Identification, interactions, nitrogen removal pathways and performances of culturable heterotrophic nitrification-aerobic denitrification bacteria from mariculture water by using cell culture and metagenomics

Evaluation of nitrogen removal performance and metabolic mechanism of a novel salt-tolerant strain Pseudomonas aeruginosa SH3

High salinity impedes efficient nitrogen removal from mariculture wastewater, which inhibits the colonization and nitrogen removal capabilities of nitrogen-removing microbes. This study aimed to isolate and characterize a salt-tolerant heterotrophic nitrification-aerobic denitrification bacterial strain. We evaluated 30 bacterial strains isolated from Portunus trituberculatus aquaculture ponds, among which Pseudomonas aeruginosa SH3 exhibited superior nitrogen removal efficiencies (99 % of NH4+-N, 71 % of NO2--N, and 85 % fof NO3--N at a salinity of 30 ‰) than the other strains. Single-factor experiments demonstrated that SH3 effectively removed either NH4+-N or NO2--N across various C/N ratios (10-20), pH levels (7-9), salinity levels (15-35 ‰), and temperatures (25-35 °C), highlighting its promising nitrogen removal capabilities under conditions suitable for mariculture. Genomic analysis showed that SH3 removes NH4+-N through ammonia assimilation and nitrification and converts NO2--N and NO3--N via denitrification and assimilatory nitrate reduction. Bioaugmentation with SH3 reduced the startup period by 14 d, addressing a common challenge of prolonged startup times in a moving-bed biofilm reactor used for nitrogen removal in marine recirculating aquaculture systems. Meanwhile, bioaugmentation maintained minimal fluctuations in nitrogen levels throughout the operational period, resulting in consistently low concentrations of NO2--N and NH4+-N, both below 1 mg/L. Therefore, strain SH3 exhibits robust nitrogen removal capabilities, demonstrating its practicality and reliability in mariculture wastewater treatment along with providing robust data support for industrial-scale applications.

High organic volumetric loading rates triggered heterotrophic nitrification in wastewater biological nutrient removal systems

This study confirmed the importance of a higher chemical oxygen demand (COD) volumetric loading rate (CODLR) for triggering heterotrophic nitrification in a lab-scale anoxic/microaerobic biological nutrient removal system for sewage treatment by analyzing the ammonia-oxidizing activities, nitrogen mass balance, and ammonia-oxidizing bacterial communities at four CODLR levels: 0.50 (Phase A), 0.75, 1.10, and 1.50 (Phase D) kg COD m-3 d-1. A higher CODLR led to a significant increase in the potential heterotrophic nitrification activity by 0.4, 0.9, 1.1, and 1.6 mg NH4±-N/g mixed liquor suspended solids h-1, respectively, contributing 7 %, 14 %, 17 %, and 21 % of the ammonia oxidization in the anoxic/microaerobic system. Furthermore, nitrogen balance analysis revealed that heterotrophic nitrifying bacteria contributed 4 % and 12 % of the ammonia oxidization in the anoxic/microaerobic system during Phases A and D, respectively, and that a more plentiful organic carbon supply in the microaerobic zone stimulated heterotrophic nitrification during Phase D.

Enrichment of a heterotrophic nitrifying and aerobic denitrifying bacterial consortium: Microbial community succession and nitrogen removal characteristics and mechanisms

This study cultivated a bacterial consortium (S60) from landfill leachate that exhibited effective heterotrophic nitrification and aerobic denitrification (HN-AD) properties. Under aerobic conditions, the removal of NH4+-N reached 100 % when the S60 consortium utilised NH4+-N either as the sole nitrogen source or in combination with NO2--N and NO3--N. Optimal HN-AD performance was achieved with sodium acetate as a carbon source and a pH of 7.0-8.0, dissolved oxygen concentration of 4.0-5.0 mg/L, and a C/N ratio of 10. Furthermore, the presence of functional genes (amoA, hao, napA, nirK, nirS, nosZ), hydroxylamine oxidase, nitrate reductase, and nitrite reductase was confirmed in the S60 consortium. Drawing from these findings, two HN-AD pathways were delineated: NH4+-N → NH2OH → NO2--N → NO3--N → NO2--N → NO → N2O → N2 and NH4+-N → NH2OH → N2O → N2. Metagenomic binning analysis of the S60 consortium uncovered complete pathways for dissimilatory nitrate reduction and denitrification within Halomonas, Zobellella, Stutzerimonas, Marinobacter, and Pannonibacter. These findings offer new insights into the application of HN-AD bacteria and their collaborative nitrogen removal in environments with varying nitrogen sources.

New Insight Into the Mechanism of Nitrite Enhancement on Heterotrophic Nitrification and Aerobic Denitrification Bacterium in Gene Expression

The growth and nitrogen metabolism of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria are affected by nitrite, but the mechanisms underlying this for strain Acinetobacter johnsonii EN-J1 are unclear. In this study, the addition of 10 mg/L nitrite increased the reduction rate of ammonium by 1.0 mg/L/h, and 20 mg/L nitrite increased the reduction rate of nitrate by 3.9 mg/L/h. Compared with the control, the nitrate reductase activity, electron transfer activity, and adenosine triphosphate content of EN-J1 were enhanced by 142.0%, 278.0% and 279.0%, respectively, in the nitrate removal process after the addition of 20 mg/L nitrite. The whole genome was annotated with nitrogen removal genes such as narGHI, narK, nsrR, nirBD, nasA, glnA, gltB, gdhA and amt. Transcriptome analysis showed that nitrite triggered significant upregulation of several key pathways, including nitrogen metabolism, the tricarboxylic acid cycle, and amino acid metabolism for enhancing denitrification. The expression of key denitrification genes (narG, narK, hmp, nirBD, glnA and nasA) was detected by real-time quantitative polymerase chain reaction. These results suggested that nitrite enhances denitrification by increasing the expression of denitrification genes, electron transfer and adenosine triphosphate levels, which is important for elucidating the mechanism of nitrite promotion of biological nitrogen removal efficiency.

