Multi-Omics Analysis Reveals the Nitrogen Removal Mechanism Induced by Electron Flow during the Start-up of the Anammox-Centered Process

Impaired denitrification of aerobic granules in response to micro/nanoplastic stress: Insights from interspecies interactions and electron transfer processes

The accumulation of micro/nanoplastics in wastewater significantly hinders denitrification in biological wastewater treatment systems, yet the intrinsic mechanisms are not fully understood. Herein, we combined signal molecule monitoring, electrochemical characterization and multi-omics analysis to investigate how quorum sensing (QS)-mediated microbial interactions influence denitrification in aerobic granular sludge systems. Results showed that after 90-day exposure to micro/nanoplastics, cross-talk between multiple signal molecules significantly declined, thereby disrupting the QS system to opportunely sense changes in the external environment. As a consequence of impaired QS, only 5 species exhibited up-regulation of the genes encoding amino acids and cofactors to sustain cross-feeding, while others, acting as "cheaters", relied on metabolites offered by cross-feeding but without reciprocating. This imbalance resulted in insufficient availability of metabolites, including redox-active metabolites such as riboflavin, and subsequently deteriorated the denitrification electron transfer process. Compared to the control group, the extracellular electron transfer capacity and denitrification electron transfer chain activity decreased to, respectively, 88.08 % and 63.33 % (microplastics-exposure) and 79.64 % and 63.75 % (nanoplastics-exposure), which directly contributed to the decline in nitrogen removal. Overall, this study provided deeper insights into the denitrification in complex microbial communities under the stress of micro/nanoplastics from the perspective of QS-mediated microbial social behavior.

Microbial and functional resilience of a flocculent sludge-based partial denitrification-anammox system against cold temperature

The anammox process offers low-carbon, cost-effective solution for municipal wastewater treatment but faces challenges in non-tropical regions due to temperature sensitivity. This study investigated the nitrogen removal performance, microbial dynamics, and functional gene responses of an integrated partial denitrification-anammox (PDA) system using flocculent sludge under gradual and sudden temperature declines. The PDA process maintained stable nitrogen removal under gradual cooling, with effluent total nitrogen (TN) <1.5 mg/L, supported by stable NO2--supply despite a decreased NO3--to-NO2- conversion rate. Anammox activity progressively declined with temperature dropped, resulting in higher TN-concentration (reactor-concentration at 2 h, <2 mg/L at >20°C, ∼19 mg/L at 15 °C and ∼26 mg/L at 10 °C). Notably, anammox bacteria (AnAOB) remained functional despite reduced abundance. However, sudden cooling caused irreversible damage to AnAOB (cell may rupture), leading to NH4+-accumulation and system failure. Overall, these findings provide important insights into extending anammox technology to non-tropical wastewater treatment, enhancing its sustainable application.

A novel micromagnetic carrier-modified integrated fixed-film activated sludge system for simultaneous efficient removal of tetracycline and mitigation of antibiotic resistance genes proliferation and dissemination

To address the challenge of antibiotic-containing wastewater, a novel micromagnetic carrier-modified integrated fixed-film activated sludge system (MC-IFAS) was developed for treating tetracycline (TC)-containing swine wastewater in this study. The magnetic effects of the MC significantly enhanced TC removal by improving TC biosorption and biodegradation in both the suspended activated sludge and the carrier-attached biofilm in the MC-IFAS. The increased electrostatic attraction and number of binding sites in both the activated sludge and the biofilm enhanced their TC biosorption capacities, particularly in the activated sludge. Additionally, the MC shifted microbial community assembly from stochastic to deterministic factors, amplifying the selection pressure induced by TC on the microbial community, thus enriching organic compound-degrading genera Dokdonella and TM7a; it also stimulated ammonia monooxygenase-mediated and cytochrome P450-mediated TC metabolisms and upregulated functional genes encoding lyases, transferases, hydrolases, and oxidoreductases- all of which enhanced TC biodegradation capacity in the MC-IFAS, particularly in the biofilm. While enhancing TC removal efficiency, the MC mitigated the proliferation and dissemination of antibiotic resistance genes (ARGs) by suppressing the abundances of ARGs hosts, the mobile genetic element intI1, and genes encoding ATP-binding cassette transporters and putative transposases. This study provides novel insights into the large-scale applications of magnetic field-enhanced TC removal strategies.

