaerobic remediation of PAH pollution and simultaneously reducing NO and N₂O emissions during bioprocesses, which has important ecological significance for amending sediment and soil PAHs contamination and potential application for the removal of PAHs in flue gas.

Fundamentals and potential environmental significance of denitrifying anaerobic methane oxidizing archaea

Many properties of denitrifying anaerobic methane oxidation (DAMO) bacteria have been explored since their first discovery, while DAMO archaea have attracted less attention. Since nitrate is more abundant than nitrite not only in wastewater but also in the natural environment, in depth investigations of the nitrate-DAMO process should be conducted to determine its environmental significance in the global carbon and nitrogen cycles. This review summarizes the status of research on DAMO archaea and the catalyzed nitrate-dependent anaerobic methane oxidation, including such aspects as laboratory enrichment, environmental distribution, and metabolic mechanism. It is shown that appropriate inocula and enrichment parameters are important for the culture enrichment and thus the subsequent DAMO activity, but there are still relatively few studies on the environmental distribution and physiological metabolism of DAMO archaea. Finally, some hypotheses and directions for future research on DAMO archaea, anaerobic methanotrophic archaea, and even anaerobically metabolizing archaea are also discussed.

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.

Stability of microbial functionality in anammox sludge adaptation to various salt concentrations and different salt-adding steps

The stability of community functioning in anaerobic ammonia oxidation (anammox) sludge adaptation to various salinity changes are concerned but not fully explored. In this study, two anammox reactors were designed in response to different salt levels and salt-adding methods. The reactor PI, run with small stepwise salt increments (0.5%-1.0%), removed >90% of nitrite and ammonium in the influent over the range of 0%-4% salt. By contrast, the reactor SI, run with a sharp salt increment (>2.5%), exhibited a reduced performance (by up to 44%) over the same salt range with a new steady state. The observed resilience times after salt perturbations indicated that the PI reactor recovered substantially and rapidly at all imposed salt levels. Principal coordinates analysis of 16S rRNA gene amplicon sequences revealed that bacterial community structures of the anammox sludge altered conspicuously in response to the salinity changes. However, quantitative PCR analysis showed that the shift in copy number of studied nitrogen-converting genes encoding hydrazine synthase (hzsA), bacterial and archaeal ammonia monooxygenases (amoA), nitrite oxidoreductase (nxrB), nitrite reductase (nirK), and nitrous oxide reductase (nosZ) was not significant (p > 0.05) in anammox sludge across the salt levels of 0.5%-4%, which suggests the stability of microbial community functioning in the osmoadaptation processes. The freshwater anammox Ca. Kuenenia showed high osmoadaptation by potentially adopting both high-salt-in and low-salt-in strategies to dominate in both reactors. The quantitative transcript analysis showed that the active anammox bacteria represented by hzsA transcripts in the SI reactor were approximately two orders of magnitude lower than those in the PI reactor during the long-term exposure to 4% salinity, manifesting the influence by the salt-increasing methods. These results provided new insight into osmo-adaptation of the anammox microbiome and will be useful for managing salinity effects on nitrogen removal processes.

Enhancement of the denitrification in low C/N condition and its mechanism by a novel isolated Comamonas sp. YSF15

A novel denitrifying bacterium YSF15 was isolated from the Lijiahe Reservoir in Xi'an and identified as Comamonas sp. It exhibited excellent nitrogen removal ability under low C/N conditions (C/N = 2.5) and 94.01% of nitrate was removed in 18 h, with no accumulation of nitrite. PCR amplification and nitrogen balance experiments were carried out, showing that 68.92% of initial nitrogen was removed as gas products and the nitrogen removal path was determined to be NO₃^--N→NO₂^--N→NO→N₂O→N₂. Scanning electron microscopy and three-dimensional fluorescence spectroscopy were used to track extracellular polymeric substances (EPS). The results show that complete-denitrification under low C/N conditions is associated with EPS, which may provide a reserve carbon source in extreme environments. These findings reveal that Comamonas sp. YSF15 can provide novel basic materials and a theoretical basis for wastewater bioremediation under low C/N conditions.

Methane emissions from aqueous sediments are influenced by complex interactions among microbes and environmental factors: A modeling study

Methane fluxes from aqueous sediments strongly influence global atmospheric methane. However, many questions still puzzle researchers; for example, why are some unstable sediments atmospheric methane sinks? In this study, a biofilm model originally developed for wastewater treatment was modified to simulate the microbial kinetics and substance conversions in aqueous surface sediments. The model was validated by the experimental data and could predict chemical profiles and microbial distributions in sediments. The model revealed complicated interactions between different microbial communities and environmental factors, including competition between aerobic methane-oxidizing bacteria, nitrite-dependent anaerobic methane-oxidizing bacteria, and anaerobic ammonia-oxidizing bacteria. The results of model simulations showed that the effects of environmental factors, especially dissolved oxygen and ammonia in overlying water, on methane fluxes are very complicated. Rapid environmental changes (which can be caused by tide, day-night alternation, or zoobenthic and human activity) and intensive competition between microbes greatly affected methane fluxes and resulted in alternation between atmospheric methane source and sink in unstable sediments. This study extends the application of a wastewater treatment model to ecological studies of microbial interactions in natural sediments and explains some problems that might be difficult to resolve by using experimental methods.