Synergetic effects of chlorinated paraffins and microplastics on microbial communities and nitrogen cycling in deep-sea cold seep sediments

Chlorinated paraffins (CPs) and microplastics (MPs) are commonly found in deep-sea cold seep sediments, where nitrogen cycling processes frequently occur. However, little is known about their combined effects on sedimentary microbial communities and nitrogen cycling in these environments. This study aimed to investigate the synergistic impacts of CPs and MPs on microbial communities and nitrogen cycling in deep-sea cold seep sediments through microcosm experiments. Our results demonstrated that the presence of CPs and MPs induced significant alterations in microbial community composition, promoting the growth of Halomonas. Furthermore, CPs and MPs were found to enhance nitrification, denitrification and anammox processes, which was evidenced by the higher abundance of genes associated with nitrification and denitrification, as well as increased activity of denitrification and anammox in the CPs and MPs-treatment groups compared to the control group. Additionally, the enhanced influence of CPs and MPs on denitrification was expected to promote nitrate-dependent and sulfate-dependent anaerobic oxidation of methane, thereby resulting in less methane released into the environment. These findings shed light on the potential consequences of simultaneous exposure to CPs and MPs on biogeochemical nitrogen cycling in deep-sea cold seep sediments.

Aerobic denitrifying bacterial community with low C/N ratio remove nitrate from micro-polluted water: Metagenomics unravels denitrification pathways

The efficient nitrogen removal from micro-polluted source water is an international challenge to be solved urgently. However, the inner denitrification mechanism of native aerobic denitrifying bacterial communities in response to carbon scarcity remains relatively unclear. Here, the bacterial community XT6, screened from an oligotrophic reservoir, exhibited aerobic denitrifying capacity under low-carbon environments. Up to 76.79-81.64 % of total organic carbon (TOC) and 51.48-67.60 % of NO3--N were removed by XT6 within 48 h at C/N ratios of 2.0-3.0. Additionally, the nitrogen balance experiments further manifested that 26.27-38.13 % of NO3--N was lost in gaseous form. As the C/N ratio decreased, XT6 tended to generate more extracellular polymeric substances (EPS), with the tightly bound EPS showing the largest increase. Pseudomonas and Variovorax were quite abundant in XT6, constituting 59.69 % and 28.65 % of the total sequences, respectively. Furthermore, metagenomics analysis evidenced that XT6 removed TOC and nitrate mainly through the tricarboxylic acid cycle and aerobic denitrification. Overall, the abovementioned results provide a deeper understanding of the nitrogen metabolic pathways of indigenous aerobic denitrifying bacterial communities with low C/N ratios and offer useful guidance for controlling nitrogen pollution in oligotrophic ecosystems.

Macrogenomics Reveals Effects on Marine Microbial Communities during Oplegnathus punctatus Enclosure Farming

(1) Background: Laizhou Bay is an important aquaculture area in the north of China. Oplegnathus punctatus is one of the species with high economic benefits. In recent years, the water environment of Laizhou Bay has reached a mild eutrophication level, while microorganisms are an important group between the environment and species. In this study, we evaluated alterations in environmental elements, microbial populations, and antibiotic resistance genes (ARGs) along with their interconnections during Oplegnathus punctatus net culture. (2) Methods: A total of 142 samples from various water layers were gathered for metagenome assembly analysis. Mariculture increases the abundance of microorganisms in this culture area and makes the microbial community structure more complex. The change had more significant effects on sediment than on seawater. (3) Results: Certain populations of cyanobacteria and Candidatus Micrarchaecta in seawater, and Actinobacteria and Thaumarchaeota in sediments showed high abundance in the mariculture area. Antibiotic resistance genes in sediments were more sensitive to various environmental factors, especially oxygen solubility and salinity. (4) Conclusions: These findings highlight the complex and dynamic nature of microorganism-environment-ARG interactions, characterized by regional specificity and providing insights for a more rational use of marine resources.

Exogenous compound bacteria enhance the nutrient removal efficiency of integrated bioremediation systems: Functional genes and microorganisms play key roles

With the continuous development of intensive mariculture, the application of the integrated bioremediation system of aquaculture wastewater (IBSAW) is increasingly promoted. However, the process and nutrients removal performance of the IBSAW need to be further optimized due to its immature technologies. In this study, exogenous compound bacteria (ECB) were added to IBSAW to investigate its pollutants removal efficiency and the relevant mechanisms. High-throughput sequencing and Geochip gene array were used to analyze the correlation between nutrients and bacteria, and the abundance of N and P cycling genes were quantified. Multivariable statistics, dimensionality reduction analysis, and network analysis were applied to explore the mechanisms of IBSAW operation. The results showed that the nutrients decreased significantly after adding ECB, with the brush treatment group significantly outperforming the ceramsite in removing NO3- and PO43-. Ceramsite has an advantage in removing NO2--N. The addition of ECB and different substrates significantly affected the composition of bacterial communities. The contents of nosZ and nirKS related to denitrification in the treatment groups were significantly higher than those in the control group, and the contents in the brush treatment group were significantly higher than that of ceramsite. The biomarkers Psychroserpens and Ruegeria on the biofilm of the brush treatment group were positively correlated with nirKS, while Mycobacterium, Erythrobacter and Paracoccus, Pseudohaliea in the ceramsite group were positively correlated with nirS and nirK, respectively. Therefore, it is speculated that the ECB significantly promoted the increase of denitrification bacteria by affecting the composition of bacterial communities, and the ECB combined with functional genera improved the efficiency of nutrients removal in the system. This study provided a reference for understanding the process and mechanism of nutrients removal, optimizing the wastewater purification technology of the IBSAW and improving the performance of the system.

Gemmobacter denitrificans sp. nov., a denitrifying bacterium, isolated from pond water for Litopenaeus vannamei

A novel Gram-stain-negative strain, designated JM10B15T, was isolated from pond water for Litopenaeus vannamei collected from Jiangmen City, Guangdong province, south PR China. Cells of the strain were aerobic, rod-shaped, and motile by lateral flagella. JM10B15T could grow at 15-40 °C, pH 6.0-9.5, and in 0-3.0 % NaCl, with optimal growth at 25-35 °C, pH 7.5-8.5, and in 0 % NaCl, respectively. Furthermore, this strain grew well on Reasoner's 2A agar but not on nutrient broth agar or Luria-Bertani agar. JM10B15T was a denitrifying bacterium capable of removing nitrites and nitrates, and three key functional genes, nasA, nirS, and nosZ, were identified in its genome. The results of phylogenetic analyses based on the 16S rRNA gene and genome sequences indicated that JM10B15T belonged to the genus Gemmobacter. JM10B15T showed the highest 16S rRNA sequence similarity to Gemmobacter lutimaris YJ-T1-11T (98.8 %), followed by Gemmobacter aquatilis IFAM 1031T (98.6 %) and Gemmobacter serpentinus HB-1T (98.1 %). The average nucleotide identity and digital DNA-DNA hybridization values between JM10B15T and the other type strains of genus Gemmobacter were 78.1-82.1 % and 18.4-22.1 %, respectively. The major fatty acids of strain JM10B15T were summed feature 8 (C18 : 1  ω6c and/or C18 : 1  ω7c) and C18 : 1  ω7c 11-methyl. In addition, the major respiratory quinone of this novel strain was Q-10, and the predominant polar lipids were phosphatidylglycerol, phosphatidylethanolamine, four unidentified phospholipids, three unidentified lipids, and an unidentified aminophospholipid. Results of analyses of the phylogenetic, genomic, physiological, and biochemical characteristics indicated that JM10B15T represents a novel species of the genus Gemmobacter, for which the name Gemmobacter denitrificans sp. nov. is proposed. The type strain is JM10B15T (=GDMCC 1.4148T=KCTC 8140T).

