Exogenous N-acyl-homoserine lactones accelerate resuscitation of starved anaerobic granular sludge after long-term stagnation

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 methane production through synergistic integration of quorum sensing Signals and microbial electrolysis in anaerobic digestion of low-quality biomass

This study explored the synergistic effects of exogenous acyl homoserine lactones (AHLs) and microbial electrolysis (ME) on enhancing anaerobic digestion (AD) of low-quality biomass. The combined AHLs + ME strategy achieved a 79.50 % increase in cumulative methane production compared to the control, outperforming individual treatments. This enhancement was attributed to accelerated substrate degradation, selective enrichment of hydrogenotrophic methanogens (Methanobrevibacter, Methanobacterium, Methanofollis), and strengthened quorum sensing, electron transfer, and methane synthesis pathways. Metagenomic analysis revealed abundance upregulation of key genes involved in AHL synthesis (bjaI, rpaI, braI, rhiI) and sensing (solR, cepR, tofR), direct interspecies electron transfer (pilA, mtrC), and hydrogenotrophic methanogenesis (ftr, mvhD, vhuD, vhcD). This study offers a novel and sustainable strategy to optimize methane production from recalcitrant biomass, advancing AD-based waste-to-energy systems.

Metagenomic insight into the diffusion signal factor mediated social traits of anammox consortia after starvation

Biomass starvation is common in biological wastewater treatment. As a social trait of microbial community, how quorum sensing (QS) regulated bacterial trade-off through interactions after starvation remains unclear. This study deciphered the mechanism of anaerobic ammonium oxidation (anammox) consortia in response to starvation, including reducing extracellular electron transfer (EET), adenosine 5'-triphosphate (ATP) content and amino acid metabolism. Metagenomic analysis has shown that the addition of the diffusion signal factor (DSF) resulted in a high abundance of antioxidant genes, which contributed to achieving redox balance in anammox bacteria. There was an enrichment of Geobacter and Methanosarcina, which were QS-responsive direct interspecific electron transfer participants. Furthermore, DSF stimulated the nitrogen and carbon metabolism of Ca. Kuenenia_stuttgartiensis, promoting syntrophy of metabolic intermediates within microbial community. This study highlighted the effect of DSF on the microbial interaction patterns and deciphered the QS-based social traits of anammox consortia after starvation, facilitating the stable operation of the anammox process.

Surface engineering-based S, N co-doped biochar for improved anaerobic digestion: Enhancing microbial-pollutant and inter-microbial electron transfer synergistic EPS protection

Enhancing extracellular electron transfer (EET) efficiency is crucial for improving the anaerobic digestion (AD) system's capability to treat recalcitrant wastewater. In this study, a novel S, N co-doped biochar (S-N-BC) was prepared through surface engineering to optimize EET within AD systems. The addition of S-N-BC significantly enhanced the performance of a mesophilic AD system treating Congo red wastewater, increasing the decolorization rate by 78 %, COD degradation rate by 82 %, and methane yield by 87 % compared to the control. Additionally, the shock resistance of anaerobic granular sludge was improved, as evidenced by the formation of the protective extracellular polymeric substances (EPS) barrier and the enhanced activities of the electron transport system. Mechanistic analysis revealed that adding S-N-BC did not alter the Congo red decolorization pathway but significantly enriched various electrochemically active bacteria and established EET pathways between microbial-pollutant and inter-microbial. This significantly accelerated EET efficiency within the AD system, ensuring stable and efficient operation under challenging conditions. This study proposed a novel approach using S-N-BC to simultaneously enhance "dual-pathway EET" between microbial-pollutant and inter-microbial while constructing an EPS protective barrier, addressing the issues of low efficiency and fragile stability of AD systems for treating recalcitrant wastewater.

Exogenous signaling molecules N-acyl-homoserine lactones promotes the reconstruction of sludge particles after impact with highly concentrated urea-formaldehyde resin microplastics

In this study, exogenous N-acyl-homoserine lactones (AHLs) was added to resist the stress by high concentration (0.5 g/L) of urea formaldehyde resin microplastics (UF-MPs) on anaerobic granular sludge (AnGS), aiming to provide a viable strategy for AnGS to withstand elevated levels of UF-MPs toxicity elucidate the intricate regulatory mechanism of AHL-mediated AnGS-QS regulation. The results showed that the three different signaling molecules (C4-HSL, C6-HSL, and C8-HSL) improved the performance of AnGS under high concentration (0.5 g/L) urea-formaldehyde resin stress, and increased sludge COD removal (4.48%, 4.76%, and 3.35%, respectively) and methanogenic activity (8.38%, 1.92%, and 18.76%, respectively). The addition of C4-HSL has the best effect on sludge particle size and strength, which is attributed to the fact that C4-HSL can significantly increase the content of polysaccharides and proteins in tightly bound extracellular polymeric substances (TB-EPS) (27.1% and 27.1%, respectively). C8-HSL most obviously promotes energy metabolism and EPS biosynthesis gene expression. Metagenomic analysis showed that trace AHLs could promote the abundance of enzymes and functional genes related to the main pathway of methane metabolism, increase the relative abundance of Methanothrix of acetophilic methanogens from 27.79% in the control group to 27.85% (C4-HSL), 28.90% (64-HSL), and 30.03% (C8-HSL), thereby improving community stability.