Mg²⁺ and magnesium ammonium phosphate (MAP)-induced anammox granulation for comparable nitrogen removal: Implementation pathways and microbial mechanisms

Anaerobic ammonium oxidation (anammox) process is a highly effective and economic technology for nitrogen removal from wastewater. However, the slow growth of anammox bacteria and sludge flotation often hinder its field application. Ion adsorption and crystal precipitation can potentially promote the sludge granulation and hence address the above issues. This study investigated two approaches to support anammox granulation through Mg2+ adsorption and magnesium ammonium phosphate (MAP) precipitation. Mg2+ addition improved the specific anammox activity (SAA) by 4.09 to 4.75-fold compared to MAP-mediated ones, which could be explained by the upregulations of nitrogen and inorganic carbon metabolisms. The active extracellular polymeric substances generation at metabolites level may also favor the granulation in Mg2+-mediated anammox. However, sludge loss halted the continuous size increase of sludge. Differently, MAP promoted granulation by physically increasing the granular density, which allowed for a greater retention of sludge within the reactor. However, the co-growth of MAP precipitates with anammox may lead to mass transfer limitations, resulting in down-regulated gene expressions and metabolites in inorganic carbon metabolism, which negatively impacted the SAA. Overall, both strategies achieved comparable nitrogen removal capacities. Nevertheless, the co-growth of MAP and anammox was promising for effectively mitigating sludge flotation. Our study provided strategies and omics-based evidences for anammox granulation and activity variations, benefiting anammox practical applications.

Sulfur-based mixotrophic denitrification: A promising approach for nitrogen removal from low C/N ratio wastewater

Sulfur-based mixotrophic denitrification has significant potential as a promising denitrification technology for treating low ratio of carbon-to‑nitrogen (C/N) wastewater. This paper provided an in-depth and comprehensive overview of the sulfur-based mixotrophic denitrification process and discussed the underlying mechanisms and functional microorganisms. Possible electron transfer pathways involved in the sulfur-based mixotrophic denitrification process are also analyzed in detail. This review focused on the various sulfur-based electron donors used in the sulfur-based mixotrophic denitrification process, including S0, S2-, S2O32-, and pyrite (FeS2), and their performances when combined with various carbon sources (such as methanol, ethanol, glucose, and woodchips) were also explored. The analysis of the contribution proportion between autotrophic and heterotrophic denitrification suggested an appropriate C/N ratio can emphasize the dominance of autotrophs, thus exerting synergistic effects and reducing the consumption of carbon sources. Additionally, three strategies, including developing new composites, new bioreactors, and new sulfur sources, were proposed to improve the performance and stability of the sulfur-based mixotrophic denitrification process. Finally, the applications (such as secondary effluent, groundwater, and agricultural/urban storm water runoff), challenges, and perspectives of the sulfur-based mixotrophic denitrification were highlighted. This review provided an in-depth insight into the coupling mechanism of sulfur-based autotrophic and heterotrophic denitrification and guidance for the future implementation of the sulfur-based mixotrophic denitrification process.

Genomic synergistic efficient carbon fixation and nitrogen removal induced by excessive inorganic carbon in the anammox-centered coupling system

Given the significance of HCO3- for autotrophic anammox bacteria (AnAOB), excessive HCO3- was always provided in anammox-related systems and engineering applications. However, its impact mechanism on anammox process at genome-level remains unknown. This study firstly established an anammox-centered coupling system that entails heterotrophic partial denitrification (PD) and hydrolytic acidification (A-PDHA) fed mainly with inorganic carbon (high HCO3- concentration and low C/N ratio). Metagenomic binning and metatranscriptomics analyses indicated that high HCO3- concentration enhanced expression of natural most efficient phosphoenolpyruvate (PEP) carboxylase within AnAOB, by up to 30.59 folds. This further induced AnAOB to achieve high-speed carbon-fixing reaction through cross-feeding of phosphate and PEP precursors with heterotrophs. Additionally, the enhanced activity of transporters and catalytic enzymes (up to 4949-fold) induced by low C/N ratio enabled heterotrophs to eliminate extracellular accumulated energy precursors mainly derived from carbon fixation products of AnAOB. This maintained high-speed carbon-fixing reaction within AnAOB and supplemented heterotrophs with organics. Moreover, assimilated energy precursors stimulated nitrogen metabolism enzymes, especially NO2- reductase (968.14 times), in heterotrophs. This established an energy-saving PD-A process mediated by interspecies NO shuttle. These variation resulted in efficient nitrogen removal (>95 %) and reduced external organic carbon demand (67 %) in A-PDHA system. This study unveils the great potential of an anammox-centered autotrophic-heterotrophic coupling system for achieving cost-effective nitrogen removal and enhancing carbon fixation under excessive HCO3- doses.