Feast/famine ratio regulates the succession of heterotrophic nitrification-aerobic denitrification and autotrophic ammonia oxidizing bacteria in halophilic aerobic granular sludge treating saline wastewater

Heterotrophic nitrification-aerobic denitrification (HN-AD) shows innovation potential of wastewater treatment process in a single tank. However, how to enrich HN-AD bacteria in activated sludge to enhance their contribution remained unknown. This study explored the impact of the feast/famine (F/F) ratio on the succession of autotrophic ammonia oxidizing bacteria (AOB) and HN-AD bacteria in a halophilic aerobic granular sludge (HAGS) system. As the F/F ratio decreased from 1/9 to 1/15, the total inorganic nitrogen (TIN) removal performance significantly decreased. The proportion of heterotrophic bacteria was dropped from 79.0 % to 33 %. Accordingly, the relative abundance of Paracoccus decreased from 70.8 % to 25.4 %, and the copy number of the napA gene was reduced from 2.2 × 1010 copies/g HAGS to 8.1 × 109 copies/g HAGS. It found the F/F ratio regulated the population succession of autotrophic AOB and HN-AD bacteria, thereby providing a solution to achieve the enrichment of HN-AD bacteria in HAGS.

Metabolic inhibitor-free assessment of the heterotrophic ammonia-oxidizing activity in activated sludge

When the contributions of three ammonia-oxidizing pathways (heterotrophic or autotrophic aerobic ammonia oxidization, and anammox) to wastewater biological nitrogen removal systems was compared by determining their ammonia-oxidizing activities, the key question is how to accurately determine the potential heterotrophic aerobic ammonia-oxidizing (PHAe) activity when the potential autotrophic aerobic ammonia-oxidizing (PAAe) activity (by ammonia-oxidizing bacteria (AOB) or archaea, or complete ammonia oxidization bacteria) also contributes to ammonia oxidization in PHAe activity assay medium. Using a AOB species and three heterotrophic AOB species as inocula, we demonstrated the feasibility of PHAe activity evaluation in the absence of a metabolic inhibitor, i.e., by subtracting the PAAe activity determined in PAAe activity assay medium from a combination of PAAe and PHAe activity determined in PHAe activity assay medium. Binary organic carbon sources (i.e., glucose and acetate) were included in the PHAe activity assay medium to fulfill the carbon requirements of most heterotrophic AOB genera. Higher ammonia-oxidizing activity in AOB biomass than heterotrophic AOB biomass (35.6 vs. 2.6-10.0 mg NH4+-N g-1 MLSS h-1) provides the remarkable advantages of autotrophic aerobic ammonia oxidization in biological nitrogen removal systems. Ammonia removal in three full-scale biological nitrogen removal systems for sewage treatment was predominantly mediated by PAAe activity (1.9-3.3 vs. 0.0-0.3 mg NH4+-N g1 MLSS h-1).

Homogeneous environmental selection mainly determines the denitrifying bacterial community in intensive aquaculture water

Nitrate reduction by napA (encodes periplasmic nitrate reductase) bacteria and nitrous oxide reduction by nosZ (encodes nitrous oxide reductase) bacteria play important roles in nitrogen cycling and removal in intensive aquaculture systems. This study investigated the diversity, dynamics, drivers, and assembly mechanisms of total bacteria as well as napA and nosZ denitrifiers in intensive shrimp aquaculture ponds over a 100-day period. Alpha diversity of the total bacterial community increased significantly over time. In contrast, the alpha diversity of napA and nosZ bacteria remained relatively stable throughout the aquaculture process. The community structure changed markedly across all groups over the culture period. Total nitrogen, phosphate, total phosphorus, and silicate were identified as significant drivers of the denitrifying bacterial communities. Network analysis revealed complex co-occurrence patterns between total, napA, and nosZ bacteria which fluctuated over time. A null model approach showed that, unlike the total community dominated by stochastic factors, napA and nosZ bacteria were primarily governed by deterministic processes. The level of determinism increased with nutrient loading, suggesting the denitrifying community can be manipulated by bioaugmentation. The dominant genus Ruegeria may be a promising candidate for introducing targeted denitrifiers into aquaculture systems to improve nitrogen removal. Overall, this study provides important ecological insights into aerobic and nitrous oxide-reducing denitrifiers in intensive aquaculture, supporting strategies to optimize microbial community structure and function.

A review of microbial nitrogen transformations and microbiome engineering for biological nitrogen removal under salinity stress

The osmotic stress caused by salinity exerts severe inhibition on the process of biological nitrogen removal (BNR), leading to the deterioration of biosystems and the discharge of nitrogen with saline wastewater. Feasible strategies to solve the bottleneck in saline wastewater treatment have attracted great attention, but relevant studies to improve nitrogen transformations and enhance the salt-tolerance of biosystems in terms of microbiome engineering have not been systematically reviewed and discussed. This work attempted to provide a more comprehensive explanation of both BNR and microbiome engineering approaches for saline wastewater treatment. The effect of salinity on conventional BNR pathways, nitrification-denitrification and anammox, was summarized at cellular and metabolic levels, including the nitrogen metabolic pathways, the functional microorganisms, and the inhibition threshold of salinity. Promising nitrogen transformations, such as heterotrophic nitrification-aerobic denitrification, ammonium assimilation and the coupling of conventional pathways, were introduced and compared based on advantages and challenges in detail. Strategies to improve the salt tolerance of biosystems were proposed and evaluated from the perspective of microbiome engineering. Finally, prospects of future investigation and applications on halophilic microbiomes in saline wastewater treatment were discussed.

