A review of quorum sensing improving partial nitritation-anammox process: Functions, mechanisms and prospects

Synthetic microbial community maintains the functional stability of aerobic denitrification under environmental disturbances: Insight into the mechanism of interspecific division of labor

Understanding how synthetic microbial community (SMC) respond to environmental disturbances is the key to realizing SMC engineering applications. Here, dibutyl phthalate (DBP) and levofloxacin (LOFX) were used as environmental disturbances to study their effects on the aerobic denitrification functional stability of SMC composed of Pseudomonas aeruginosa N2 (PA), Acinetobacter baumannii N1(AC) and Aeromonas hydrophila (AH). The results showed that aerobic denitrification efficiency could be maintained at about 93 % under DBP or LOFX disturbance, and interspecific communication was mainly carried out through N-butyryl-L-homoserine lactone (C4-HSL) and N-(3-oxododecanoyl)-L-homoserine lactone (3OC12-HSL), correspondingly. DBP and LOFX induced the acceleration of tricarboxylic acid (TCA) cycle, which facilitated the energy flux and extracellular polymeric substances (EPS) production, thereby allowing SMC to adapt to disturbances. Under DBP disturbance, DBP stimulated phenazine-1-carboxylic acid production to accelerate electron transfer from the quinone pool to complex III, resulting in an increase in electron transfer activity. Up-regulation of complex I, complex III and heme synthesis genes under LOFX disturbance led to enhanced denitrification enzymes expression and electron transfer efficiency. SMC re-regulated different metabolic pathways to build metabolic networks to maintain normal metabolic activity under different disturbances. Overall, SMC maintained functional stability through the labor division in modulation of interspecific communication, formation of defensive barriers, promotion of energy flux, directional transfer of electron flux, and reconstruction of metabolic networks. DBP stimulated AH and PA to occupy functional dominance, while LOFX induced AC and PA to play a major role. The understanding of the stability mechanism under different environmental disturbances provides valuable guidance for stability maintenance and engineering applications of SMC.

Roles and opportunities of quorum sensing in natural and engineered anaerobic digestion systems

Anaerobic digestion (AD) is a biological process in which anaerobic microorganisms convert organic matter into methane-rich gas, contributing to the cycling of carbon and other nutrients. Quorum sensing (QS), a microbial communication mechanism, plays a critical role in regulating population-level behaviors within AD systems. This review systematically examines the roles and applications of QS in AD, emphasizing its importance in enhancing process efficiency. The review begins by exploring the pathways and characteristics of QS in key functional microorganisms involved in AD. We analyze the response mechanisms of QS to key environmental variables and their effects on the structure and function of microbe communities and extracellular polymeric substances secretion. Potential applications of QS in engineered AD systems are discussed, with a focus on promoting system startup, improving operational efficiency, and enhancing resistance and stability. The use of exogenous signaling molecules and quorum quenching reagents to optimize AD performance is also evaluated. Additionally, the ecological significance of QS in natural environments, such as seafloor sediments and wetlands, is explored, emphasizing its role in regulating AD-related microorganisms within complex microbial communities. Finally, the review identifies current knowledge gaps and outlines future research directions in AD, including QS database development, QS-engineered bacteria excavation, and advanced analytical methods assistants. This comprehensive review aims to bridge existing gaps in QS-related knowledge in AD and provide fresh perspectives for studying microbial communication and collaboration through QS.

Enhanced nitrogen removal from low C/N municipal wastewater in a step-feed integrated fixed-film activated sludge system: Synergizing anammox and partial denitrification with sludge fermentation liquid supplementation

The scarcity of rapidly biodegradable organics, which serve as essential electron donors for the partial denitrification (PD) process, significantly hinders the combined application of PD coupled with anammox (PDA) in municipal wastewater treatment plants. This study innovatively applied, for the first time, a step-feed strategy combined with the use of sludge fermentation liquid (SFL) as an external carbon source in an integrated fixed-film activated sludge (IFAS) system, successfully driving full nitrification and PDA to achieve advanced nitrogen removal from low C/N real municipal wastewater. Moreover, the associated nitrogen removal mechanism of this system was systematically analyzed. By employing second-step SFL feed as a supplementary carbon source, the nitrogen removal efficiency reached 92.26 ± 2.77 % and the effluent total inorganic nitrogen was 6.43 ± 2.23 mg/L, with anammox contributing approximately 70 % to total inorganic nitrogen removal. 16S rRNA gene sequencing and fluorescence in situ hybridization analysis unveiled the extensive cooperation and synergistic interactions among anammox bacteria, denitrifying bacteria, and nitrifying bacteria, with Candidatus Brocadia being highly enriched in biofilms with a relative abundance of 2.21 %. Metagenomic sequencing confirmed that the relative abundance of the narGHI gene was greater than that of the nirS gene, providing stable nitrite accumulation conditions for the anammox process. Overall, this study proposes an innovative synergistic treatment scheme that utilizes a step-feed full nitrification-PDA process driven by SFL to achieve advanced nitrogen removal in municipal wastewater treatment plants. This approach is characterized by low energy consumption, low operational costs and a high nitrogen removal efficiency.

Characterization of nitrogen transformation and microbial interactions of floc sludge and biofilms in single-stage gas-lift partial nitritation/anammox reactor with hollow cylindrical carriers

Floc sludge and biofilms are common biomass types used in the partial nitritation/anaerobic ammonium oxidation (anammox) (PN/A) process to treat ammonium wastewater; however, the interactions between these coexisting biomass types for stable nitrogen removal have not yet been well characterized. This study employed single-stage PN/A reactor with hollow cylindrical polypropylene carriers to investigate nitrogen transformation and microbial interactions in floc sludge and biofilms during long-term stable nitrogen removal. Floc sludge primarily drove ammonia oxidation, demonstrating 11.6-fold higher efficiency than biofilms, whereas biofilms mediated the anammox reaction, 3.6-fold more efficiently than floc sludge. Floc sludge and biofilms contributed 18.4 % and 65.9 % to nitrogen removal, respectively. Ammonia-oxidizing bacteria dominated the floc sludge (Nitrosomonas) and cooperated with anammox biofilms (Candidatus Kuenenia) to enable stable nitrogen removal. These findings deepen understanding of nitrogen transformation and microbial interactions across different biomass types and facilitate the optimization of PN/A efficiency through flexible control strategies.