Achieving nitrite shunt using in-situ free ammonia enriched by natural zeolite: Pilot-scale mainstream anammox with flexible nitritation strategy

The mainstream partial nitritation/anammox (PN/A) process represents a significant innovation in decarbonizing municipal wastewater treatment. However, its implementation is considerably hampered by the challenge of stable nitrite supply. In this study, a pilot-scale PN/A system receiving real sewage (20 m3) was operated at room temperature for nearly one year. Remarkable PN performance with relatively high nitrite accumulation ratio of 75.04 ± 10.05 % was obtained via in-situ free ammonia (FA) strategy. The ammonium concentration enriched in the zeolite increased significantly by 548.8 times compared to that in the aqueous phase by ion exchange. This substantial increase robustly inhibited nitrite-oxidizing bacteria (NOB), resulting in high relative abundance ratio of ammonia-oxidizing bacteria (AOB) to NOB of 37.93 ± 12.61 in the zeolite biofilm, compared to 10.22 ± 1.67 in suspended floc sludge. The significant differences in FA concentrations between zeolite biofilm and suspended floc sludge resulted in distinct spatial distribution disparities of AOB and NOB, which were central to achieving stable nitrite accumulation without complex multiple selective pressures. Consequently, compliant effluent with total nitrogen of 10.91 ± 4.23 mg N/L was achieved at 10.4-31.1 °C without external carbon source addition. The biocarriers in the anammox process played a key role in enhancing functional genes and electron flow, supporting anammox-dominated nitrogen removal. This study presents a flexible and adaptable strategy for mainstream nitrite shunting, highlighting its potential for large-scale implementation of mainstream anammox treatment.

Significantly Enhanced Nitrate and Phosphorus Removal by Pyrite/Sawdust Composite-Driven Mixotrophic Denitrification with Boosted Electron Transfer: Comprehensive Evaluation of Water-Gas-Biofilm Phases during a Long-Term Study

Further reducing total nitrogen (TN) and total phosphorus (TP) in the secondary effluent needs to be realized effectively and in an eco-friendly manner. Herein, four pyrite/sawdust composite-based biofilters were established to treat simulated secondary effluent for 304 days. The results demonstrated that effluent TN and TP concentrations from biofilters under the optimal hydraulic retention time (HRT) of 3.5 h were stable at <2.0 and 0.1 mg/L, respectively, and no significant differences were observed between inoculated sludge sources. The pyrite/sawdust composite-based biofilters had low N2O, CH4, and CO2 emissions, and the effluent's DOM was mainly composed of five fluorescence components. Moreover, mixotrophic denitrifiers (Thiothrix) and sulfate-reducing bacteria (Desulfosporosinus) contributing to microbial nitrogen and sulfur cycles were enriched in the biofilm. Co-occurrence network analysis deciphered that Chlorobaculum and Desulfobacterales were key genera, which formed an obvious sulfur cycle process that strengthened the denitrification capacity. The higher abundances of genes encoding extracellular electron transport (EET) chains/mediators revealed that pyrite not only functioned as an electron conduit to stimulate direct interspecies electron transfer by flagella but also facilitated EET-associated enzymes for denitrification. This study comprehensively evaluates the water-gas-biofilm phases of pyrite/sawdust composite-based biofilters during a long-term study, providing an in-depth understanding of boosted electron transfer in pyrite-based mixotrophic denitrification systems.

Interspecies cooperation-driven photogenerated electron transfer processes and efficient multi-pathway nitrogen removal in the g-C3N4-anammox consortia biohybrid system

Photocatalytic materials-microbial biohybrid systems pave the way for solar-driven wastewater nitrogen removal. In this study, interspecies cooperation in photogenerated electron transfer and efficient nitrogen removal mechanism in the g-C3N4-anammox consortia biohybrid system were first deciphered. The results indicated that the essential extracellular electron carriers (cytochrome c and flavin) for anammox genomes were provided by associated bacteria (BACT3 and CHLO2). This cooperation, regulated by the ArcAB system and electron transfer flavoprotein, made anammox bacteria the primary photogenerated electron sink. Furthermore, an efficient photogenerated electron harness was used to construct a reductive glycine pathway (rGlyP) in anammox bacteria inventively, which coexisted with the Wood-Ljungdahl pathway (WLP), constituting a dual-pathway carbon fixation model, rGlyP-WLP. Carbon fixation products efficiently contributed to the tricarboxylic acid cycle, while inhibiting electron diversion in anabolism. Photogenerated electrons were targeted channeled into nitrogen metabolism-available electron carriers, enhancing anammox and dissimilatory nitrate reduction to ammonium (DNRA) processes. Moreover, ammonia assimilation by the glycine cleavage system in rGlyP established an alternative ammonia removal route. Ultimately, multi-pathway nitrogen removal involving anammox, DNRA, and rGlyP achieved 100 % ammonia removal and 94.25 % total nitrogen removal efficiency. This study has expanded understanding of anammox metabolic diversity, enhancing its potential application in carbon-neutral wastewater treatment.