Mass transfer affects reactor performance, microbial morphology, and community succession in the methane-dependent denitrification and anaerobic ammonium oxidation co-culture

Denitrifying anaerobic methane oxidation (DAMO) combining anaerobic ammonium oxidation (Anammox) process is a novel nitrogen removal technology. However, the roles of methane transfer (gas phase) and nitrogen transfer (liquid phase) in the heterogeneous process remain unclear. In this study, granular DAMO and Anammox co-cultures were inoculated from a hollow-fiber membrane bioreactor into a sequence batch reactor (SBR). Since the methane transfer became limited in SBR, the nitrate removal rate first decreased and then increased to 10 mg/(L∙day), while the ammonium removal rate did not recover and was around 2 mg/(L∙day). The activity of DAMO archaea and Anammox bacteria decreased noticeably. Furthermore, granular aggregates dispersed into small granules and ultimately became flocs with poor settleability in SBR. The content of extracellular polymeric substances decreased, especially that of proteins and humics. DAMO archaea decreased by 94.6% and Anammox bacteria decreased by 72%. In summary, the limitation of methane transfer affected DAMO and Anammox processes more notably than nitrogen transfer, resulting in lower nitrogen removal, granule disruption, and microbial community succession.

Regulation of coastal methane sinks by a structured gradient of microbial methane oxidizers

Coastal wetlands are widely recognized as atmospheric methane sources. However, recent field studies suggest that some coastal wetlands could also act as methane sinks, but the mechanism is not yet clear. Here, we investigated methane oxidation with different electron acceptors (i.e., oxygen, nitrate/nitrite, sulfate, Fe(III) and Mn(IV)) in four coastal wetlands in China using a combination of molecular biology methods and isotopic tracing technologies. The geochemical profiles and in situ Gibbs free energies suggest that there was significant nitrite-dependent anaerobic oxidation of methane (nitrite-AOM) in the sub-surface sediments; this was subsequently experimentally verified by both the microbial abundance and activity. Remarkably, the methanotrophic communities seemed to exist in the sediments as layered structures, and the surface aerobic methane-oxidizing bacteria were able to take up atmospheric methane at a rate of 0.10-0.18 nmol CH₄ day^-1 cm^-2, while most, if not all, sedimentary methane was being completely consumed by anaerobic methanotrophs (23-58% by methane oxidizers in phylum NC10). These results suggest that coastal methane sinks might be governed by diverse microbial communities where NC10 methane oxidizers contributed significantly. This finding helps to better understand and predict the coastal methane cycle and reduce uncertainties in the estimations of the global methane flux.

High-efficiency inorganic nitrogen removal by newly isolated Pannonibacter phragmitetus B1

An aerobic heterotrophic nitrogen removal bacterium strain, B1, was isolated from aquaculture water and identified as Pannonibacter phragmitetus (99% similarity) by 16S rRNA sequencing analysis. When ammonium, nitrite or nitrate was the sole nitrogen source, with an initial nitrogen concentration of 14 mg/L, the nitrogen removal efficiencies were 98.66%, 99.96% and 98.73%, respectively, and the corresponding maximum removal rates reached as high as 1.16, 0.77 and 0.81 mg/L/h, respectively. In the presence of NH₄⁺-N, the removal efficiency of 56 mg/L NO₂^--N within 27 h increased by 83.50%, and the corresponding removal rate reached as high as 1.72 mg/L/h. Additionally, different carbon sources (dl-malic acid, sucrose, sodium citrate, and glucose) could be utilized in nitrogen removal. Sequence amplification indicates that the denitrification genes nirK, norB and narG are present in strain B1. All results demonstrate that strain B1 has high promise for future applications of removing inorganic nitrogen from wastewater.

Optimization of methane-dependent oxygenic denitrification in sequencing batch reactors by insights into the microbial interactions

Methane-dependent oxygenic denitrification (O2DN) is a promising technology used for reducing greenhouse gas emissions of nitrous oxide (N₂O) during wastewater treatment. Heterotrophic bacteria are associated with methane-dependent O2DN bacteria, and it has been proposed that metabolic cross-feeding occurs between the two populations above. In this study, a mathematical model was developed to describe the microbial processes and interactions between methane-dependent O2DN bacteria and associated heterotrophic bacteria in a sequencing batch reactor (SBR). A growth factor-dependent decoupling of metabolism and growth of methane-dependent O2DN bacteria was introduced into the model. Effects of influent substrates, operating parameters, and initial biomass on microbial community and reactor performance were then investigated, and the above parameters were optimized using the model. Results surprisingly show that organic matter in the influent greatly stimulated the growth of methane-dependent O2DN bacteria but slightly limited the increase of heterotrophic bacteria. This effect could be explained by the increased excretion of growth factors by heterotrophic bacteria and the intensified competition for nitrite when methane-dependent O2DN bacteria increased. These results will assist in providing a new understanding of microbial interactions in methane-dependent O2DN systems and offer a new and efficient strategy for operating methane-dependent O2DN reactors.