Denitrification Research Articles

Microbial Inoculants Modify the Functions of Resident Soil Microbes to Expedite the Field Restoration of the Abandoned Mine

ABSTRACTGlobal‐scale mining activities have had significant deleterious impacts on local ecosystems and the overall environment, which will necessitate robust restoration efforts. A practical approach includes combining microbial inoculants with the technology of external soil spray seeding. This approach holds the potential for sustainable abandoned mine site restoration by enhancing plant growth through the modulation of soil nutrients and microbial communities. Nonetheless, the detailed effects of microbial inoculants on specific aspects of soil microbial community functions and their complex interactions with plant growth remain underexplored, particularly in the context of restoration efforts. To bridge this gap, we performed a four‐year field study at an abandoned carbonate mine location, using metagenomic sequencing to evaluate the influence of microbial inoculants on soil microbial functionality. Our research revealed that introducing microbial inoculants greatly enhanced essential soil parameters and notably increased plant biomass. Additionally, these inoculants altered the functional gene makeup of the microbial community, significantly boosting the relative abundance of processes such as nitrogen fixation, nitrification, denitrification, assimilatory nitrate reduction (ANRA), dissimilatory nitrate reduction (DNRA), and organic phosphorus mineralization. Conversely, there was a decrease in the relative abundance of carbon degradation, phosphorus regulation, and transport processes. We observed strong correlations between the abundance of nitrogen and phosphorus cycles and plant biomass. Crucially, microbial inoculants affect plant biomass by initially altering soil properties and subsequently coordinating nitrogen and phosphorus cycles. These findings provide valuable insights into the role of microbial inoculants in mine site restoration and offer a theoretical foundation for their broader practical application.

Research progress of nanoscale zero-valent iron in synergistic combinations of denitrifying bacteria for nitrate removal

Nitrate (NO3--N) is a common inorganic nitrogen pollutant in water. Excessive NO3--N can lead to water eutrophication and threaten human health. Nanoscale zero-valent iron (nZVI) has attracted much attention in NO3--N removal due to its high specific surface and excellent electron donor properties. The combination of nZVI and denitrifying bacteria (DNB) demonstrates high efficiency in converting NO3--N into N2. This approach not only substantially enhances the removal rate of NO3--N but also exhibits superior environmental sustainability compared with conventional chemical denitrification methods. Accordingly, it holds substantial promise for mitigating NO3--N pollution and warrants further exploration in the pollution control. Therefore, it is necessary to understand the interaction mechanism between nZVI and DNB for NO3--N removal. This paper details the factors affecting the removal of NO3--N by nZVI combined with DNB, reviews the latest research progress in this field, elaborates on the interaction mechanism between nZVI and DNB for NO3--N removal, and discusses the challenges and future research directions of NO3--N removal by nZVI combined with DNB. This review aims to provide a theoretical basis for the development of efficient approaches for the remediation of NO3--N pollution.

Impact of Agroforestry Practices on Soil Microbial Diversity and Nutrient Cycling in Atlantic Rainforest Cocoa Systems

Microorganisms are critical indicators of soil quality due to their essential role in maintaining ecosystem services. However, anthropogenic activities can disrupt the vital metabolic functions of these microorganisms. Considering that soil biology is often underestimated and traditional assessment methods do not capture its complexity, molecular methods can be used to assess soil health more effectively. This study aimed to identify the changes in soil microbial diversity and activity under different cocoa agroforestry systems, specially focusing on taxa and functions associated to carbon and nitrogen cycling. Soils from three different cocoa agroforestry systems, including a newly established agroforestry with green fertilization (GF), rubber (Hevea brasiliensis)–cocoa intercropping (RC), and cocoa plantations under Cabruca (cultivated under the shave of native forest) (CAB) were analyzed and compared using metagenomic and metaproteomic approaches. Samples from surrounding native forest and pasture were used in the comparison, representing natural and anthropomorphic ecosystems. Metagenomic analysis revealed a significant increase in Proteobacteria and Basidiomycota and the genes associated with dissimilatory nitrate reduction in the RC and CAB areas. The green fertilization area showed increased nitrogen cycling activity, demonstrating the success of the practice. In addition, metaproteomic analyses detected enzymes such as dehydrogenases in RC and native forest soils, indicating higher metabolic activity in these soils. These findings underscore the importance of soil management strategies to enhance soil productivity, diversity, and overall soil health. Molecular tools are useful to demonstrate how changes in agricultural practices directly influence the microbial community, affecting soil health.