AHL-mediated quorum sensing drives microbial community succession and metabolic pathway in algal-bacterial biofilm system

Microalgae, ammonia-oxidizing bacteria (AOB), and anaerobic ammonium-oxidizing bacteria (AnAOB) have been proven to form an integrated algal-bacterial biofilm system with over 93 % of total nitrogen removal. Compared to conventional nitrification-denitrification process, this system operated without additional organic carbon or aeration. In order to understand the interaction mechanisms between bacteria and algae, this study investigated microbial community succession, the changes in metabolic pathways and the potential role of acyl-homoserine lactone (AHL)-mediated quorum sensing (QS) during the formation of the algae/partial nitrification/anammox biofilm system. Within this algal-bacterial symbiotic biofilm, the dominant genera identified were Candidatus_Brocadia (AnAOB), Nitrosomonas (AOB), and Geitlerinema (microalgae), with relative abundances of 13.86 %, 6.37 %, and 2.88 %, respectively. Compared with the first two stages, the abundance of genes related to nitrogen metabolism pathways (anaerobic ammonium oxidation, denitrification, and ammonia assimilation) increased, indicating enhanced nitrogen transformation capacity in the algal-bacterial symbiotic stage. Co-occurrence network analysis also revealed enhanced microbial interactions, with increased negative correlations (from 36.07 % to 39.38 %), high average standard betweenness centrality (from 0.193 to 0.304), and reduced community vulnerability (from 0.037 to 0.028), contributing to biofilm stability and resilience. The variations in AHLs provided direct evidence for more frequent interspecies communication, facilitating the ecological reconfiguration in the biofilm. Overall, the close synergistic relationship between microalgae and bacteria supports stable biofilm development and high nitrogen removal efficiency.

Deciphering the dead zone on anammox system in biofilters

In an anammox biofilm reactor, long-term operation inevitably leads to the repeated formation of localized dead zones. Once these dead zones (DZs) occur, the anammox reactor's nitrogen removal efficiency is severely reduced. However, the mechanisms and intrinsic reasons for the transformation of DZs remain unexplored. In this study, the pilot-scale biofilters were classified into biologically active zones (BZs), transition zones (TZs), and DZs. The results indicated that microbial communities undergo accelerated succession from the TZ. Biofilms respond to environmental stress from the DZs by altering the levels of signaling molecules, triggering a series of cascading reactions. These reactions alter the abundance of genes involved in nitrogen removal, promote substance transformation, and speed up the succession of microbial communities. This study demonstrates the objectives and self-healing mechanisms of the anammox biofilm process in the presence of dead zones, which could support the long-term application of anammox technology.

Enhancement of the reactivation process of long-term starved anammox granular sludge with gravel balls: Microbial succession and metabolic impact

Anaerobic ammonium oxidation (Anammox) process is an economical and energy-efficient method of wastewater nitrogen removal. However, they are highly susceptible to starvation stress caused by sudden environmental changes. Rapid reactivation of starved anammox sludge is a crucial method to address seed sludge shortages and expand practical applications. This study investigated the impact of gravel balls on the reactivation of long-term starved anammox granular sludge (628 days). The results showed that gravel balls enhanced the recovery of nitrogen removal performance in starved anammox sludge, with nitrogen removal efficiency being 19.88% higher than the control group at the end of the recovery phase. The gravel balls also increased extracellular polymeric substance (EPS) secretion, contributing to the stability of the anammox system. Furthermore, the gravel balls promoted the proliferation of anammox bacteria, with the relative abundance of anammox bacteria reaching 38.25% on the 80th day. The analyses of microbial functions indicated that gravel balls facilitated cross-feeding and co-metabolism among microbes, while enhancing quorum sensing associated with anammox bacteria, forming a multifunctional community network centered on anammox bacteria. This indicates that gravel balls can effectively accelerate the reactivation process of long-term starved anammox sludge, aiding the reutilization of long-term starved anammox sludge.

Life history strategies determine response to SRT driven crash in anammox bioreactors

Anaerobic ammonium oxidation (anammox) is a biological process often applied in wastewater treatment plants for nitrogen removal from highly concentrated side-stream effluents from anaerobic digesters. However, they are vulnerable to process instability prompted by operational shocks and microbial community imbalances, resulting in lengthy recovery times. These issues are further compounded by a lack of understanding of how sustained press disturbances influence the microbial ecology of the system. Here we investigate the response and recovery of an anammox membrane bioreactor to a solids retention time (SRT)-induced reactor crash using 16S rRNA gene and shotgun metagenomic sequencing. We observed a strong selection of bacterial groups based on reproduction strategies, with the Orders Rhodospirillales and Sphingobacteriales increasing from 1.0 % and 11.9 % prior to the crash to 31.9 % and 18.1 % during the crash respectively. The Orders Brocadiales and Anaerolineales decreased from 17.3 % and 28.3 % to 7.3 % and 1.4 % over the same time period, respectively. Metagenomic and metatranscriptomic analyses revealed differential crash responses in metabolically distinct groups of bacteria, with increased expression of genes for extracellular carbohydrate active enzymes, peptidases and membrane transporters. Following the crash, the reactor recovered to its prior state of nitrogen removal performance and pathway analysis demonstrated increased expression of genes related to exopolysaccharide biosynthesis and quorum sensing during the reactor recovery period. This study highlights the effects of reactor perturbations on microbial community dynamics in anammox bioreactors and provides insight into potential recovery mechanisms from severe disturbance.

Microbial coadaptation drives the dynamic stability of microecology in mainstream and sidestream anammox systems under exposure of progesterone

Microbial cooperation determines the efficacy of wastewater biological treatment, and the adaptability of microorganisms to environmental stresses varies. Recently, extensive use of hormones results in their inevitable discharge into aquatic environment. Therefore, mainstream and sidestream anammox reactors were constructed in this study to evaluate their removal performance of progesterone and nitrogen simultaneously, the adaptability of anammox consortia to progesterone stress and the corresponding regulation mechanism. Both anammox processes had the resilience to progesterone stress, with the average nitrogen removal efficiency exceeding 90 %. At the same time, progesterone removal efficiency also exceeded 70 %. In contrast, microbial community in the mainstream reactors was more susceptible to progesterone interference. The adaptation of anammox consortia mainly depended on microbial cooperation and molecular regulation. Initially, bacteria secreted more extracellular polymeric substances to detain progesterone. Biodegradation also contributed to mitigating the side effect of progesterone, which was demonstrated by the proliferation of potential degrading bacteria such as Bacillus salacetis, Bacillus wiedmannii and Rhodococcus erythropolis. In addition, the enhancement of microbial interaction intensity drove their cooperation to enhance adaptability and maintain stable performance. Combined with metagenomic and metatranscriptomic analyses, such microbial adaptability was enhanced through molecular regulations, including the energy redistribution for amino acid synthesis and alteration of key metabolic pathways. Related functional gene expressions and microbial interactions were, in turn, regulated by quorum sensing. This work verifies the feasibility of anammox process in hormone-containing wastewater treatment and provides a holistic understanding of molecular mechanism of microbial interaction and coadaptation to stress.