Selective genes expression and metabolites transformation drive a robust nitrite accumulation during nitrate reduction under alternating feast-famine condition

Nitrite production via denitrification has been regarded as a key approach for survival of anaerobic ammonium oxidation (anammox) bacteria. Despite the important carbon substrate, little is known about the role of differential genes expression and extracellular metabolite regulation among diverse microbial communities. In this study, a novel alternating feast-famine strategy was proposed and demonstrated to efficiently accumulate nitrite in a low-nitrogen loading rate (NLR) (0.2∼0.8 kg N/m3/d) denitrification system. Highly selective expression of denitrifying genes was revealed as key regulators. Interestingly, in absence of carbon source (ACS) condition, the expression of narG and narI/V genes responsible for reduction of nitrate to nitrite jumped to 2.5 and 5.1 times higher than that in presence of carbon source (PCS) condition with carbon to nitrate ratio of 3.0. This fortunately facilitated a rapid nitrite accumulation once acetate was added, despite a significantly down-regulated narG and narI/narV and up-regulated nirS/nirK. This strategy selected Thauera as the most dominant denitrifier (50.2 %) with the highest contribution to narG and narI/narV genes, responsible for the high nitrite accumulation. Additionally, extracellular xylose, pyruvate, and glucose jointly promoted carbon-central metabolic pathway of key denitrifiers in ACS stage, playing an important role in the process of self-growth and selective enrichment of functional bacteria. The relatively rapid establishment and robust performance obtained in this study shows an engineering-feasible and economically-favorable solution for the regulation of partial denitrification in practical application.

Simultaneous ammonia and nitrate removal by novel integrated partial denitrification-anaerobic ammonium oxidation-bioelectrochemical system

The current study explored the performance of an integrated partial denitrification-anaerobic ammonium oxidation (anammox)-bioelectrochemical system on simultaneous removal of ammonia nitrogen and nitrate nitrogen. Different operational conditions were selected to optimize critical parameters of the process for improving nitrogen removal. The results indicated that more than 90 % of total inorganic nitrogen removal efficiency was achieved under the optimal conditions: ammonia nitrogen/nitrate nitrogen ratio of 1:2, external resistance of 200 Ω and inoculation volume ratio of anammox bacteria/denitrifying at 2:1. Improved nitrogen removal under the optimal conditions were confirmed by microbial community changes (Candidatus Brocadia and Thiobacillus) and enhanced of nitrogen metabolism-related genes (hao, hzsA/C and hdh). Increases of Limnobacter indicated an enhanced electron transfer efficiency. Overall, high-efficiency and stable nitrogen removal efficiency without nitrite nitrogen accumulation could be achieved by the integrated system under the optimal conditions, providing novel insights for simultaneous treatment of domestic wastewater and groundwater.

Temporal differentiation in the adaptation of functional bacteria to low-temperature stress in partial denitrification and anammox system

Despite reliable nitrite supply through partial denitrification, the adaptation of denitrifying bacteria to low temperatures remains elusive in partial denitrification and anammox (PDA) systems. Here, temporal differentiations of the structure, activity, and relevant cold-adaptation mechanism of functional bacteria were investigated in a lab-scale PDA bioreactor at decreased temperature. Although distinct denitrifying bacteria dominated after low-temperature stress, both short- and long-term stresses exerted differential selectivity towards the species with close phylogenetic distance. Species Azonexus sp.149 showed high superiority over Azonexus sp.384 under short-term stress, and long-term stress improved the adaptation of Aquabacterium sp.93 instead of Aquabacterium sp.184. The elevated transcription of nitrite reductase genes suggested that several denitrifying bacteria (e.g., Azonexus sp.149) could compete with anammox bacteria for nitrite. Species Rivicola pingtungensis and Azonexus sp.149 could adapt through various adaptation pathways, such as the two-component system, cold shock protein (CSP), membrane alternation, and electron transport chain. By contrast, species Zoogloea sp.273 and Aquabacterium sp.93 mainly depended on the CSP and oxidative stress response. This study largely deepens our understanding of the performance deterioration in PDA systems during cold shock and provides several references for efficient adaptation to seasonal temperature fluctuation.