Mechanisms of Irrigation Water Levels on Nitrogen Transformation and Microbial Activity in Paddy Fields

Nitrogen is a vital nutrient for rice growth; however, its inefficient use often results in nutrient loss, environmental degradation, and the emission of greenhouse gases. In this study, a rice paddy simulation was conducted under different water levels (1–4 cm), incorporating a comprehensive analysis of nitrogen dynamics, environmental factors, and microbial communities to evaluate the impact of water levels on nitrogen concentrations and microbial composition. The results indicated that the water level had a greater impact on nitrogen concentrations in surface water than in soil water. Compared to low water level conditions (1 cm), the average concentrations of ammonium nitrogen, nitrate nitrogen, and nitrite nitrogen in surface water under 2–4 cm water levels decreased by approximately 53.8%, 36.7%, and 78.9%, respectively. Water levels also influenced the microbial composition and nitrogen cycling in paddy soil, with lower water levels promoting aerobic processes such as nitrification, while higher water levels facilitated anaerobic processes such as denitrification and dissimilatory nitrate reduction to ammonium. Correspondingly, microbial composition shifted, with aerobic bacteria predominating in shallow water conditions and anaerobic bacteria flourishing under deeper water. These findings suggest that optimized water management, particularly through shallow irrigation, may mitigate nitrogen loss and improve nitrogen use efficiency. Nevertheless, additional field studies are necessary to validate these results and explore their interaction with other agricultural practices.

Exploring NRB Biofilm Adhesion and Biocorrosion in Oil/Water Recovery Operations Within Pipelines.

Microbially influenced corrosion represents a critical challenge to the integrity and durability of carbon steel infrastructure, particularly in environments conducive to biofilm formation by nitrate-reducing bacteria (NRB). This study investigated the impact of NRB biofilms on biocorrosion processes within oil/water recovery operations in Algerian pipelines. A comprehensive suite of experimental and analytical techniques, including microbial analysis, gravimetric methods, and surface characterization, were employed to elucidate the mechanisms of microbially influenced corrosion (MIC). Weight loss measurements revealed that carbon steel samples exposed to injection water exhibited a corrosion rate of 0.0125 mm/year, significantly higher than the 0.0042 mm/year observed in crude oil environments. The microbial analysis demonstrated that injection water harbored an average of (4.4 ± 0.56) × 106 cells/cm2 for sessile cells and (3.1 ± 0.25) × 105 CFU/mL for planktonic cells, in stark contrast to crude oil, which contained only (2.4 ± 0.34) × 103 cells/cm2 for sessile cells and (4.5 ± 0.12) × 102 CFU/mL for planktonic cells, thereby highlighting the predominant role of injection water in facilitating biofilm formation. Contact angle measurements of injection water on carbon showed 45° ± 2°, compared to 85° ± 4° for crude oil, suggesting an increased hydrophilicity associated with enhanced biofilm adhesion. Scanning electron microscopy further confirmed the presence of thick biofilm clusters and corrosion pits on carbon steel exposed to injection water, while minimal biofilm and corrosion were observed in the crude oil samples.

Accelerated corrosion by nitrate-reducing bacteria and gaseous hydrogen permeation of X80 steel

Effects of hydrogen gas and Nitrate-Reducing Bacteria (NRB) on the corrosion and hydrogen permeation dynamics of X80 steel in simulated pipeline condensate water was investigated. Weight-loss analysis and electrochemical tests show a direct correlation between NRB-induced corrosion of X80 steel and the presence of hydrogen gas, particularly evident with escalating hydrogen blending ratios. Notably, corrosion primarily manifested through the formation of localized pitting, exhibiting a positive correlation with hydrogen blending ratios. Furthermore, the hydrogen permeating results show there is a synergistic enhancement of hydrogen permeating facilitated by the coexistence of hydrogen gas and NRB. Quantitative assessments of hydrogen permeability parameters highlighted significant amplifications in hydrogen diffusion flux (J) and apparent hydrogen concentration (Capp) under the combined influence of hydrogen and NRB, underscoring the complex interplay between these factors.