Deciphering the formation of granules by n-DAMO and Anammox microorganisms

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) process is a promising wastewater treatment technology, but the slow microbial growth rate greatly hinders its practical application. Although high-level nitrogen removal and excellent biomass accumulation have been achieved in n-DAMO granule process, the formation mechanism of n-DAMO granules remains unresolved. To elucidate the role of functional microbes in granulation, this study attempted to cultivate granules dominated by n-DAMO microorganisms and granules coupling n-DAMO with anaerobic ammonium oxidation (Anammox). After long-term operation, dense granules were developed in the two systems where both n-DAMO archaea and n-DAMO bacteria were enriched, whereas granulation did not occur in the other system dominated by n-DAMO bacteria. Extracellular polymeric substances (EPS) measurement indicated the critical role of EPS production in the granulation of n-DAMO process. Metagenomic and metatranscriptomic analyses revealed that n-DAMO archaea and Anammox bacteria were active in EPS biosynthesis, while n-DAMO bacteria were inactive. Consequently, more EPS were produced in the systems containing n-DAMO archaea and Anammox bacteria, leading to the successful development of n-DAMO granules. Furthermore, EPS biosynthesis in n-DAMO systems is potentially regulated by acyl-homoserine lactones and c-di-GMP. These findings not only provide new insights into the mechanism of granule formation in n-DAMO systems, but also hint at potential strategies for management of the granule-based n-DAMO process.

Metabolomic reveals the responses of sludge properties and microbial communities to high nitrite stress in denitrifying phosphorus removal systems

Nitrite, as an electron acceptor, plays a good role in denitrifying phosphorus removal (DPR); however, high nitrite concentration has adverse affects on sludge performance. We investigated the precise mechanisms of responses of sludge to high nitrite stress, including surface characteristics, intracellular and extracellular components, microbial and metabolic responses. When the nitrite stress reached 90 mg/L, the sludge settling performance was improved, but the activated sludge was aging. FTIR and XPS analysis revealed a significant increase in the hydrophobicity of the sludge, resulting in improve settling performance. However, the intracellular carbon sources synthesis was inhibited. In addition, the components in the tightly bound extracellular polymeric substances (TB-EPS) of sludge were significantly reduced and indicated the disturb of metabolism. Notably, Exiguobacterium emerged as a new genus when face high nitrite stress that could maintaining survival in hostile environments. Moreover, metabolomic analysis demonstrated strong biological response to nitrite stress further supported above results that include the inhibited of carbohydrate and amino acid metabolism. More importantly, some lipids (PS, PA, LysoPA, LysoPC and LysoPE) were significantly upregulated that related enhanced membrane lipid remodeling. The comprehensive analyses provide novel insights into the high nitrite stress responses mechanisms in activated sludge systems.

Recovery strategies and mechanisms of anammox reaction following inhibition by environmental factors: A review

Anaerobic ammonium oxidation (anammox) is a promising biological method for treating nitrogen-rich, low-carbon wastewater. However, the application of anammox technology in actual engineering is easily limited by environmental factors. Considerable progress has been investigated in recent years in anammox restoration strategies, significantly addressing the challenge of poor reaction performance following inhibition. This review systematically outlines the strategies employed to recover anammox performance following inhibition by conventional environmental factors and emerging pollutants. Additionally, comprehensive summaries of strategies aimed at promoting anammox activity and enhancing nitrogen removal performance provide valuable insights into the current research landscape in this field. The review contributes to a comprehensive understanding of restoration strategies of anammox-based technologies.

Communication leads to bacterial heterogeneous adaptation to changing conditions in partial nitrification reactor

Recently, it is reported that bacterial communication coordinates the whole consortia to jointly resist the adverse environments. Here, we found the bacterial communication inevitably distinguished bacterial adaptation among different species in partial nitrification reactor under decreasing temperatures. We operated a partial nitrification reactor under temperature gradient from 30 °C to 5 °C and found the promotion of bacterial communication on adaptation of ammonia-oxidizing bacteria (AOB) was greater than that of nitrite-oxidizing bacteria (NOB). Signal pathways with single-component sensing protein in AOB can regulate more genes involved in bacterial adaptation than that with two-component sensing protein in NOB. The negative effects of bacterial communication, which were seriously ignored, have been highlighted, and Clp regulator downstream diffusible signal factor (DSF) based signal pathways worked as transcription activators and inhibitors of adaptation genes in AOB and NOB respectively. Bacterial communication can induce differential adaptation through influencing bacterial interactions. AOB inclined to cooperate with DSF synthesis bacteria as temperature declined, however, cooperation between NOB and DSF synthesis bacteria inclined to get weakening. According to the regulatory effects of signal pathways, bacterial survival strategies for self-protection were revealed. This study hints a potential way to govern niche differentiation in the microbiota by bacterial communication, contributing to forming an efficient artificial ecosystem.

Anammox process in anaerobic baffled biofilm reactors with columnar packings: Characteristics of flow field and microbial community

The enrichment of anammox bacteria is a key issue in the application of anammox processes. A new type of reactor － anaerobic baffle biofilm reactor (ABBR) developed from anaerobic baffle reactor (ABR) was filled with columnar packings and established for effective enrichment of anammox bacteria. The flow field analysis showed that, compared with ABR, ABBR narrowed the dead zone so as to improve the substrate transferring performances. Two ABBRs with different types of columnar packings (Packings 1 and Packings 2) were constructed to culture anammox biofilms. Packings 1 consisted of the single-form honeycomb carriers while Packings 2 was modular composite packings consisting of non-woven fabric and honeycomb carriers. The effects of different types of columnar packings on microbial community and nitrogen removal were studied. The ABBR filled with Packings 2 had a higher retention rate of biomass than the ABBR filled with Packings 1, making the anammox start-up period be shortened by 21.28%. The enrichment of anammox bacteria were achieved and the dominant anammox bacteria were Candidatus Brocadia in both R1 and R2. However, there were four genera of anammox bacteria in R2 and one genus of anammox bacteria in R1, and the cell density of anammox bacteria in R2 was 95% higher than that in R1. R2 has the advantage of maintaining excellent and stable nitrogen removal performance at high nitrogen loading rate. The results revealed that the packings composed of two types of carriers may have a better enrichment effect on anammox bacteria. This study is of great significance for the rapid enrichment of anammox bacteria and the technical promotion of anammox process.