Efficient heterotrophic nitrification by a novel bacterium Sneathiella aquimaris 216LB-ZA1-12T isolated from aquaculture seawater

High concentration of ammonia poses a common threat to the healthy breeding of marine aquaculture organisms. Since aquaculture water is rich in organic matter, heterotrophic nitrifying bacteria might play a crucial role in ammonia removal. However, their roles in ammonia oxidation remain unknown. Here, we report a novel strain isolated from shrimp aquaculture seawater, identified as Sneathiella aquimaris 216LB-ZA1-12T, capable of heterotrophic nitrification. It is the first characterized heterotrophic nitrifier of the order Sneathiellales in the class Alphaproteobacteria. It exhibits high activity in heterotrophic nitrification, removing nearly 94% of ammonium-N under carbon-constrained conditions in 8 days with no observed nitrite accumulation. The heterotrophic nitrification pathway, inferred based on detection and genomic data was as follows: NH4+→NH2OH→NO→NO2-→NO3-. While this pathway aligns with the classical nitrification pathway, while the significant difference lies in the absence of classical HAO and HOX encoding genes in the genome, which is common in heterotrophic nitrifying bacteria. In summary, this bacterium is not only valuable for studying the nitrifying mechanism, but also holds potential for practical applications in ammonia removal in marine aquaculture systems and saline wastewater.

Exploring influence mechanism of small-molecule carbon source on heterotrophic nitrification-aerobic denitrification process from carbon metabolism, nitrogen metabolism and electron transport process

The heterotrophic nitrification-aerobic denitrification (HNAD) process can remove nitrogen and organic carbon under aerobic conditions. To get the in-depth mechanism of the HAND process, a strain named Acinetobacter johnsonii ZHL01 was isolated, and enzyme activity, electron transport, energy production, and gene expression of the strain were studied with small-molecule carbon sources, including sodium citrate, sodium acetate, sodium fumarate, and sodium succinate. The HNAD pathway of ZHL01 was NH4+→NH2OH → NO, and nitrogen balance analysis shows that ZHL01 could assimilate and denitrify 58.29 ± 1.05 % and 16.58 ± 1.07 % of nitrogen, respectively. The assimilation, the nitrification/denitrification, and the respiration processes were regulated by the concentration of reduced nicotinamide adenine dinucleotide (NADH) produced from the different metabolic pathways of small-molecule carbon sources. The HNAD process occurs to reduce intracellular redox levels related to NADH concentrations. This discovery provides a theoretical basis for the practical application of HAND bacteria.

Removal of ammonia nitrogen from residual ammonium leaching solution by heterotrophic nitrification-aerobic denitrification process

The aftermath of mining weathered crust elution-deposited rare earth ore produces a large amount of residual ammonium leaching solution, which causes ammonia and nitrogen pollution to the mine site. Recently, denitrification by heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria has attracted much attention. However, limited studies exist regarding the denitrification process of HN-AD bacteria. In this study, we combined four strains of HN-AD bacteria, Pseudomonas fulva K3, Pseudomonas mosselii K17, Klebsiella oxytoca A12, and Enterobacter hormaechei A16, obtained from rare earth element leaching sites, to select the best microbial consortium for ammonia nitrogen removal. We designed an ammonia removal process applicable to HN-AD bacteria to directly remove ammonia nitrogen from acidic leaching solutions. The experimental results demonstrated that the most efficient microbial consortium for ammonia nitrogen removal to be K3 + K17 + A16, with a removal efficiency of 89.68% for 8 h. In this process, considering the influencing factors of the ammonia removal process, the larger the influent flow rate and influent ammonia nitrogen concentration, the greater the ammonia nitrogen accumulation and pH decrease in the reactor. In consecutive multi-batch experiments, the ammonia removal process was used to remove ammonia nitrogen, at concentrations of 100-600 mg/L, from the simulated leaching solution at pH 4-7, whereby the effluent ammonia nitrogen concentration was lower than 15 mg/L. The results demonstrate that the ammonia removal process is highly feasible and stable. These findings will provide new ideas for the application of HN-AD bacteria and new methods for the removal of ammonia nitrogen from acidic leaching solutions.

Aerobic denitrifying bacterial-fungal consortium mediating nitrate removal: Dynamics, network patterns and interactions

In recent years, nitrogen removal by mixed microbial cultures has received increasing attention owing to cooperative metabolism. A natural bacterial-fungal consortium was isolated from mariculture, which exhibited an excellent aerobic denitrification capacity. Under aerobic conditions, nitrate removal and denitrification efficiencies were up to 100% and 44.27%, respectively. High-throughput sequencing and network analysis suggested that aerobic denitrification was potentially driven by the co-occurrence of the following bacterial and fungal genera: Vibrio, Fusarium, Gibberella, Meyerozyma, Exophiala and Pseudoalteromonas, with the dominance of Vibrio and Fusarium in bacterial and fungal communities, respectively. In addition, the isolated consortium had a high steady aerobic denitrification performance in our sub-culturing experiments. Our results provide new insights on the dynamics, network patterns and interactions of aerobic denitrifying microbial consortia with a high potential for new biotechnology applications.

Domestication and pilot-scale culture of mixed bacteria HY-1 capable of heterotrophic nitrification-aerobic denitrification

To further investigate the potential of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria for practical applications, the HN-AD mixed bacteria HY-1 were enriched and domesticated in this study. After five generations of domestication, the mixture was able to remove 98% of ammonia nitrogen (400 mg/L) and 81.9% of mixed nitrogen source (nitrate, nitrite). Changes in community structure in the domestication process of mixed microorganisms were studied using 16S rDNA-seq. The results indicated an increase in the abundance of Acinetobacter from 16.9% to 80%. The conditions for the expanded culture of the HY-1 were also optimized. Moreover, A pilot-scale expanded reactor with a capacity of 1000L was constructed, and the HY-1 was successfully expanded from 0.1L to 800L. The community structures of the HY-1 remained stable after the expanded culture, with Acinetobacter as the dominant species. Moreover, the HY-1 demonstrated adaptability to actual high ammonia nitrogen wastewater and showed potential for practical application.