Directional Electron Transfer in Enzymatic Nano‐Bio Hybrids for Selective Photobiocatalytic Conversion of Nitrate

Semi‐artificial photosynthetic system (SAPS) that combines enzymes or cellular organisms with light‐absorbing semiconductors, has emerged as an attractive approach for nitrogen conversion, yet faces the challenge of reaction pathway regulation. Herein, we find that photoelectrons can transfer from the ‐C≡N groups at the edge of cyano‐rich carbon nitride (g‐C3N4‐CN) to nitrate reductase (NarGH), while the direct electron transfer to nitrite reductase (cd1NiR) is inhibited due to the physiological distance limit of active sites (> 14 Å). By means of the directional electron transfer between g‐C3N4‐CN and extracted biological enzymes, the product of the denitrification reaction was switched from inert N2 to usable nitrite with an unprecedented selectivity of up to 95.3%. The converted nitrite could be further utilized by anammox microbiota and dissimilatory nitrate reduction to ammonia (DNRA) microorganisms, doubling the efficiency of total nitrogen removal (96.5 ± 2.3 %) for biological nitrogen removal and ammonia generation (12.6 mg NH4+‐N L‐1 h‐1), respectively. Thus, our work paves an appealing way for the sustainable treatment and utilization of nitrate for ammonia fuel production by strategically regulating the electron transfer pathway across the biotic‐abiotic interface.

Directional Electron Transfer in Enzymatic Nano-Bio Hybrids for Selective Photobiocatalytic Conversion of Nitrate.

Semi-artificial photosynthetic system (SAPS) that combines enzymes or cellular organisms with light-absorbing semiconductors, has emerged as an attractive approach for nitrogen conversion, yet faces the challenge of reaction pathway regulation. Herein, we find that photoelectrons can transfer from the -C≡N groups at the edge of cyano-rich carbon nitride (g-C3N4-CN) to nitrate reductase (NarGH), while the direct electron transfer to nitrite reductase (cd1NiR) is inhibited due to the physiological distance limit of active sites (> 14 Å). By means of the directional electron transfer between g-C3N4-CN and extracted biological enzymes, the product of the denitrification reaction was switched from inert N2 to usable nitrite with an unprecedented selectivity of up to 95.3%. The converted nitrite could be further utilized by anammox microbiota and dissimilatory nitrate reduction to ammonia (DNRA) microorganisms, doubling the efficiency of total nitrogen removal (96.5 ± 2.3 %) for biological nitrogen removal and ammonia generation (12.6 mg NH4+-N L-1 h-1), respectively. Thus, our work paves an appealing way for the sustainable treatment and utilization of nitrate for ammonia fuel production by strategically regulating the electron transfer pathway across the biotic-abiotic interface.

Functional Genes and Metabolic Pathways of Nitrogen Metabolism Microorganisms in Lake Sediments：A Case Study of Hongfeng Lake, Guizhou Province

The nitrogen cycle is of great importance for material circulation and energy flow in lake ecosystems. It is driven by microorganisms in lake sediments and can contribute to balancing lake ecosystems. In this study, physical and chemical properties of the sediments sampled from Hongfeng Lake in Guizhou Province were assayed and analyzed using metagenomics to reveal relevant microorganisms, functional genes, metabolic pathways, and their relationships throughout nitrogen metabolism. The results showed that bacteria were dominant, and the top three relative abundant genera were Thiobacillus （16.64%）, Rubrivivax（9.43%）, and Nitrospira （7.09%）. Only six pathways, including nitrogen fixation, nitrification, denitrification, assimilatory nitrate reduction, dissimilatory nitrate reduction, and complete nitrification, were detected in total, of which denitrification and dissimilatory nitrate reduction were the primary processes, but anaerobic ammonia oxidation was not detected. Bacteria and archaea participated in these six pathways, while eukaryotes only functioned in dissimilatory nitrate reduction, denitrification, and complete nitrification. Ammonia nitrogen, nitrate nitrogen, and total phosphorus, as main environmental factors affecting the distribution of functional genes for nitrogen metabolism, differentiated with each other in their respective real-world conditions. A positive correlation （95.04%） was observed between the functional genes and microorganisms, and narG, narZ, and nxrA possessed the highest abundance and the highest host genes. On this basis, these findings are expected to further elucidate the nitrogen cycle of typical karst lakes in Guizhou Province.