Facilitating nitrification and biofilm formation of Vibrio sp. by N-acyl-homoserine lactones in high salinity environment

The N-acyl-homoserine lactones (AHLs)-mediated quorum-sensing (QS) system played a crucial role in regulating biological nitrogen removal and biofilm formation. However, the regulatory role of AHLs on nitrogen removal bacteria in high salinity environment has remained unclear. This study evaluated the roles and release patterns of AHLs in Vibrio sp. LV-Q1 under high salinity condition. Results showed that Vibrio sp. primarily secretes five AHLs, and the AHLs activity is strongly correlated with the bacterial density. Exogenous C10-HSL and 3OC10-HSL were found to significantly enhance ammonium removal, while making a minor contribution to the growth rate. Both the C10-HSL and 3OC10-HSL promoted the biofilm formation of Vibrio sp. with an enhancement of 1.64 and 1.78 times, respectively. The scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM) observations confirmed the biofilm-enhancing effect of AHLs. Further analysis revealed that AHLs significantly improved bacterial self-aggregation and motility, as well as the level of extracellular polymeric substances (EPS). These findings provide significant guidance on construction of nitrification system at high salinity.

Effect of biochar on the SPNA system at ambient temperatures

Biochar has been extensively studied in wastewater treatment systems. However, the role of biochar in the single-stage partial nitritation anammox (SPNA) system remains not fully understood. This study explored the impact of biochar on the SPNA at ambient temperatures (20 °C and 15 °C). The nitrogen removal rate of the system raised from 0.43 to 0.50 g N/(L·d) as the biochar addition was raised from 2 to 4 g/L. Metagenomic analysis revealed that gene abundances of amino sugar metabolism and nucleotide sugar metabolism, amino acid metabolism, and quorum sensing were decreased after the addition of biochar. However, the gene abundance of enzymes synthesizing NADH and trehalose increased, indicating that biochar could stimulate electron transfer reactions in microbial metabolism and assist microorganisms in maintaining a steady state at lower temperatures. The findings of this study provide valuable insights into the mechanism behind the improved nitrogen removal facilitated by biochar in the single-stage partial nitritation anammox system.

Regulation of extracellular polymers based on quorum sensing in wastewater biological treatment from mechanisms to applications: A critical review

Extracellular polymeric substances (EPS) regulated by quorum sensing (QS) could directly mediate adhesion between microorganisms and form tight microbial aggregates. Besides, EPS have redox properties, which can facilitate electron transfer for promoting electroactive bacteria. Currently, the applications research on improving wastewater biological treatment performance based on QS regulated EPS have been widely reported, but reviews on the level of QS regulated EPS to enhance EPS function in microbial systems are still lacking. This work proposes the potential mechanisms of EPS synthesis by QS regulation from the viewpoint of material metabolism and energy metabolism, and summarizes the effects of QS on EPS synthesis. By synthesizing the role of QS in EPS regulation, we further point out the applications of QS-regulated EPS in wastewater biological treatment, which involve a series of aspects such as strengthening microbial colonization, mitigating membrane biofouling, improving the shock resistance of microbial metabolic systems, and strengthening the electron transfer capacity of microbial metabolic systems. According to this comprehensive review, future research on QS-regulated EPS should focus on the exploration of the micro-mechanisms, and economic regulation strategies for QS-regulated EPS should be developed, while the stability of QS-regulated EPS in long-term production experimental research should be further demonstrated.

Quantitative effect of adding percentages of anammox granules on the start-up process and microbial community analysis

The anaerobic ammonium oxidation (anammox) process is challenging due to its long start-up duration and high demand for mature anammox seed sludge. However, adding a small amount of anammox sludge to the inoculum can be a reasonable solution. This study investigated the effect of adding percentage of anammox granules (0, 1, 2, 4, and 8%) in the seed sludge on the anammox start-up process. The anammox process was achieved in all five reactors after 55, 6, 5, 3 and 0 days. Increasing the adding percentage effectively shortened the duration of lag phase and cell lysis, but had little effect on the final nitrogen removal performance, except for 4% adding percentage. Families of Brocadiaceae, Burkholderiaceae, Ignavibacteriaceae, SJA-28, and Rhodocyclaceae were dominant, with a core microbiota of eight operational taxonomic unites (OTUs), and Candidatus Brocadia fulgida became the dominant anammox species. Seven synergistic members with anammox bacteria were identified by correlation network analysis. Major potential functional groups involved in C and N cycle were also observed by FAPROTAX. Together with the qPCR and sequencing results, it was suggested that more than 2% of adding percentages would result in a short lag phase, rapid growth rate in elevation stage, high final performances, and anammox bacteria abundance comparable to that in the anammox seed sludge. This crucial finding indicated the feasibility of economical and rapid start-up of the anammox process with a minimum amount of anammox seed sludge.

Metagenomics reveals the microbial community and functional metabolism variation in the partial nitritation-anammox process: From collapse to recovery

Mainstream partial nitritation-anammox (PNA) process easily suffers from performance instability and even reactor collapse in application. Thus, it is of great significance to unveil the characteristic of performance recovery, understand the intrinsic mechanism and then propose operational strategy. In this study, we combined long-term reactor operation, batch tests, and metagenomics to reveal the succession of microbial community and functional metabolism variation from system collapse to recovery. Proper aeration control (0.10-0.25 mg O2/L) was critical for performance recovery. It was also found that Candidatus Brocadia became the dominant flora and its abundance increased from 3.5% to 11.0%. Significant enhancements in carbon metabolism and phospholipid biosynthesis were observed during system recovery, and the genes abundance related to signal transduction was dramatically increased. The up-regulation of sdh and suc genes showed the processes of succinate dehydrogenation and succinyl-CoA synthesis might stimulate the production of amino acids and the synthesis of proteins, thereby possibly improving the activity and abundance of AnAOB, which was conducive to the performance recovery. Moreover, the increase in abundance of hzs and hdh genes suggested the enhancement of the anammox process. Changes in the abundance of key genes involved in nitrogen metabolism indicated that nitrogen removal pathway was more diverse after system recovery. The achievement of performance recovery was driven by anammox, nitrification and denitrification coupled with dissimilatory nitrate reduction to ammonium. These results provide deeper insights into the recovery mechanism of PNA system and also provide a potential regulation strategy for the stable operation of the mainstream PNA process.