Lumen air pressure regulated multifunctional microbiotas in membrane-aerated biofilm reactors for simultaneous nitrogen removal and antibiotic elimination from aquaculture wastewater

In this study, two membrane-aerated biofilm reactors (MABRs) were constructed: one solely utilizing biofilm and another hybrid MABR (HMABR) incorporating both suspended-sludge and biofilm to treat low C/N aquaculture wastewater under varying lumen air pressure (LAP). Both HMABR and MABR demonstrated superior nitrogen removal than conventional aeration reactors. Reducing LAP from 10 kPa to 2 kPa could enhance denitrification processes without severely compromising nitrification, resulting in an increase in total inorganic nitrogen (TIN) removal from 50.2±3.1 % to 71.6±1.0 %. The HMABR exhibited better denitrification efficacy than MABR, underscoring its potential for advanced nitrogen removal applications. A decline in LAP led to decreased extracellular polymeric substance (EPS) production, which could potentially augment reactor performance by minimizing mass transfer resistance while maintaining microbial matrix stability and function. Gene-centric metagenomics analysis revealed decreasing LAP impacted nitrogen metabolic potentials and electron flow pathways. The enrichment of napAB at higher LAP and the presence of complete ammonia oxidation (Comammox) Nitrospira at lower LAP indicated aerobic denitrification and Comammox processes in nitrogen removal. Multifunctional microbial communities developed under LAP regulation, diversifying the mechanisms for simultaneous nitrification-denitrification. Increased denitrifying gene pool (narGHI, nirK, norB) and enzymatic activity at a low LAP can amplify denitrification by promoting denitrifying genes and electron flow towards denitrifying enzymes. Sulfamethoxazole (SMX) was simultaneously removed with efficiency up to 80.2 ± 3.7 %, mainly via biodegradation, while antibiotic resistome and mobilome were propagated. Collectively, these findings could improve our understanding of nitrogen and antibiotic removal mechanisms under LAP regulation, offering valuable insights for the effective design and operation of MABR systems in aquaculture wastewater treatment.

NH2-MIL-101(Fe)-mediated photo-Fenton reaction enhanced simultaneous removal of nitrogen and refractory organics in anammox process through interfacial electron transfer

In this study, H2O2 (0.1 ‰) and NH2-MIL-101(Fe)-driven (150 mg/L) photo-Fenton-coupled anammox were proposed to simultaneously improve the removal efficiency of nitrogen and humic acid. Long-term experiments showed that the total nitrogen removal efficiency was increased by the photo-Fenton reaction to 91.9 ± 1.5 % by altering the bioavailability of refractory organics. Correspondingly, the total organic carbon removal efficiency was significantly increased. Microbial community analyses indicated that Candidatus_Brocadia maintained high activity during photo-Fenton reaction and was the most abundant genus in the reactor. Dissimilatory nitrate reduction to ammonium process and denitrification process were enhanced, resulting in reduced NO3--N production. The establishment of electron transfer between microorganisms and NH2-MIL-101 (Fe) improved the charge separation efficiency of the quantum dots and increased the intracellular adenosine triphosphate content of anammox bacteria. These results indicated that photo-Fenton-anammox process promoted the removal of nitrogen and refractory organics in one reactor which had good economic value and application prospects.

Insight into microbial adaptability in continuous flow anaerobic ammonium oxidation process for low-strength sewage treatment

Organic matter concentration is a critical factor influencing the adaptability of anaerobic ammonium oxidation (anammox) bacteria to low-strength sewage treatment. To address this challenge and achieve stable anammox activity, a micro-aeration partial nitrification-anammox process was developed for continuous-flow municipal sewage treatment. Under limited ammonium conditions, the effective utilization of organics in denitrification promoted the stable accumulation of nitrite and enhanced anammox activity. This, in turn, led to enhanced nitrogen removal efficiency, reaching approximately 87.7%. During the start-up phase, the protein content of extracellular polymeric substances (EPS) increased. This enhanced EPS intensified the inhibitory effect of denitrifying bacteria (DNB) on nitrite-oxidizing bacteria through competition for nitrite, thereby facilitating the proliferation of anammox bacteria (AnAOB). Additionally, several types of DNB capable of utilizing slowly biodegradable organics contributed to the adaptability of AnAOB. These findings provide valuable insights for ensuring efficient anammox performance and robust nitrogen removal in the treatment of low-strength sewage.