Impact of water depth and flow velocity on organic matter removal and nitrogen cycling in floating constructed wetlands

The configuration of water depth (WD) and flow velocity (FV) in floating constructed wetlands (FCWs) reflects the complex interactions between hydraulic conditions and wastewater treatment performance. Properly configuring hydraulic conditions can optimize performance. This study explored the removal and degradation of organic matter and nitrogen (N) in FCWs with three different WDs (50 cm (SD), 70 cm (MD), and 95 cm (DD)) under four FV conditions (0, 0.3, 1.0, and 1.5 m/d). It also analyzed the regulatory effects of hydraulic conditions on microbial community structure to provide new insights for configuring hydraulic conditions in FCWs. The results indicate that variations in WD and FV do not significantly affect COD removal (p > 0.05), with removal efficiency (RE) consistently reaching 90 % across all conditions. However, hydraulic conditions do influence the degradation rate (k) of COD, with increased FV enhancing the k in MD and DD. In contrast, WD significantly impacts the removal of NH4+−N (p < 0.05), with RE in SD exceeding 80 % across all four FVs. The primary reason is that increased WD reduced the abundance of nitrifying bacteria (Nitrosomonas and Nitrospira) and functional genes (hao and pmoABC-amoABC). Meanwhile, increased WD and FV significantly enriched the denitrifying bacteria genera (such as Massilia, Bacillus, Ensifer, and Rhodobacter) and certain functional genes (nap and nir), favoring the denitrification process and enhancing the dissimilatory nitrate reduction to ammonium (DNRA) process. Furthermore, microbial community diversity, richness, and the number of operational taxonomic units (OTUs) were highest in SD, but decreased with increased WD. This is mainly related to changes in oxygen (O2) transmission and nutrient concentration. These findings help us understand the mechanisms by which hydraulic regulation affects the removal of organic matter and N in FCWs. They also propose potential solutions from a hydraulics perspective to enhance N removal.

Fimbriimonadales performed dissimilatory nitrate reduction to ammonium (DNRA) in an anammox reactor

Bacteria belonging to the order Fimbriimonadales are frequently detected in anammox reactors. However, the principal functions of these bacteria and their potential contribution to nitrogen removal remain unclear. In this study, we aimed to systematically validate the roles of Fimbriimonadales in an anammox reactor fed with synthetic wastewater. High-throughput 16S rRNA gene sequencing analysis revealed that heterotrophic denitrifying bacteria (HDB) were the most abundant bacterial group at the initial stage of reactor operation and the abundance of Fimbriimonadales members gradually increased to reach 38.8 % (day 196). At the end of reactor operation, Fimbriimonadales decreased to 0.9 % with an increase in anammox bacteria. Correlation analysis demonstrated nitrate competition between Fimbriimonadales and HDB during reactor operation. Based on the phylogenetic analysis, the Fimbriimonadales sequences acquired from the reactor were clustered into three distinct groups, which included the sequences obtained from other anammox reactors. Metagenome-assembled genome analysis of Fimbriimonadales allowed the identification of the genes narGHI and nrfAH, responsible for dissimilatory nitrate reduction to ammonium (DNRA), and nrt and nasA, responsible for nitrate and nitrite transport. In a simulation based on mass balance equations and quantified bacterial groups, the total nitrogen concentrations in the effluent were best predicted when Fimbriimonadales was assumed to perform DNRA (R2 = 0.70 and RMSE = 18.9). Moreover, mass balance analysis demonstrated the potential contribution of DNRA in enriching anammox bacteria and promoting nitrogen removal. These results prove that Fimbriimonadales compete with HDB for nitrate utilization through DNRA in the anammox reactor under non-exogenous carbon supply conditions. Overall, our findings suggest that the DNRA pathway in Fimbriimonadales could enhance anammox enrichment and nitrogen removal by providing substrates (nitrite and/or ammonium) for anammox bacteria.

Model the evolutionary pattern of N species and pool size in groundwater continuum by utilizing measured source and sink rates of nitrate and ammonium

Nitrate and ammonium are primary nitrogen (N) contaminants in groundwater and effective restoration strategies depend on understanding the interactions of N transformation processes along redox gradients. Utilizing the 15N tracing technique, we assess nitrate removal rates, focusing on denitrification and anammox in a N-rich groundwater of the Hetao Basin, a typical semiarid region in western China. Results showed that N removal rate (0.36–22.01 µM N d−1) was composed mainly of denitrification (73 ± 18 %), with rates increasing from upstream oxidizing environment to downstream reducing areas. In reducing downstream, both denitrification and anammox adhered to substrate-driven Michaelis-Menten kinetics. Integrating data on all source and sink rates of nitrate and ammonium pools (denitrification, anammox, dissimilatory nitrate reduction ammonia, nitrification, mineralization), we constructed a N-transfer-dynamics model based on chemical stoichiometry. This model effectively captured the observed spatial N transfer patterns and highlighted that the balance of oxidants and biodegradable organic N inputs influences N species retention and removal in groundwater. Our combined experimental and modeling approach underscores the importance of reducing organic N and/or adding oxidants to mitigate groundwater N pollution. These findings provide crucial insights for optimizing high N groundwater remediation strategies and potentially inform for wastewater management practices.