Potential role of AgNPs within wastewater in deteriorating sludge floc structure and settleability during activated sludge process: Filamentous bacteria and quorum sensing

Excellent sludge floc structure and settleability are essential to maintain the process stability and excellent effluent quality during the activated sludge process. The underlying effect of silver nanoparticles (AgNPs) within wastewater on sludge floc structure and settleability is still unclear. The potential role of AgNPs in promoting filamentous bacterial proliferation and deteriorating sludge floc structure and settleability based on quorum sensing (QS) were investigated in this study. The results indicated that N-acyl homoserine lactose (AHL) concentration sharply increased from 23.56 to 108.41 ng/g VSS in the sequencing batch reactor with 1 mg/L AgNPs. AgNPs strengthened communication between filamentous bacteria, which triggered the filamentous bacterial QS system involving the synthetic gene hdtS and sensing genes traR and lasR. Filamentous bacterial proliferation was promoted by the triggered QS via positively regulating its cell cycle progression including chromosomal replication and divisome formation. In addition, extracellular protein production was obviously increased from 43.56 to 97.91 mg/g VSS through QS by regulating arginine and tyrosine secretion during filamentous bacterial proliferation under 1 mg/L AgNPs condition, which led to an increase in the negative charge and hydrophily at the cell surface. AgNPs resulted in an obvious increase in the surface energy barrier (WT) between bacteria. The change in the physicochemical properties of extracellular polymeric substance (EPS) induced by QS among filamentous bacteria obviously inhibited bacterial aggregation between filamentous bacteria and floc-forming bacteria under AgNPs condition, thus resulting in serious deterioration of the sludge floc structure and settleability. This study provided new insights into the microcosmic mechanism for the effect of AgNPs on sludge floc structure and settleability.

Roles and regulation of quorum sensing in anaerobic granular sludge: Research status, challenges, and perspectives

Anaerobic granular sludge (AnGS) has a complex and important internal microbial communication system due to its unique microbial layered structure. As a concentration-dependent communication system between bacterial cells through signal molecules, QS (quorum sensing) is widespread in AnGS and exhibits great potential to regulate microbial behaviors. Therefore, the universal functions of QS in AnGS have been systematically summarized in this paper, including the influence on the metabolic activity, physicochemical properties, and microbial community of AnGS. Subsequently, the common QS-based AnGS regulation approaches are reviewed and analyzed comprehensively. The regulation mechanism of QS in AnGS is analyzed from two systems of single bacterium and mixed bacteria. This review can provide a comprehensive understanding of QS functions in AnGS systems, and promote the practical application of QS-based strategies in optimization of AnGS treatment process.

Enhancing anammox bacteria enrichment in integrated fixed-film activated sludge partial nitritation/anammox process via floc retention control

A promising technology for partial nitritation/anammox (PN/A) processes to treat ammonium wastewater is integrated fixed-film activated sludge (IFAS). For practical applications, achieving efficient enrichment of anammox bacteria (AnAOB) remains a challenge. In this study, membranes were temporarily used to separate solid and liquid components to induce changes in the mixed liquor suspended solids of the flocs. With membrane separation, AnAOB proliferated rapidly with a seven-fold increase in the maximum specific growth rate (μ) (from 0.009 to 0.072 d-1) and a three-fold increase in the nitrogen removal rate (from 0.91 to 3.20 kg N/(m3·d)). Moreover, microbial community analysis showed significant changes in bacterial species richness and diversity with and without membrane separation. Overall, the regulation of flocs significantly influenced the microbial community structure of both flocs and biofilms leading to improved nitrogen removal efficiency in the IFAS-PN/A system.

Deciphering the role of granular activated carbon (GAC) in anammox: Effects on microbial succession and communication

Anaerobic ammonium oxidation (anammox) offered an energy-efficient option for nitrogen removal from wastewater. Granular activated carbon (GAC) addition has been reported that improved biomass immobilization, but the role of GAC in anammox reactors has not been sufficiently revealed. In this study, it was observed that GAC addition in an upflow anaerobic sludge blanket (UASB) reactor led to the significantly shortened anammox enrichment time (shortened by 45 days) than the reactor without GAC addition. The nitrogen removal rate was 0.83 kg N/m³/day versus 0.76 kg N/m³/day in GAC and non-GAC reactors, respectively after 255 days' operation. Acyl-homoserine lactone (AHL) quorum sensing signal molecule C8-HSL had comparable concentrations in both anammox reactors, whereas the signal molecule C12-HSL was more pervasive in the reactor containing GAC than the reactor without GAC. Microbial analysis revealed distinct anammox development in both reactors, with Candidatus Brocadia predominant in the reactor that did not contain GAC, and Candidatus Kuenenia predominant in the reactor that contained GAC. Denitrification bacteria likely supported anammox metabolism in both reactors. The analyses of microbial functions suggested that AHL-dependent quorum sensing was enhanced with the addition of GAC, and that GAC possibly augmented the extracellular electron transfer (EET)-dependent anammox reaction.

Succession of microbial communities reveals the inevitability of anammox core in the development of anammox processes

The lack of anammox seeds is regarded as the bottleneck of anammox-based processes. Although the interactions in anammox consortia have attracted increasing attention, little is known about the influence of inoculated sludge populations on the growth of anammox bacteria. In this study, four sludge of distinct communities mixed with anammox sludge (the relative abundance of Ca. Kuenenia was 1.96 %) were used as the seeds, respectively for the start-up of anammox processes. Notably, all these mixed microbial communities tend to form a similar microbial community, defined as the anammox core, containing anammox-bacteria (22.9 ± 5.9 %), ammonia-oxidizing-bacteria (0.8 ± 0.7 %), nitrite-oxidizing-bacteria (0.2 ± 0.2 %), Chloroflexi-bacteria (0.7 ± 0.4 %), and heterotrophic-denitrification-bacteria (0.3 ± 0.2 %). It also elucidated that the communities of Nitrosomonas-dominated sludge were the closest to the anammox core, and achieved the highest nitrogen-removal rate of 0.73 kg-N m^-3 d^-1. This study sheds light on the solution to the shortage of anammox seeds in the full-scale wastewater treatment application.