A new strategy to simultaneous removal and recovery of nitrogen from wastewater without N2O emission by heterotrophic nitrogen-assimilating bacterium

Biological assimilation that recovery the nitrogen from wastewater in the form of biomass offers a more environmentally friendly solution for the limitations of the conventional wastewater treatments. This study reported the simultaneous removal and recovery of nitrogen from wastewater without N₂O emission by a heterotrophic nitrogen-assimilating Acinetobacter sp. DN1 strain. Nitrogen balance, biomass qualitative analysis, genome and enzyme studies have been performed to illustrate the mechanism of nitrogen conversion by strain DN1. Results showed that the ammonium removal followed one direct pathway (GOGAT/GDH) and three indirect pathways (NH₄⁺ → NH₂OH → NO → NO₂^- → NH₄⁺ → GOGAT/GDH; NH₄⁺ → NH₂OH → NO → NO₂^- → NO₃^- → NO₂^- → NH₄⁺ → GOGAT/GDH; NH₄⁺ → NH₂OH → NO → NO₃^- → NO₂^- → NH₄⁺ → GOGAT/GDH). Nitrogen balance and biomass qualitative analysis showed that over 70 % of the ammonium in the wastewater was converted into intracellular nitrogen-containing compounds and stored in the cells of strain DN1. Traditional denitrification pathway was not detected and the ammonium was removed through assimilation, which makes it more energy-saving for nitrogen recovery when compared with Haber-Bosch process. This study provides a new direction for simultaneous nitrogen removal and recovery without N₂O emission by the heterotrophic nitrogen-assimilating bacterium.

The Effects of Shaking Duration on the Abundance and the Community of Aerobic Denitrifying Bacteria in Shrimp Pond Water and Sediment Samples

With the rapid development of intensive aquaculture, the considerable release of nitrogenous organic compounds has become a serious threat to aquatic organisms. Currently, isolating autochthonous aerobic denitrifying bacteria (ADB) from aquaculture environments is essential for the biological elimination of nitrogenous pollutants. In this study, the enrichment of ADB from shrimp pond water and sediment samples was conducted under different shaking durations. The absolute abundance of total bacteria, nosZ-type, and the napA-type ADB was measured using qPCR. High-throughput sequencing of 16S rRNA, nosZ, and napA genes was performed to reveal the community structure of bacteria and ADB, respectively. Our data revealed that absolute abundance and the community structure of the total bacteria, nosZ-type and napA-type ADB, were significantly altered under different shaking durations. Specifically, the order Pseudomonadales, possessing both nosZ and napA genes, was significantly enriched in water and sediment samples under both 12/12 and 24/0 shaking/static cycles. However, in water samples, compared to the 24/0 shaking/static cycles, the 12/12 shaking/static cycles could lead to a higher enrichment rate of aerobic denitrification bacteria indicated by the higher absolute abundance of bacteria and the higher accounting percentage of orders Oceanospirillales and Vibrionales. Moreover, although the order Pseudomonadales notably increased under the 12/12 of shake/static cycle compared to the 24/0 shaking/static cycle, considering the relative higher abundance of ADB in 24/0 shaking/static cycle, the enrichment of ADB in sediment may be efficient with the 24/0 shaking/static cycle.

Novel insights into aerobic denitrifying bacterial communities augmented denitrification capacity and mechanisms in lake waters

Scanty attention has been paid to augmenting the denitrification performance of polluted lake water by adding mix-cultured aerobic denitrifying bacterial communities (Mix-CADBCs). In this study, to solve the serious problem of nitrogen pollution in lake water bodies, aerobic denitrifying bacteria were added to lake water to enhance the nitrogen and carbon removal ability. Three Mix-CADBCs were isolated from lake water and they could remove >94 % of total nitrogen and dissolved organic carbon, respectively. The balance of nitrogen analysis shown that >70 % of the initial nitrogen was converted to gaseous nitrogen, and <11 % of the initial nitrogen was converted into microbial biomass. The batch experiments indicated that three Mix-CADBCs could perform denitrification under various conditions. According to the results of nirS-type sequencing, the Hydrogenophaga sp., Prosthecomicrobium sp., and Pseudomonas sp. were dominated genera of three Mix-CADBCs. The analysis of network indicated Pseudomonas I.Bh25.14 and Vogsella LIG4 were correlated with the removal of total nitrogen (TN) and dissolved organic carbon in the Mix-CADBCs. Compared with lake raw water, the addition of three Mix-CADBCs could promote the denitrification capacity (the removal efficiencies of TN > 78.72 %), microbial growth (optical density increased by 0.015-0.138 and the total cell count increased by 2 times), and organic degradation ability (the removal efficiency chemical oxygen demand >38 %) of lake water. In general, the findings of this study demonstrated that Mix-CADBCs could provide a new perspective for biological treatment lake water body.

Biofilm stratification and autotrophic-heterotrophic interactions in a structured bed reactor (SBRIA) for carbon and nitrogen removal

The structured-bed reactor with intermittent aeration (SBRIA) is a promising technology for simultaneous carbon and nitrogen removal from wastewater. An in depth understanding of the microbiological in the reactor is crucial for its optimization. In this research, biofilm samples from the aerobic and anoxic zones of an SBRIA were analyzed through 16S rRNA sequencing to evaluate the bacterial community shift with variations in the airflow and aeration time. The control of the airflow and aeration time were essential to guarantee reactor performances to nitrogen removal close to 80%, as it interfered in nitrifying and denitrifying communities. The aeration time of 1.75 h led to establishment of different nitrogen removal pathways by syntrophic relationships between nitrifier, denitrifier and anammox species. Additionally, the predominance of these different species in the internal and external parts of the biofilm varied according to the airflow.

Effect of dissolved oxygen on high C/N wastewater treatment in moving bed biofilm reactors based on heterotrophic nitrification and aerobic denitrification: Nitrogen removal performance and potential mechanisms

In this study, it was investigated the nitrogen removal (NR) performance and potential mechanism for high C/N wastewater treatment under different dissolved oxygen (DO) concentrations. The results showed that DO concentration significantly affected the removal efficiency of total nitrogen (TN). When the initial DO increased from 0.5 to 1.5 mg/L, TN removal efficiency significantly increased from 65 % to 85 %. However, a further DO increase did not promote TN removal, and the NR was only 80 % with an initial DO concentration of 3.5 mg/L. The effect of DO concentration on NR was influenced by the combined action of functional bacteria and electron flow. Excessive DO concentration did not positively affect NR efficiency but promoted electron utilization and respiratory proliferation. When the DO concentration was 1.5 mg/L, more electrons generated by sodium acetate metabolism were transferred to the aerobic denitrification process, compared to when the DO concentration was 3.5 mg/L.