Identifying the major metabolic potentials of microbial-driven carbon, nitrogen and sulfur cycling on stone cultural heritage worldwide

Microbial activities and biochemical reactions are responsible for the biodeterioration of stone cultural heritage, but information on microbial metabolic potentials remains elusive. Here we profiled microbial community signatures and its functional traits on stone cultural heritage from different climate zones globally using sequencing datasets available publicly. Bacterial community on stone cultural heritage shows a significant separation between BSk (cold semi-arid climate) and Cfb (temperate oceanic climate) with Aw (tropical savanna climate) as a transition region. Importantly, the ubiquity of ammonia oxidizers and nitrite oxidizers on stone cultural heritage under different climates supports the active production and accumulation of nitrates while ammonia/ammonium can be supplied by dinitrogen fixation and dissimilatory nitrate reduction to ammonium (DNRA), together with the hydrolysis of urea, arginine, formamide and cyanate. Sulfate accumulation on stone cultural heritage is mainly resulted from the microbial-driven transformation of organosulfur and thiosulfate, with little dissimilatory reduction of sulfate. Pseudorhodoplanes was identified and reported in elemental sulfur turnover for the first time. Notably, carbon sequestration via the reductive tricarboxylic acid (rTCA) cycle and an incomplete 3-hydroxypropionate/4-hydroxybutynate (HP/HB) cycle other than the Calvin Benson-Bassham (CBB) cycle is also significant on stone cultural heritage under relatively humid climate. These results advance our understanding of microbial metabolic potentials and their genetical partitioning patterns on stone cultural heritage of different climate zones globally.

Removal of nitrate nitrogen by aerobic denitrification granular sludge under strongly alkaline conditions: Performance and mechanism

Aerobic granular sludge (AGS) technology faces challenges in treating strongly alkaline wastewater because high pH affects sludge settleability and microbial metabolic activity. In this study, aerobic denitrification granular sludge (ADGS) was cultivated with strongly autoaggregating aerobic denitrifying bacteria as the sludge source at an influent pH of 11.0 under aerobic conditions. The formation characteristics of ADGS and its pollutant removal efficiency under strongly alkaline conditions were explored. Compared with ADGS cultivated under neutral conditions, ADGS cultivated under strongly alkaline conditions resulted in superior particle formation rates, settleability, particle strengths and particle stabilities. Nitrate nitrogen was almost completely removed at a pH of 11.0 under aerobic conditions. Protein, polysaccharides and humic acid in extracellular polymeric substances (EPS) under strongly alkaline conditions were involved in the ADGS formation process. In addition, the presence of metallic elements in the EPS and inorganic nuclei in the sludge facilitated the formation of ADGS. The strongly alkaline environment enriched the alkali-resistant aerobic denitrifying bacteria, such as unclassified_f__Rhodobacteraceae, Tessaracoccus, Halomonas, and Pseudofulvimonas, as well as specialized alkali-resistant genera. Furthermore, the strongly alkaline environment increased the abundance of key enzymes involved in carbon metabolism and upregulated the expression of nitrogen metabolism genes in the ADGS to maintain their metabolic activity. In addition, the microorganisms upregulated genes that encoded reverse transporter protein genes (mnhACEFG, nhaACP, and phaACDEFG) and K+ uptake protein genes (trkAHG) and secreted greater amounts of acidic functional groups and free amino groups to help maintain intracellular and extracellular osmolality and acid–base homeostasis under strongly alkaline stress. This research provides a possible option and innovative strategy for the management of highly alkaline nitrate-containing wastewater.