Long-term adaptation of two anammox granules with different ratios of Candidatus Brocadia and Candidatus Jettenia under increasing salinity and their application to treat saline wastewater

Nitrogen removal in saline wastewater is a challenge of the anaerobic ammonium oxidation (anammox) process, which is dominated by freshwater anammox bacteria (FAB). Candidatus Brocadia and Candidatus Jettenia, the most widely used FABs, have been separately applied and evaluated for their ability to treat saline wastewater. To understand the effect of salinity on nitrogen removal capability when they present together in an anammox granule, we compared two anammox granules: GRN1 was evenly dominated by Ca. Brocadia (42 %) and Ca. Jettenia (43 %), while GRN2 was dominated with mostly Ca. Brocadia (90 %) and a small amount of Ca. Jettenia (1 %). Each granule was inoculated into a continuous column reactor to treat artificial wastewater containing 150 mg NH₄⁺-N/L and 150 mg NO₂^--N/L under increasing saline conditions for 250 days. GRN1 showed superior and more stable nitrogen removal than GRN2 under saline conditions of up to 15 g NaCl/L. Under high-saline conditions, both the granules' sizes decreased (larger GRN1 than GRN2 in initial). The mass percent of Na salt increased (more in GRN2) and mineral contents decreased more in GRN1. High-throughput sequencing for microbial community analysis showed that Planctomycetes in GRN1 (85 %) and GRN2 (92 %) decreased to 14 % and 12 %, respectively. The ratio of Ca. Brocadia and Ca. Jettenia in GRN1 changed to 37 % and 63 %, respectively, whereas the ratio in GRN2 (99 % and 1 %, respectively) did not change. Both salt-adapted granules were applied to the two-stage partial nitritation and anammox (PN/A) process to treat high strength ammonium (400 mg/L) wastewater under high saline condition (15 g NaCl/L). The PN/A process containing GRN1 showed more stable nitrogen removal performance during approximately 100 days of operation. These results suggest that the anammox granules evenly dominated by two FABs, Ca. Brocadia and Ca. Jettenia, would be advantageous to treat high-strength NH₄⁺ wastewater under high-saline conditions.

Regulating performance of CANON process via adding external quorum sensing signal molecules in membrane bioreactor

In this study, the regulation effect of the external quorum sensing signals, N-dodecanoyl homoserine lactone (C12-HSL) on CANON process were investigated in a membrane bioreactor. C12-HSL significantly enhanced the aerobic ammonia-oxidizing bacteria and improved the ammonia monooxygenase activity to 0.134 from 0.076 μg NO₂^--N mg^-1 protein min^-1, while suppressed anaerobic ammonia-oxidizing bacteria and limited the TN removal to 0.07 from 0.22 kg m^-3 d^-1. Key enzymes synthesis were enhanced during the operation without C12-HSL addition, enabling the resistance of CANON system to high C12-HSL. As a result, the hydroxylamine oxidoreductase and nitrite reductase activity reached 35.9 EU g^-1 SS and 1.28 μg NO₂^--N mg^-1 protein min^-1, respectively; Nitrosomonas and Candidatus Kuenenia, with the abundance as 12.5 % and 22.9 %, cooperatively contributed to the TN removal, which maintained at 0.19 kg m^-3 d^-1. C12-HSL was profitable for aerobic ammonia oxidation, which could be adopted for regulating the nitrite production rate.

Using an innovative umbrella-shape membrane module to improve MBR for PN-ANAMMOX process

Membrane fouling has been a key factor limiting the applications of membrane bioreactor (MBR). In this study, a novel umbrella-shape membrane module was applied to construct two MBRs for two-stage partial nitrification-anaerobic ammonia oxidation (PN-ANAMMOX) process. After 55 days operation, the ANAMMOX process was started and the PN process was well controlled. Then, the ANAMMOX and PN process were successfully coupled to run the PN-ANAMMOX process. On 103 days, the best nitrogen removing effect was achieved with the maximum nitrogen loading rate (NLR) of 0.4 kg N·(m³·d)^-1 and the corresponding maximum total nitrogen removal rate (TNRR) of 75.23%. The umbrella-shape membrane module in both reactors only needed to be cleaned once during the operation for 105 days, indicating that the membrane module had better resistance to membrane fouling. The functional bacteria were cultivated in suspension state; moreover, the cell densities of ammonia oxidizing bacteria (AOB) and ANAMMOX bacteria (AnAOB) reached 58.32 × 10¹² copies/g sludge and 28.39 × 10¹² copies/g sludge. Their abundances reached 73.25% and 57.80% of the total bacteria, respectively. MBR improved by umbrella-shape membrane module could realize the rapid start-up of ANAMMOX process, effective control of PN process, and stable operation of PN-ANAMMOX process. This study provided a novel approach to control membrane fouling by optimizing the membrane module shape and widened applications of MBRs in PN-ANAMMOX process.

Metagenomic and extracellular polymeric substances analysis reveals the mechanism of exogenous N-hexanoyl-L-homoserine lactone in alleviating the inhibition of perfluorooctanoic acid on anammox process

To alleviate the negative effects of perfluorooctanoic acid (PFOA) on nitrogen removal via anaerobic ammonia oxidation (anammox), an exogenous signaling factor (N-hexanoyl-L-homoserine lactone, C₆-HSL) was introduced into an anammox reactor. Results showed that 2 μmol/L C₆-HSL promoted the nitrogen removal efficiency of the anammox reactor under PFOA stress, with the removal efficiencies of ammonia and nitrite increasing from 79.7 ± 4.8 % and 80.8 ± 3.8 %, to 94.4 ± 4.3 % and 97.1 ± 3.8 %. Exogenous C₆-HSL enhanced the compactness of the extracellular proteins, and improved the sludge hydrophobicity. Meanwhile, C₆-HSL resulted in a microbial shift, with the relative abundance of Planctomycetes increasing from 30.2 % to 49.5 %. Candidatus Kuenenia stuttgartiensis replaced Candidatus Brocadia sp. BL1 as the dominant species, while the available space for other nitrogen-removing bacteria was reduced. Exogenous C₆-HSL promoted the expression of anammox-related genes, such as hzsB and hdh, while denitrifying genes were down-regulated. In addition, the relative abundance of HdtS, which synthesizes AHLs, increased by 0.02446%.

A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal

The novel biological nitrogen removal process has been extensively studied for its high nitrogen removal efficiency, energy efficiency, and greenness. A successful novel biological nitrogen removal process has a stable microecological equilibrium and benign interactions between the various functional bacteria. However, changes in the external environment can easily disrupt the dynamic balance of the microecology and affect the activity of functional bacteria in the novel biological nitrogen removal process. Therefore, this review focuses on the microecology in existing the novel biological nitrogen removal process, including the growth characteristics of functional microorganisms and their interactions, together with the effects of different influencing factors on the evolution of microbial communities. This provides ideas for achieving a stable dynamic balance of the microecology in a novel biological nitrogen removal process. Furthermore, to investigate deeply the mechanisms of microbial interactions in novel biological nitrogen removal process, this review also focuses on the influence of quorum sensing (QS) systems on nitrogen removal microbes, regulated by which bacteria secrete acyl homoserine lactones (AHLs) as signaling molecules to regulate microbial ecology in the novel biological nitrogen removal process. However, the mechanisms of action of AHLs on the regulation of functional bacteria have not been fully determined and the composition of QS system circuits requires further investigation. Meanwhile, it is necessary to further apply molecular analysis techniques and the theory of systems ecology in the future to enhance the exploration of microbial species and ecological niches, providing a deeper scientific basis for the development of a novel biological nitrogen removal process.