Mining the landfill soil metagenome for denitrifying methanotrophic taxa and validation of methane oxidation in microcosm

In the present study, the microbial community residing at different depths of the landfill was characterized to assess their roles in serving as a methane sink. Physico-chemical characterization revealed the characteristic signatures of anaerobic degradation of organic matter in the bottom soil (50-60 cm) and, active process of aerobic denitrification in the top soil (0-10 cm). This was also reflected from the higher abundance of bacterial domain in the top soil metagenome represented by dominant phyla Proteobacteria and Actinobacteria which are prime decomposers of organic matter in landfill soils. The multiple fold higher relative abundances of the two most abundant genera; Streptomyces and Intrasporangium in the top soil depicted greater denitrifying taxa in top soil than the bottom soil. Amongst the aerobic methanotrophs, the genera Methylomonas, Methylococcus, Methylocella, and Methylacidiphilum were abundantly found in the top soil metagenome that were essential for oxidizing methane generated in the landfill. On the other hand, the dominance of archaeal domain represented by Methanosarcina and Methanoculleus in the bottom soil highlighted the complete anaerobic digestion of organic components via acetoclasty, carboxydotrophy, hydrogenotrophy, methylotrophy. Functional characterization revealed a higher abundance of methane monooxygenase gene in the top soil and methyl coenzyme M reductase gene in the bottom soil that correlated with the higher relative abundance of aerobic methanotrophs in the top soil while methane generation being the active process in the highly anaerobic bottom soil in the landfill. The activity dependent abundance of endogenous microbial communities in the different zones of the landfill was further validated by microcosm studies in serum bottles which established the ability of the methanotrophic community for methane metabolism in the top soil and their potential to serve as sink for methane. The study provides a better understanding about the methanotrophs in correlation with their endogenous environment, so that these bacteria can be used in resolving the environmental issues related to methane and nitrogen management at landfill site.

Soil Microbial Co-Occurrence Patterns under Controlled-Release Urea and Fulvic Acid Applications

The increasing amount of agricultural applications of controlled-release urea (CRU) and fulvic acids (FA) demands a better understanding of FA’s effects on microbially mediated nitrogen (N) nutrient cycling. Herein, a 0–60 day laboratory experiment and a consecutive pot experiment (2016–2018) were carried out to reveal the effects of using CRU on soil microbial N-cycling processes and soil fertility, with and without the application of FA. Compared to the CRU treatment, the CRU+FA treatment boosted wheat yield by 22.1%. To reveal the mechanism of CRU+FA affecting the soil fertility, soil nutrient supply and microbial community were assessed and contrasted in this research. From 0–60 days, compared with the CRU treatment, leaching NO₃⁻-N content of CRU+FA was dramatically decreased by 12.7–84.2% in the 20 cm depth of soil column. Different fertilizers and the day of fertilization both have an impact on the soil microbiota. The most dominant bacterial phyla Actinobacteria and Proteobacteria were increased with CRU+FA treatment during 0–60 days. Network analysis revealed that microbial co-occurrence grew more intensive during the CRU+FA treatment, and the environmental change enhanced the microbial community. The CRU+FA treatment, in particular, significantly decreased the relative abundance of Sphingomonas, Lysobacter and Nitrospira associated with nitrification reactions, Nocardioides and Gaiella related to denitrification reactions. Meanwhile, the CRU+FA treatment grew the relative abundance of Ensifer, Blastococcus, and Pseudolabrys that function in N fixation, and then could reduce NH₄⁺-N and NO₃⁻-N leaching and improve the soil nutrient supply. In conclusion, the synergistic effects of slow nutrition release of CRU and growth promoting of FA could improve the soil microbial community of N cycle, reduce the loss of nutrients, and increase the wheat yield.

Nitrogen and sulfamethoxazole removal in a partially saturated vertical flow constructed wetland treating synthetic mariculture wastewater

This study investigated the removal of nitrogen and sulfamethoxazole (SMX), and the microbial communities in a partially saturated vertical flow constructed wetland (PS-VFCW) fed with synthetic mariculture wastewater operated at different saturated zone depths (SZDs), i.e. 51, 70, and 60 cm. Removal efficiencies were 99.8%-100.0% for COD, 34.1%-100.0% for NH4+-N, 67.8%-97.3% for total inorganic nitrogen (TIN), and 29.8%-57.2% for SMX. Excellent nitrification performance was achieved at the SZDs of 51 and 60 cm. Denitrification performed well at 70 and 60 cm SZDs. The highest TIN removal efficiency (97.3%) was achieved as the SZD was 60 cm. SMX removal was significantly influenced by SZD and was promoted by higher SZD. The removal of organics, nitrogen, and SMX mainly occurred in the unsaturated zone. Ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, denitrifying bacteria, and SMX-degrading bacteria were detected in the unsaturated and saturated zones, and showed an increasing trend in abundance along the depth.

Nitrogen Removal Performance of Novel Isolated Bacillus sp. Capable of Simultaneous Heterotrophic Nitrification and Aerobic Denitrification

The control of nitrogenous pollutants is a key concern in aquaculture production. Bacillus spp. are commonly used as probiotics in aquaculture, but only a few reports have focused on the simultaneous heterotrophic nitrification and aerobic denitrification (SND) capacity of Bacillus sp. strains. In order to improve nitrogen biodegradation efficiency in the aquaculture industry, the SND capacity of Bacillus sp. strains was evaluated using both individual and mixed nitrogen sources and different sources of organic carbon. Twelve Bacillus sp. isolates were screened from aquaculture pond sediments and shrimp guts for nitrogen biodegradation. Six strains exhibited especially efficient inorganic nitrogen removal capacities in media with individual and mixed nitrogen sources. These strains comprise K8, N2, and N5 (B. subtilis), HYS (B. albus), H4 (B. amyloliquefaciens), and S1 (B. velezensis). The strains grew better when the sole nitrogen source was NH₄⁺-N, but degraded nitrogen in the following order: nitrite nitrogen (NO₂^--N), ammonium nitrogen (NH₄⁺-N), and nitrate nitrogen (NO₃^--N). There was no associated NO₂^--N accumulation, regardless of the nitrogen source. The optimal carbon source for nitrogen removal varied based on different nitrogen sources and associated metabolic pathways. The optimal carbon sources for the removal of NO₃^--N, NO₂^--N, and NH₄⁺-N were sodium citrate, sodium acetate, and sucrose, respectively. The application of H4 in recirculating aquaculture water further demonstrated that NO₂^--N and NH₄⁺-N could be effectively removed. This study thus provides valuable technical support for the bioremediation of aquaculture water.