Multipathway of Nitrogen Metabolism Revealed by Genome-Centered Metatranscriptomics from Pyrite-Guided Mixotrophic Partial Denitrification/Anammox Installations

Further reducing the organic requirements is essential for the sustainable development of partial denitrification/anammox technology. Here, an innovative mixotrophic partial denitrification/anammox (MPD/A) installation fed with pyrite and few organics was realized, and the average nitrogen and phosphorus removal rates were as high as 96.24 ± 0.11% and 79.23 ± 2.06%, respectively, with a C/N ratio of 0.5. To understand the nature by which MPD/A achieves efficient nitrogen removal and organic conservation, the electron transfer-dependent nitrogen escape and energy metabolism were first elucidated using multiomics analysis. Apart from heterotrophic denitrification and anammox, the results revealed some unexpected metabolic couplings of MPD/A systems, in particular, putative nitrate-dependent organic and pyrite oxidation among nominally heterotrophic Denitratisoma (PRO3) strains, which accelerated nitrate gasification with a low-carbon supply. Additionally, Candidatus Brocadia (AMX) employed extracellular solid-state electron acceptors as terminal electron sinks for high-rate ammonium removal. AMX transported ammonium electrons to extracellular γFeO(OH) (generated from pyrite oxidation) through the transient storage of menaquinoline pools, cytoplasmic migration via multiheme cytochrome(s), and OmpA protein/nanowires-mediated electron hopping on cell surfaces. Further investigation observed that extracellular electron flux resulted in the transfer of more energy from the increased oxidation of the electron donor to the ATP, supporting nitrite-independent ammonium removal.

Novel insights into carbon nanomaterials enhancing anammox for nitrogen removal: Effects and mechanisms

Carbon nanomaterials (CNMs) possess the properties including large specific surface area, high porosity, and stable chemical structures, presenting significant application advantages in wastewater treatment. Indeed, CNMs are considered to be added to anammox systems to strengthen anammox function, especially to resolve the challenge of anammox technology, i.e., the slow growth rate of anammox bacteria, as well as its high environmental sensitivity. This paper systematically reviews the promotion effects and mechanisms of CNMs on the nitrogen removal performance of anammox system. Among the zero-, one-, and two-dimensional CNMs, two-dimensional CNMs have best promoting effect on the nitrogen removal performance of anammox system due to its excellent conductivity and abundant functional groups. Then, the promotion effects of CNMs on anammox process are summarized from the perspective of anammox activity and bacteria abundance. Furthermore, CNMs not only enhance the anammox process, but also stimulate the coupling of denitrification pathways with anammox, as well as the improvement of system operational stability (alleviating the inhibitions of low temperature and pH fluctuation), thus contributing to the promoted nitrogen removal performance. Essentially, CNMs are capable of facilitating microbial immobilization and electron transfer, which favor to improve the efficiency and stability of anammox process. Finally, this review highlights the gap in knowledge and future work, aiming to provide a deeper understanding of how CNMs can strengthen the anammox system and provide a novel perspective for the engineering of the anammox process.

A pilot study of situ sludge fermentation-driven multiple biological nitrogen removal pathways (SFBNR): Revealing microbial synergy mechanism based on co-occurrence network analysis

The sludge fermentation-driven biological nitrogen removal (SFBNR) has garnered increasing attention due to its efficient carbon resource utilization from waste activated sludge (WAS). This study successfully extended the application of this technique to a 38 m3 reactor, facilitating a daily ultra-low carbon to nitrogen ratio (<1) wastewater treatment capacity of 16 tons and a WAS capacity of 500 L. After 185-days operation, the system demonstrated commendable performance with a denitrification efficiency (DNE) of 93.22 % and a sludge reduction efficiency (SRE) of 72.07 %. To better understand the potential mechanisms, various functional bacteria interactions were revealed by co-occurrence network analysis. The results unveiled module hubs (e.g., Anaerolineaceae, Denitratisoma, and Candidatus Brocadia) and connectors (e.g., Tuaera and Candidatus Alysiosphaera) in the network exhibited synergistic relationships facilitated by carbon metabolism and nitrogen cycling. Furthermore, the interaction between biofilm sludge (BS) and suspended sludge (SS) contributed to the in-situ enrichment of anaerobic ammonium oxidizing bacteria (AnAOB), whose abundance in BS reached 1.8 % (200-times higher than in SS) after six months, and the suspend-biofilm interface served as a hotspot for anammox activity.