Low-concentration methanol effect on the microorganisms, nitrogen removal, and recovery of the completely autotrophic nitrogen removal over nitrite

Methanol has a significant effect on the performance of the completely autotrophic nitrogen removal over the nitrite (CANON) process. In this research, the effect of low-concentration methanol on the functional microorganisms and nitrogen removal and recovery in the CANON system is investigated. The result shows that the anaerobic ammonium-oxidizing bacteria (AnAOB) was suppressed with low-concentration methanol addition, and the phylum Planctomycetes was hidden. The genus Candidatus Brocadia was restrained, and the relative abundances reduced from 25.5 to 15.0% in the upper biofilm and from 20.3 to 14.3% in the bottom biofilm, respectively. However, low-concentration methanol promoted the nitrifying oxidizing bacteria (NOB) activity. This phenomenon reduced the average ammonium nitrogen removal rate from 95.0 to 70.7%, and the average total nitrogen removal rate decreased from 81.3 to 43.6%, respectively. The results demonstrated that the low-concentration methanol as an organic carbon matter harmed the CANON process. Fortunately, the CANON system had an excellent self-healing ability when the methanol was stopped, with the average ammonium nitrogen removal rate and total nitrogen removal rate returning to 95.5 and 80.9%, respectively. This research supplies a reference for practical engineering design and application by improving the understanding of the effects of low-concentration methanol on CANON process performance.

Activation of N-acyl-homoserine lactone-mediated quorum sensing system improves long-term preservation of anammox microorganisms by vacuum lyophilization

The long-term preservation of anaerobic ammonium oxidation (anammox) microorganisms via vacuum lyophilization process would help commercialize the technique. In this study, vacuum lyophilization was evaluated for the cost-effective long-term preservation of such microorganisms. Skim milk was found to be the most effective cryoprotectant for maintaining the physiological properties (heme c, EPS, and the PN/PS ratio) of anammox microorganisms. Conversely, the vacuum lyophilization technique was shown to cause serious damage to the quorum sensing (QS) system of anammox, so that anammox activity was not adequately recovered afterwards. To overcome this limitation, activation of the AHL-mediated QS system were applied to the vacuum lyophilization process. Endogenous (i.e., fresh anammox sludge of 10%) and exogenous (i.e., C6-HSL of 60 mg/L) QS autoinducers significantly increased anammox activity to 88.2 ± 12.2 and 130.0 ± 12.2 mgTN/gVSS/d, respectively, after 56 d of reactivation. In addition, nitrogen removal potentials were estimated to be 123.5 and 87.5 gTN/m³/d, respectively. The effect of the exogenous QS autoinducer on anammox reactivation was reconfirmed through the comparison experiment. The results of this study will be greatly significant to this field since they improve the feasibility of the once-underestimated vacuum lyophilization technique.

Achieving Partial Nitritation by Treating Sludge With Free Nitrous Acid: The Potential Role of Quorum Sensing

Partial nitritation is increasingly regarded as a promising biological nitrogen removal process owing to lower energy consumption and better nitrogen removal performance compared to the traditional nitrification process, especially for the treatment of low carbon wastewater. Regulating microbial community structure and function in sewage treatment systems, which are mainly determined by quorum sensing (QS), by free nitrous acid (FNA) to establish a partial nitritation process is an efficient and stable method. Plenty of research papers reported that QS systems ubiquitously existed in ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), and various novel nitrogen removal processes based on partial nitritation were successfully established using FNA. Although the probability that partial nitritation process might be achieved by the regulation of FNA on microbial community structure and function through the QS system was widely recognized and discussed, the potential role of QS in partial nitritation achievement by FNA and the regulation mechanism of FNA on QS system have not been reviewed. This article systematically reviewed the potential role of QS in the establishment of partial nitritation using FNA to regulate activated sludge flora based on the summary and analysis of the published literature for the first time, and future research directions were also proposed.

Sustainable upgrading of biological municipal wastewater treatment based on anammox: From microbial understanding to engineering application

Anaerobic ammonium oxidation (anammox) has drawn increasing attention as a promising option to energy-neutral wastewater treatment. While anammox process still faces challenges in the low-strength and organics-contained municipal wastewater due to its susceptibility and the technical gaps in substrate supply. Effective strategies for extensive implementation of anammox in municipal wastewater treatment plants (WWTPs) remain poorly summarized. In view of the significance and necessity of introducing anammox into mainstream treatment, the growing understanding not only at level of microbial interactions but also on view of upgrading municipal WWTPs with anammox-based processes need to be considered urgently. In this review, the critical view and comprehensive analysis were offered from the perspective of microbial interactions within partial nitrification- and partial denitrification-based anammox processes. To minimize the microbial competition and enhance the cooperation among anammox bacteria and other functional bacteria, targeted control strategies were systematically evaluated. Based on the comprehensive overview of recent advances, the combination of flexible regulation of input organic carbon with anaerobic/oxic/anoxic process and the integration of sludge fermentation with anoxic biofilms in anaerobic/anoxic/oxic process were proposed as promising solutions to upgrade municipal WWTPs with anammox technology. Furthermore, a new perspective of coupling anammox with denitrifying dephosphatation was proposed as a promising method for in-depth nutrients removal from carbon-limit municipal wastewater in this study. This review provides the critical and comprehensive viewpoints on anammox engineering in municipal wastewater and paves the way for the anammox-based upgrading of municipal WWTPs.

Quorum sensing - a promising tool for degradation of industrial waste containing persistent organic pollutants

Restoring an environment contaminated with persistent organic pollutants (POPs) is highly challenging. Biodegradation by biofilm-forming bacteria through quorum sensing (QS) is a promising treatment process to remove these pollutants and promotes eco-restoration. QS plays an important role in biofilm formation, solubilization, and biotransformation of pollutants. QS is a density-based communication between microbial cells via signalling molecules, which coordinates specific characters and helps bacteria to acclimatize against stress conditions. Genetic diversification of a biofilm offers excellent opportunities for horizontal gene transfer, improves resistance against stress, and provides a suitable environment for the metabolism of POPs. To develop this technology in industrial scale, it is important to understand the fundamentals and ubiquitous nature of QS bacteria and appreciate the role of QS in the degradation of POPs. Currently, there are knowledge gaps regarding the environmental niche, abundance, and population of QS bacteria in wastewater treatment systems. This review aims to present up-to-date and state-of-the-art information on the roles of QS and QS-mediated strategies in industrial waste treatment including biological treatments (such as activated sludge), highlighting their potentials using examples from the pulp and paper mill industry, hydrocarbon remediation and phytoremediation. The information will help to provide a throughout understanding of the potential of QS to degrade POPs and advance the use of this technology. Current knowledge of QS strategies is limited to laboratory studies, full-scale applications remain challenging and more research is need to explore QS gene expression and test in full-scale reactors for wastewater treatment.