Heterotrophic ammonium assimilation: An important driving force for aerobic denitrification of Rhodococcus erythropolis strain Y10

Studies on microbial ammonium removal have focused on the heterotrophic nitrification of microorganisms and have rarely studied the role of ammonium assimilation. In this study, Rhodococcus erythropolis strain Y10 with the capacity of aerobic denitrification was screened from the surface flow constructed wetlands that treat high-strength ammonium swine wastewater. Instead of through nitrification, this strain removed ammonium through heterotrophic ammonium assimilation, with the removal rate of 9.69 mg/L/h. The KEGG nitrogen metabolism pathway analysis combined with nitrogen balance calculation manifested that the removal of nitrate and nitrite by R. erythropolis Y10 was achieved through two pathways: 1) assimilation reduction to biomass nitrogen and 2) aerobic denitrification reduction to gaseous nitrogen. Ammonium addition improved the aerobic denitrification rate of nitrate and nitrite. The maximal reduction rates of nitrate and nitrite increased from 7.82 and 7.23 mg/L/h to 9.09 and 8.09 mg/L/h respectively, when 100 mg/L ammonium was separately added to 150 mg/L nitrate and nitrite. Furthermore, the removal efficiency of total nitrogen increased from 69.80% and 77.65% to 89.19% and 91.88%, respectively. Heterotrophic ammonium assimilation promoted the aerobic denitrification efficiency of Rhodococcus erythropolis strain Y10.

The removal of ammonia nitrogen via heterotrophic assimilation by a novel Paracoccus sp. FDN-02 under anoxic condition

A novel strain FDN-02 was isolated from a sequencing batch biofilm reactor. FDN-02 was identified as Paracoccus sp., and the Genbank Sequence_ID was MW652628. Comparing with the removal efficiency of ammonia nitrogen (NH₄⁺-N) by bacterium FDN-02 under different growth conditions, the optimal initial pH, carbon source, and C/N ratio were 7.0, sucrose, and 16, respectively. The maximum removal efficiency and rate of NH₄⁺-N were respectively 96.2% and 10.06 mg-N/L/h within 8 h under anoxic condition when the concentration of NH₄⁺-N was 44.87 mg/L. Specifically, 71.9% of NH₄⁺-N was utilized by strain FDN-02 through heterotrophic assimilation to synthetize organic nitrogen, and approximately 24.1% of NH₄⁺-N was lost in the form of gaseous nitrogen without the emission of nitrous oxide. Bacterium FDN-02 was also found to be a denitrifying organism, and nitrate nitrogen and nitrite nitrogen of lower concentrations were removed by denitrification after the enlargement of biomass. Further investigation showed that the biomass after the removal of NH₄⁺-N by strain FDN-02 had resource utilization potential, and the contents of proteins and amino acids were 635 and 192.97 mg/g, respectively, especially for the usage as an alternative nutrient source for livestock and organic fertilizers. This study provided a promising environmentally friendly biological treatment method for the removal of NH₄⁺-N in the wastewater.

Mutual impacts and interactions of antibiotic resistance genes, microcystin synthetase genes, graphene oxide, and Microcystis aeruginosa in synthetic wastewater

The physiological impacts and interactions of antibiotic resistance gene (ARG) abundance, microcystin synthetase gene expression, graphene oxide (GO), and Microcystis aeruginosa in synthetic wastewater were investigated. The results demonstrated that the absolute abundance of sul1, sul2, tetW, and tetM in synthetic wastewater dramatically increased to 365.2%, 427.1%, 375.2%, and 231.7%, respectively, when the GO concentration was 0.01 mg/L. Even more interesting is that the sum gene copy numbers of mcyA-J also increased to 243.2%. The appearance of GO made the significant correlation exist between ARGs abundance and mcyA-J expression. Furthermore, M. aeruginosa displayed better photosynthetic performance and more MCs production at 0.01 mg/L GO. There were 65 pairs of positive correlations between the intracellular differential metabolites of M. aeruginosa and the abundance of sul1, sul2, tetM, and tetW with various GO concentrations. The GO will impact the metabolites and metabolic pathway in M. aeruginosa. The metabolic changes impacted the ARGs, microcystin synthetase genes, and physiological characters in algal cells. Furthermore, there were complex correlations among sul1, sul2, tetM, tetW, mcyA-J, MCs, photosynthetic performance parameters, and ROS. The different concentration of GO will aggravate the hazards of M. aeruginosa by promoting the expression of mcyA-J, producing more MCs; simultaneously, it may cause the spread of ARGs.

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

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

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

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

Simultaneous ammonium and sulfate biotransformation driven by aeration: Nitrogen/sulfur metabolism and metagenome-based microbial ecology

The present study aimed to clarify the effect of oxygen respiration on biotransformation of alternative electron acceptors (e.g., nitrate and sulfate) underlying the simultaneous removal of ammonium and sulfate in a single aerated sequencing batch reactor. Complete nitrification was achieved in feast condition, while denitrification was carried out in both feast and famine conditions when aeration intensity (AI) was higher than 0.22 L/(L·min). Reactors R1 [0.56 L/(L·min)], R2 [0.22 L/(L·min)], and R3 [0.08 L/(L·min)] achieved 72.39% sulfate removal efficiency in feast condition, but H₂S release occurred in R3. Following exogenous substrate depletion, sulfate concentration increased again and exceeded the influent value in R1, indicating that sulfate transformation was affected by oxygen intrusion. Metagenomic analysis showed that a higher AI promoted sulfate reduction by switching from dissimilatory to assimilatory pathway. Lower AI-acclimated microorganisms (R3) produced H₂S and ammonium, while higher AI-acclimated microorganisms (R1) accumulated nitrite, which confirmed that biotransformation of N and S was strongly regulated by redox imbalance driven by aeration. This implied that respiration control, a microbial self-regulation mechanism, was linked to the dynamic imbalance between electron donors and electron acceptors. Aerobic nitrate (sulfate) reduction, as one of the effects of respiration control, could be used as an alternative strategy to compensate for dynamic imbalance, when supported by efficient endogenous metabolism. Moderate aeration induced microorganisms to change their energy conservation and survival strategy through respiration control and inter-genus protection of respiratory activity among keystone taxa (including Azoarcus in R1, Thauera in R2, and Thiobacillus, Ottowia, and Geoalkalibacter in R3) to form an optimal niche in response to oxygen intrusion and achieve benign biotransformation of C, N, and S without toxic intermediate accumulation. This study clarified the biotransformation mechanism of ammonium and sulfate driven by aeration and provided theoretical guidance for optimizing existing aeration-based techniques.