Anammox Coupled with Photocatalyst for Enhanced Nitrogen Removal and the Activated Aerobic Respiration of Anammox Bacteria Based on cbb3-Type Cytochrome c Oxidase

This study introduced photogenerated electrons into the anammox system by coupling them to a g-C3N4 nanoparticle photocatalyst. A high nitrogen removal efficiency (94.25%) was achieved, exceeding the biochemical limit of 89% imposed by anammox stoichiometry. Photogenerated electrons boosted anammox metabolic activity by empowering key enzymes (NIR, HZS, and WLP-related proteins) and triggered rapid algal enrichment by enhancing the algal Calvin cycle, thus developing multiple anammox-algae synergistic nitrogen removal processes. Remarkably, the homologous expression of cbb3-type cytochrome c oxidase (CcO) in anammox bacteria was discovered and reported in this study for the first time. This conferred aerobic respiration capability to anammox bacteria and rendered them the principal oxygen consumer under 7.9-19.8 mg/L dissolved oxygen, originating from algal photosynthesis. Additionally, photogenerated electrons selectively targeted the cb1 complex and cbb3-type CcO as activation sites while mobilizing the RegA/B regulatory system to activate the expression of cbb3-type CcO. Furthermore, cbb3-type CcO blocked oxidative stress in anammox by depleting intracellular oxygen, a substrate for reactive oxygen species synthesis. This optimized the environmental sensitivity of anammox bacteria and maintained their high metabolic activity. This study expands our understanding of the physiological aptitudes of anammox bacteria and provides valuable insights into applying solar energy for enhanced wastewater treatment.

Enhancing plant growth in biofertilizer-amended soil through nitrogen-transforming microbial communities

Biofertilizers have immense potential for enhancing agricultural productivity. However, there is still a need for clarification regarding the specific mechanisms through which these biofertilizers improve soil properties and stimulate plant growth. In this research, a bacterial agent was utilized to enhance plant growth and investigate the microbial modulation mechanism of soil nutrient turnover using metagenomic technology. The results demonstrated a significant increase in soil fast-acting nitrogen (by 46.7%) and fast-acting phosphorus (by 88.6%) upon application of the bacterial agent. This finding suggests that stimulated soil microbes contribute to enhanced nutrient transformation, ultimately leading to improved plant growth. Furthermore, the application of the bacterial agent had a notable impact on the accumulation of key genes involved in nitrogen cycling. Notably, it enhanced nitrification genes (amo, hao, and nar), while denitrification genes (nir and nor) showed a slight decrease. This indicates that ammonium oxidation may be the primary pathway for increasing fast-acting nitrogen in soils. Additionally, the bacterial agent influenced the composition and functional structure of the soil microbial community. Moreover, the metagenome-assembled genomes (MAGs) obtained from the soil microbial communities exhibited complementary metabolic processes, suggesting mutual nutrient exchange. These MAGs contained widely distributed and highly abundant genes encoding plant growth promotion (PGP) traits. These findings emphasize how soil microbial communities can enhance vegetation growth by increasing nutrient availability and regulating plant hormone production. This effect can be further enhanced by introducing inoculated microbial agents. In conclusion, this study provides novel insights into the mechanisms underlying the beneficial effects of biofertilizers on soil properties and plant growth. The significant increase in nutrient availability, modulation of key genes involved in nitrogen cycling, and the presence of MAGs encoding PGP traits highlight the potential of biofertilizers to improve agricultural practices. These findings have important implications for enhancing agricultural sustainability and productivity, with positive societal and environmental impacts.

A critical review of improving mainstream anammox systems: Based on macroscopic process regulation and microscopic enhancement mechanisms

Full-scale anaerobic ammonium oxidation (anammox) engineering applications are vastly limited by the sensitivity of anammox bacteria to the complex mainstream ambience factors. Therefore, it is of great necessity to comprehensively summarize and overcome performance-related challenges in mainstream anammox process at the macro/micro level, including the macroscopic process variable regulation and microscopic biological metabolic enhancement. This article systematically reviewed the recent important advances in the enrichment and retention of anammox bacteria and main factors affecting metabolic regulation under mainstream conditions, and proposed key strategies for the related performance optimization. The characteristics and behavior mechanism of anammox consortia in response to mainstream environment were then discussed in details, and we revealed that the synergistic nitrogen metabolism of multi-functional bacterial genera based on anammox microbiome was conducive to mainstream anammox nitrogen removal processes. Finally, the critical outcomes of anammox extracellular electron transfer (EET) at the micro level were well presented, carbon-based conductive materials or exogenous electron shuttles can stimulate and mediate anammox EET in mainstream environments to optimize system performance from a micro perspective. Overall, this review advances the extensive implementation of mainstream anammox practice in future as well as shedding new light on the related EET and microbial mechanisms.