The challenges of monitoring and manipulating anaerobic microbial communities

Mixed anaerobic microbial communities are a key component in valorization of waste biomass via anaerobic digestion. Similar microbial communities are important as soil and animal microbiomes and have played a critical role in shaping the planet as it is today. Understanding how individual species within communities interact with others and their environment is important for improving performance and potential applications of an inherently green technology. Here, the challenges associated with making measurements critical to assessing the status of anaerobic microbial communities are considered. How these measurements could be incorporated into control philosophies and augment the potential of anaerobic microbial communities to produce different and higher value products from waste materials are discussed. The benefits and pitfalls of current genetic and molecular approaches to measuring and manipulating anaerobic microbial communities and the challenges which should be addressed to realise the potential of this exciting technology are explored.

Quorum-Sensing Molecules: Sampling, Identification and Characterization of N-Acyl-Homoserine Lactone in Vibrio sp

Quorum sensing (QS) is a mechanism by which gram-negative bacteria regulate their gene expression by making use of cell density. QS is triggered by a small molecule known as an autoinducer. Typically, gram-negative bacteria such as Vibrio produce signaling molecules called acyl homoserine lactones (AHLs). However, their levels are very low, making them difficult to detect. We used thin layer chromatography (TLC) to examine AHLs in different Vibrio species, such as Vibrio alginolyticus, Vibrio parahemolyticus, and Vibrio cholerae, against a standard- Chromobacterium violaceum. Further, AHLs were characterised by high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC–MS). C4-HSL (N- butanoyl- L- homoserine lactone), C6-HSL (N- hexanoyl- L- homoserine lactone), 3-oxo-C8-HSL (N-(3-Oxooctanoyl)-DL-homoserine lactone), C8-HSL (N- octanoyl- L- homoserine lactone), C₁10-HSL (N- decanoyl- L- homoserine lactone), C12-HSL (N- dodecanoyl- L- homoserine lactone) and C14-HSL (N- tetradecanoyl- L- homoserine lactone) were identified from Vibrio. These results may provide a basis for blocking the AHL molecules of Vibrio, thereby reducing their pathogenicity and eliminating the need for antimicrobials.

Advances in Studies on Microbiota Involved in Nitrogen Removal Processes and Their Applications in Wastewater Treatment

The discharge of excess nitrogenous pollutants in rivers or other water bodies often leads to serious ecological problems and results in the collapse of aquatic ecosystems. Nitrogenous pollutants are often derived from the inefficient treatment of industrial wastewater. The biological treatment of industrial wastewater for the removal of nitrogen pollution is a green and efficient strategy. In the initial stage of the nitrogen removal process, the nitrogenous pollutants are converted to ammonia. Traditionally, nitrification and denitrification processes have been used for nitrogen removal in industrial wastewater; while currently, more efficient processes, such as simultaneous nitrification-denitrification, partial nitrification-anammox, and partial denitrification-anammox processes, are used. The microorganisms participating in nitrogen pollutant removal processes are diverse, but information about them is limited. In this review, we summarize the microbiota participating in nitrogen removal processes, their pathways, and associated functional genes. We have also discussed the design of efficient industrial wastewater treatment processes for the removal of nitrogenous pollutants and the application of microbiome engineering technology and synthetic biology strategies in the modulation of the nitrogen removal process. This review thus provides insights that would help in improving the efficiency of nitrogen pollutant removal from industrial wastewater.

Nano zero-valent iron improves anammox activity by promoting the activity of quorum sensing system

The addition of nano zero-valent iron (nZVI) has been proven to improve the efficiency of the anammox process, however, the mechnism is not clear. Here, the effect of nZVI on anammox microbial community was studied by metagenomic sequencing methods. It was found that 50 mg/L nZVI indeed promoted the removal of NH₄⁺ and NO₂^- of the anammox reactor and significantly improved the relative abundance of AnAOB (Ca. Brocadia) from 42.1% to 52.5%. What's more, 50 mg/L nZVI increased the abundance of c-di-GMP synthesized protein from 148 rpmr to 252 rpmr in the microbial community and decreased the abundance of c-di-GMP degradation protein from 238 rpmr to 204 rpmr, which indirectly led to the enrichment of c-di-GMP in the microbial community. The enrichment of c-di-GMP reduced the motility of microorganisms in the reactor and promoted the secretion of extracellular polymers by bacteria, which is beneficial to the formation of sludge particles in the anammox reactor. In conclusion, this research clarified the mechanism of nZVI promoting the anammox process and provided theoretical guidance for the engineering application of anammox.

Deciphering bacterial social traits via diffusible signal factor (DSF) -mediated public goods in an anammox community

Both the benefits of bacterial quorum sensing (QS) and cross-feeding for bio-reactor performance in wastewater treatment have been recently reported. As the social traits of microbial communities, how bacterial QS regulating bacterial trade-off by cross-feeding remains unclear. Here, we find diffusion signal factor (DSF), a kind of QS molecules, can bridge bacterial interactions through regulating public goods (extracellular polymeric substances (EPS), amino acids) for metabolic cross-feedings. It showed that exogenous DSF-addition leads to change of public goods level and community structure dynamics in the anammox consortia. Approaches involving meta-omics clarified that anammox and a Lautropia-affiliated species in the phylum Proteobacteria can supply costly public goods for DSF-Secretor species via secondary messenger c-di-GMP regulator (Clp) after sensing DSF. Meanwhile, DSF-Secretor species help anammox bacteria scavenge extracellular detritus, which creates a more suitable environment for the anammox species, enhances the anammox activity, and improves the nitrogen removal rate of anammox reactor. The trade-off induces discrepant metabolic loads of different microbial clusters, which were responsible for the community succession. It illustrated the potential to artificially alleviate metabolic loads for certain bacteria. Deciphering microbial interactions via QS not only provides insights for understanding the social behavior of microbial community, but also creates new thought for enhancing treatment performance through regulating bacterial social traits via quorum sensing-mediated public goods.