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

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

Bacterial community compositions and nitrogen metabolism function in a cattle farm wastewater treatment plant revealed by Illumina high-throughput sequencing

Bacteria play an important role in pollutant transformation in activated sludge-based wastewater treatment plants (WWTPs). Exploring the microbial community structure and diversity is essential to improving the performance of wastewater treatment processes. This study employed Illumina MiSeq high-throughput sequencing to investigate the microbial community composition and diversity in a cattle farm wastewater treatment plant (Cf-WWTP). The results showed that the dominant phyla in the whole process were Proteobacteria, Bacteroidetes, and Firmicutes. The principal coordinate analysis (PCoA) indicated that the different stages had a significant impact on the microbial community structure; Bacteroidetes was the dominant phylum in the anearobic stage and Proteobacteria was the dominant phylum in the anoxic-oxic stage. Redundancy analysis (RDA) revealed that total phosphorus (TP) was the most significant factor that regulated the microbial community composition, followed by chemical oxygen demand (COD), total nitrogen (TN), and pH. Proteobacteria, Patescibacteria, and Chloroflexi were simultaneously negatively correlated with TN, COD, and TP. Nitrogen metabolic pathway and transformation mechanism was elucidated by a complete denitrification function predicted with phylogenetic investigation of communities with reconstruction of unobserved states (PICRUSt), as well as detection of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). These results provide new insights into our understanding of microbial community and metabolic functions of Cf-WWTP.

Genomic studies on natural and engineered aquatic denitrifying eco-systems: A research update

Excess nitrogenous compounds in municipal or industrial wastewaters can stimulate growth of denitrifying bacteria, in return, to convert potentially hazardous nitrate to inorganic nitrogen gas. To explore the community structure, distributions and succession of functional strains, and their interactions with other microbial communities, contemporary studies were performed based on detailed genomic analysis. This mini-review updated contemporary genomic studies on denitrifying genes in natural and engineered aquatic systems, with the constructed wetlands being the demonstrative system for the latter. Prospects for the employment of genomic studies on denitrifying systems for process design, optimization and development of novel denitrifying processes were discussed.

A fungus-bacterium co-culture synergistically promoted nitrogen removal by enhancing enzyme activity and electron transfer

The fungus Penicillium citrinum WXP-2 and the bacterium Citrobacter freundii WXP-9 were isolated and found to have poor denitrification performance. Surprisingly, co-culture of the two strains which formed fungus-bacterium pellets (FBPs) promoted the removal efficiency of nitrate (NO₃^--N; 95.78%) and total nitrogen (TN; 81.73%). Nitrogen balance analysis showed that excess degraded NO₃^--N was primarily converted to N₂ (77.53%). Moreover, co-culture increased the dry weight to 0.74 g/L. The diameter of pellets and cell viability also increased by 1.49 and 1.78 times, respectively, indicating that the co-culture exerted a synergistic effect to promote growth. The increase in electron-transmission system activity [99.01 mg iodonitrotetrazolium formazan/(g·L)] and nitrate reductase activity [8.65 mg N/(min·mg protein)] were responsible for denitrification promotion. The FBPs also exhibited the highest degradation rate at 2:1 inoculation ratio and 36 h delayed inoculation of strain WXP-9. Finally, recycling experiments of FBP demonstrated that the high steady TN removal rate could be maintained for five cycles.

Nitrogen removal by heterotrophic nitrifying and aerobic denitrifying bacterium Pseudomonas sp. DM02: Removal performance, mechanism and immobilized application for real aquaculture wastewater treatment

A bacterial strain was isolated and identified as Pseudomonas sp. DM02 from an aquaculture system. Strain DM02 showed efficient heterotrophic nitrification-aerobic denitrification capability. Total ammonia nitrogen (TAN, 10 mg/L) could be completely removed by strain DM02 within 12 h under low nutrient condition. Nitrogen mass balance indicated that 70.8% of the initial TAN was translated into gaseous nitrogen and 28.1% was converted into intracellular nitrogen. Various carbon sources can be used for nitrate removal (>95% within 28 h). The optimal conditions for TAN, nitrate and nitrite removal were pH 7 with carbon/nitrogen (C/N) ratios of 8, 12 and 12, respectively. The napA, nirK, and nosZ functional genes were successful amplified from strain DM02. Both bioaugmentation and immobilized technology of strain DM02 present ability (>88%) for continuous treatment of real aquaculture wastewater. This research indicated a great potential for practical application of Pseudomonas sp. DM02 in aquaculture wastewater treatment.

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

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

One-pot degradation of urine wastewater by combining simultaneous halophilic nitrification and aerobic denitrification in air-exposed biocathode microbial fuel cells (AEB-MFCs)

Urine wastewater is used as fuel in microbial fuel cells to generate power for several applications. However, the knowledge on the removal efficiencies of pollutants and bacterial composition of electrode biofilm is still lacking. In this study, two air-exposed biocathode microbial fuel cells (AEB-MFCs) were constructed and some nitrogen-removing consortium were inoculated to fabricate multifunctional AEBs for urine treatment and energy recovery. Results demonstrated that urine wastewater can be degraded through one-pot degradation without positive aeration. The removal efficiencies of NH₄⁺-N, total nitrogen and chemical oxygen demand reached 86.8% ± 1.5%, 62.7% ± 2.3%, and 52.7% ± 1.6% respectively. Cyclic voltammetry illustrated several catalytic activities related to C/N metabolism occurred in both biofilms and varied with the operation continuing in a single stable cycle. In addition, the community structure analysis revealed that many active microorganisms, including nitrogen-removing bacteria, heterotrophs, and electrochemically active bacteria were enriched in both electrodes, especially many halophilic nitrifiers/denitrifiers occupied in AEBs and directed the system toward the integrated pathways of halophilic nitrogen removal and energy recovery. This study presented a novel method for the energy conversion and effective degradation of urine, which can serve as a promising technology for urine wastewater treatment.

Aerobic denitrifying bacterial communities drive nitrate removal: Performance, metabolic activity, dynamics and interactions of core species

Three novel mix-cultured aerobic denitrifying bacteria (Mix-CADB) consortia named D14, X21, and CL exhibited excellent total organic carbon (TOC) removal and aerobic denitrification capacities. The TOC and nitrate removal efficiencies were higher than 93.00% and 98.00%. The results of Biolog demonstrated that three communities displayed high carbon metabolic activity. nirS gene sequencing and ecological network model revealed that Pseudomonas stutzeri, Paracoccus sp., and Paracoccus denitrificans dominated in the D14, X21, and CL communities. The dynamics and co-existence of core species in communities drove the nutrient removal. Response surface methodology showed the predicted total nitrogen removal efficiency reached 99.43% for D14 community. The three Mix-CADB consortia have great potential for nitrogen-polluted aquatic water treatment because of their strong adaptability and removal performance. These results will provide new understanding of co-existence, interaction and dynamics of Mix-CADB consortia for nitrogen removal in nitrogen-polluted aquatic ecosystems.