Light-Driven Electron Uptake from Nonfermentative Organic Matter to Expedite Nitrogen Dissimilation by Chemolithotrophic Anammox Consortia

Nonphotosynthetic microorganisms are typically unable to directly utilize light energy, but light might change the metabolic pathway of these bacteria indirectly by forming intermediates such as reactive oxygen species (ROS). This work investigated the role of light on nitrogen conversion by anaerobic ammonium oxidation (anammox) consortia. The results showed that high intensity light (>20000 lx) caused ca. 50% inhibition of anammox activity, and total ROS reached 167% at 60,000 lx. Surprisingly, 200 lx light was found to induce unexpected promotion of the nitrogen conversion rate, and ultraviolet light (<420 nm) was identified as the main contributor. Metagenomic and metatranscriptomic analyses revealed that the gene encoding cytochrome c peroxidase was highly expressed only under 200 lx light. 15N isotope tracing, gene abundance quantification, and external H2O2 addition experiments showed that photoinduced trace H2O2 triggered cytochrome c peroxidase expression to take up electrons from extracellular nonfermentative organics to synthesize NADH and ATP, thereby expediting nitrogen dissimulation of anammox consortia. External supplying reduced humic acid into a low-intensity light exposure system would result in a maximal 1.7-fold increase in the nitrogen conversion rate. These interesting findings may provide insight into the niche differentiation and widespread nature of anammox bacteria in natural ecotopes.

Nitrite-resistance mechanisms on wastewater treatment in denitrifying phosphorus removal process revealed by machine learning, co-occurrence, and metagenomics analysis

Nitrite is a key intermediate in nitrogen metabolism that determines microbial transformations of N and P, greenhouse gas (N₂O) emissions, and system nutrient removal efficiency. However, nitrite also exerts toxic effects on microorganisms. A lack of understanding of high nitrite-resistance mechanisms at community- and genome-scale resolutions hinders the optimization for robustness of wastewater treatment systems. Here, we established nitrite-dependent denitrifying and phosphorus removal (DPR) systems under a gradient concentration of nitrite (0, 5, 10, 15, 20, and 25 mg N/L), relying on 16S rRNA gene amplicon and metagenomics to explore high nitrite-resistance mechanism. The results demonstrated that specific taxa were adopted to change the metabolic relationship of the community through phenotypic evolution to resist toxic nitrite contributing to the enhancement of denitrification and inhibition of nitrification and phosphorus removal. The key specific species, Thauera enhanced denitrification, whereas Candidatus Nitrotoga decreased in abundance to maintain partial nitrification. The extinction of Candidatus Nitrotoga induced a simpler restructuring-community, forcing high nitrite-stimulating microbiome to establish a more focused denitrification rather than nitrification or P metabolism in response to nitrite toxicity. Our work provides insights for understanding microbiome adaptation to toxic nitrite and giving theoretical support for operation strategy of nitrite-based wastewater treatment technology.

Mitigating mechanism of nZVI-C on the inhibition of anammox consortia under long-term tetracycline hydrochloride stress: Extracellular polymeric substance properties and microbial community evolution

In this study, activated carbon-loaded nano-zero-valent iron (nZVI-C) composites were added to anaerobic ammonium oxidation bacteria (AnAOB) to overcome the inhibition of tetracycline hydrochloride (TCH). Results showed that 500 mg L^-1 nZVI-C effectively mitigated the long-term inhibition of 1.5 mg L^-1 TCH on AnAOB and significantly improved the total nitrogen removal efficiency (TNRE) (from 65.27% to 86.99%). Spectroscopic analysis revealed that nZVI-C increased the content of N-H and CO groups in EPS, which contributed to the adsorption of TCH. The accumulation of humic acid-like substances in EPS was also conducive to strengthening the extracellular defense level. In addition, TCH-degrading bacteria (Clostridium and Mycobacterium) were enriched in situ, and the abundance of Ca. Brocadia was significantly increased (from 10.69% to 18.59%). Furthermore, nZVI-C increased the abundance of genes encoding tetracycline inactivation (tetX), promoted mineralization of TCH by 90%, weakening the inhibition of TCH on microbial nitrogen metabolism. nZVI-C accelerated the electron consumption of anammox bacteria by upregulating the abundance of electron generation genes (nxrA, hdh) and providing electrons directly.