Intra/extracellular electron transfer and metagenomic analysis elucidated the roles of magnetic iron powder (Fe3O4) on mixotrophic denitrification system

Elemental iron provides a viable strategy to improve the denitrification efficiency by expediting electron transport. However, the roles of magnetic iron powder (Fe3O4) on mixotrophic denitrification remains unknown. In this study, the intra/extracellular electron transfer (IET/EET) and microbial metabolism mechanisms were explored in a Fe3O4-mediated sulfide-autotrophic and heterotrophic denitrification system. The results showed that Fe3O4 promoted the formation of dense clump structure with filamentous cross-linking in activated sludge. Fe3O4 could increase the coenzyme Q activity in IET and the content of free riboflavin and cytochrome c in EET. Metagenomic analysis indicated that denitrification, sulfide oxidation and sulfate reduction were the main pathways of nitrogen and sulfur metabolism, and the enriched denitrifying bacteria (Halomonas and Hypobacterium) and sulfur-oxidizing bacteria (Marinicella) could stably support nitrate removal. This study expands our understanding of the IET/EET during Fe3O4-mediated mixotrophic denitrification process, providing a novel insight for nitrogen removal from marine recirculating aquaculture wastewater.

Detection of Nitrate-Reducing/Denitrifying Bacteria from Contaminated and Uncontaminated Tallgrass Prairie Soil: Limitations of PCR Primers.

Contamination of soil by spills of crude oil and oilfield brine is known to affect the species composition and functioning of soil microbial communities. However, the effect of such contamination on nitrogen cycling, an important biogeochemical cycle in tallgrass prairie soil, is less well known. Detecting nitrate-reducing (NR) and denitrifying (DN) bacteria via PCR amplification of the genes essential for these processes depends on how well PCR primers match the sequences of these bacteria. In this study, we enriched for NR and DN bacteria from oil/brine tallgrass prairie soil contaminated 5-10 years previously versus those cultured from uncontaminated soil, confirmed the capacity of 75 strains isolated from the enrichments to reduce nitrate and/nitrite, then screened the strains with primers specific to seven nitrogen cycle functional genes. The strains comprised a phylogenetically diverse group of NR and DN bacteria, with proportionately more γ-Proteobacteria in oil-contaminated sites and more Bacilli in brine-contaminated sites, suggesting some residual effect of the contaminants on the NR and DN species distribution. Around 82% of the strains shown to reduce nitrate/nitrite would not be identified as NR and DN bacteria by the battery of NR and DN primers used. Our results indicate an urgent need to expand the NR/DN functional gene primer database by first identifying novel NR/DN strains through their capacity to reduce nitrate/nitrite.

Application of flow airlift fluidized bed (CAFB) - Aerobic denitrification bioreactor using polycaprolactone (PADR) as organic carbon source and biofilm carrier for synthetic marine aquaculture wastewater treatment

Aerobic denitrification, a novel method for complete nitrate removal process, relies on a constant carbon supply, and carbon availability can be a limiting factor for the process. Three biodegradable polymers - polybutylene succinate (PBS), polycaprolactone (PCL), and polylactic acid (PLA) were used as carbon source of aerobic denitrifying bacteria, Halomonas venusta for treating recirculating aquaculture wastewater in batch tests, respectively. The group utilizing PCL displayed the greatest efficiency in nitrate removal. Afterwards, a continuous flow airlift fluidized bed (CAFB) - aerobic denitrification bioreactor using PCL(PADR) as bio-carriers and carbon source was started up. The effect of shifting temperature on the nitrate removal and microbial community of CAFB-PADR was investigated under different hydraulic retention times (HRT). The results showed that no significant difference was detected in denitrification activity at 25 and 30 °C condition, where the denitrification rate was 1.1–1.8 times higher than that at 15 °C. However, the nitrogen removal efficiency was above 50 % and there was barely accumulation of nitrite at 15 °C, indicating the good nitrate removal performance in CAFB-PADR at all three temperature conditions. Microbial composition analyses revealed that as the temperature decreased, the relative abundance of Gammaproteobacteria increased, while those of Alphaproteobacteria and Bacteroidia diminished decreased. The existence of denitrifying function genera (Marinobacter, Pseudomonas, Halomonas and Hydrogenophaga) ensured the rapid removal of nitrogen in the biofilter. When the temperature dropped to 15 °C, Pseudomonas progressively took the position of Marinobacter, both of which had denitrification and degradation activities. The relative abundance of the denitrifying bacteria Halomonas that we inoculated has been less than 1 % since phase II. Overall, the PCL-supported CAFB-PADR demonstrated in this study shows potential for removing high concentrations of nitrate from recirculating marine aquaculture wastewater.

Characterisation of Pseudomonas mosselii H6 for aerobic denitrification: stoichiometry and reaction kinetics

The application of aerobic denitrifying bacteria in biological nitrogen removal has been of great interest. However, the mechanism of nitrogen conversion by aerobic denitrifying bacteria is unclear due to the lack of stoichiometry and kinetic studies. Therefore, this study aimed to investigate the stoichiometry and kinetics of aerobic denitrification using Pseudomonas mosselii H6 as a model strain. The results showed that the stoichiometric equations of strain H6 under different nitrogen source conditions were obtained by nitrogen balance analysis; the concentrations of different limiting substrates were set and combined with the multi-substrate Monod kinetic equation to fit the half-saturation constants of COD, ammonia, nitrate and nitrite nitrogen for strain H6, which were 230.1008, 25.3435, 4.1009 and 23.4601 mg/L, respectively; based on the model of “two-step denitrification”, the numerical simulation of the growth and aerobic denitrification of strain H6 under different limiting substrate concentrations was carried out by using AQUASIM software to test the validity of stoichiometric coefficients and kinetic constants and perfect fitting results were obtained. These parameters will help to simulate the growth and denitrification performance of aerobic denitrifying bacteria and provide an experimental basis for further quantitatively elucidating the mechanism of action in the aerobic denitrification process.

Bifunctional sludge-derived redox carbon dots with photoelectron storage and delivery properties for ammonia production by photosensitized Shewanella oneidensis MR-1

Combining the light-harvesting capabilities of photosensitizers with microbial catalysis shows great potential in solar-driven biomanufacturing. However, little information is available about the effects of photosensitizers on the photoelectron transport during the dissimilatory nitrate reduction to ammonium (DNRA) process. Herein, redox carbon dots (CDs-500) were prepared from sludge via the pyrolysis-Fenton reaction and then used to construct a photosynthetic system with Shewanella oneidensis MR-1. The MR-1/CDs-500 photosynthetic system showed a 5.9-fold increase in ammonia production (4.9 mmol(NH3)·g−1(protein)·h−1) with a high selectivity of 94.0 %. The photoelectrons were found to be stored in CDs-500 and transferred into the cells. During the inward electron transport, the intracellular CDs-500 could be used as the direct photoelectron transfer stations between outer membrane cytochrome c and DNRA-related enzymes without the involvement of CymA and MtrA. This work provides a new method for converting waste into functional catalysts and increases solar-driven NH3 production to a greater extent.

Artificial Cultivation of Aquatic Plants Promotes Nitrogen Transformation and the Abundance of Key Functional Genes in Agricultural Drainage Ditch Sediments in the Yellow River Irrigation Area in China

Excess nitrogen in agricultural drainage poses a serious threat to the water quality safety of the Yellow River basin. Utilizing aquatic plants to modify the rhizosphere microbial community structure and facilitate nitrogen transformation is a crucial strategy for mitigating regional water eutrophication. We here compare key processes of nitrogen transformation occurring in the rhizosphere of sediments of a ditch artificially planted with a mix of species (Phragmites australis, Typha orientalis, Nymphaea tetragon) with the rhizosphere of a ditch occupied by naturally occurring aquatic vegetation, dominated either by P. australis or T. orientalis. Our results revealed a species effect, with an increased denitrification rate (DR) and dissimilatory nitrate reduction to ammonium rate (DNRAR) in the cultivated ditch for P. australis, compared to the naturally occurring T. orientalis vegetation. The nitrogen fixation rate (NFR) increased in the artificial setting with T. orientalis in comparison to natural P. australis vegetation. The richness of the bacterial community and the relative abundances of Bacteroidota, Firmicutes, and Geobacter were significantly greater in the rhizosphere of the artificially cultivated ditch due a greater availability in nitrogen and organic carbon. In the artificially cultivated ditch, the dominant functional genes affecting DRNARs in the rhizosphere sediments of P. australis were nrfC and nrfA, whereas DRs were driven mainly by norB and napA, which were influenced by the nitrogen and carbon levels. The dominant functional genes affecting NFRs in the rhizosphere sediments of T. orientalis were nifD, nifK, and nifH. Our results provide a scientific basis for the use of aquatic plants for mitigating excess nitrogen levels in agricultural drainage.