An in-situ biochar-enhanced anaerobic membrane bioreactor for swine wastewater treatment under various organic loading rates

A biochar-assisted anaerobic membrane bioreactor (BC-AnMBR) was conducted to evaluate the performance in treating swine wastewater with different organic loading rates (OLR) ranging from 0.38 to 1.13 kg-COD/(m3.d). Results indicated that adding spent coffee grounds biochar (SCG-BC) improved the organic removal efficiency compared to the conventional AnMBR, with an overall COD removal rate of > 95.01%. Meanwhile, methane production of up to 0.22 LCH4/gCOD with an improvement of 45.45% was achieved under a high OLR of 1.13 kg-COD/(m3.d). Furthermore, the transmembrane pressure (TMP) in the BC-AnMBR system was stable at 4.5 kPa, and no irreversible membrane fouling occurred within 125 days. Microbial community analysis revealed that the addition of SCG-BC increased the relative abundance of autotrophic methanogenic archaea, particularly Methanosarcina (from 0.11% to 11.16%) and Methanothrix (from 16.34% to 24.05%). More importantly, Desulfobacterota and Firmicutes phylum with direct interspecific electron transfer (DIET) capabilities were also enriched with autotrophic methanogens. Analysis of the electron transfer pathway showed that the concentration of c-type cytochromes increased by 38.60% in the presence of SCG-BC, and thus facilitated the establishment of DIET and maintained high activity of the electron transfer system even at high OLR. In short, the BC-AnMBR system performs well under various OLR conditions and is stable in the recovery energy system for swine wastewater.

Regulation and application of quorum sensing on anaerobic digestion system

Quorum sensing (QS) plays an important role in the social behavior of microbial communities. Anaerobic digestion (AD) is a biological process using anaerobic microorganisms to degrade organic macromolecules into small molecules for biogas and biofertilizer production. In AD, the QS signaling molecule N-acyl homoserine lactones (AHLs) induces bacterial metabolism, improving AD process efficiency. However, there are fewer systematic reports about QS regulation of microbial behavior in AD. In this report, the effects of signaling molecules on extracellular polymer secretion, biofilm formation, granulation of granular sludge and bacterial metabolism in AD were investigated in detail. At present, the regulation behavior of QS on AD is a group phenomenon, and there are few in-depth studies on the regulation pathway. Therefore, we conducted an in-depth analysis of the pure culture system, granular sludge and reactor in the AD. Then we pointed out that the future application potential of QS in the AD may be combined with quorum quenching (QQ) and omics technology, which is of great significance for the future application of AD.

Removal of organic matter from food wastewater using anaerobic digestion at low temperatures enhanced by exogenous signaling molecule N-hexanoyl-homoserine lactone enhancement: Insight to extracellular polymeric substances and key functional genes

This experiment aimed to study the effects of adding the exogenous signaling molecule N-hexanoyl-homoserine lactone (C6-HSL) on the anaerobic digestion of food wastewater at low temperature (15 °C). Daily addition of 0.4 μmol C6-HSL increased the average chemical oxygen demand removal from 45.98% to 94.92%, while intermittent addition (adding 2 μmol C6-HSL every five days) increased it from 45.98% to 72.44%. These two modes of C6-HSL addition increased protease and acetate kinase activity by 47.99%/8.04% and 123.26%/127.91% respectively, and increased coenzyme F420 concentrations by 15.79% and 63.16%, respectively. The regulation of loosely bound extracellular polymeric substances synthesis was influenced by C6-HSL, which increased protein and polysaccharide content in sludge. The relative abundance of Firmicutes and Bacteroidetes increased following addition of C6-HSL. After continuous addition of C6-HSL, the relative abundance of related functional genes such as amy, apgM, aceE, and accC increased, indicating that methanogens obtained sufficient substrate. The abundance of glycolysis-related functional genes such as glk, pfk, pgi, tpiA, gap, pgk, gpmA, eno, and pyk increased after the addition of C6-HSL, ensuring the efficient transformation and absorption of organic matter by anaerobic sludge at low temperatures. This study provides new comprehensive insights into the mechanism behind the enhancement of food wastewater anaerobic digestion by C6-HSL at low temperature.

Attached indigenous microalgal-bacterial consortium with greater stress-resistance facilitated recovery of integrated fixed-film system after experiencing short-term stagnation inhibition

Stability of integrated fixed-film indigenous microalgal-bacterial consortium (IF-IMBC) requires further investigation. This study focused on the influence of short-term stagnation (STS), caused by influent variations or equipment maintenance, on IF-IMBC. Results showed that the IF-IMBC system experienced initial inhibition followed by subsequent recovery during STS treatment. Enhanced organics utilization was believed to contribute to system recovery. It is proposed that the attached IMBC possessed greater stress resistance. On the one hand, a higher increase in bacteria potentially participating in organic degradation was observed. Moreover, the dominant eukaryotic species significantly decreased in suspended IMBC while its abundance remained stable in the attached state. On the other hand, increased abundance for most functional enzymes was primarily observed in the attached bacteria. This fundamental research aims to bridge the knowledge gap regarding the response of IMBC to variations in operational conditions.

Boron induced multiple quorum-sensing circuits in parallel to assist in anaerobic digestion recovery from volatile fatty acids accumulation

Exogenous quorum sensing (QS) molecular can regulate the activity and granulation process of anaerobic sludge in anaerobic digestion process, but would be impractical as a standalone operation. Here we demonstrated that application of 1 mg L-1 boric acid assisted in an upflow anaerobic sludge blanket (UASB) reactor recovery from volatile fatty acids (VFAs) accumulation. After VFAs accumulation, the chemical oxygen demand (COD) removal suddenly reduced from 78.98% to 55.86%. The relative abundance of acetoclastic methanogens decreased from 55.79% to 68.28%-23.14%∼25.41%, and lead to the acetate accumulate as high as 1317.03 mg L-1. Granular sludge disintegrated and the average size of sludge decreased to 586.38 ± 42.45 μm. Application of 1 mg L-1 boric acid activated the interspecies QS signal (AI-2) and then induced the secretion of intraspecies QS signal (N-acyl-homoserine lactones, AHLs). AHLs were then stimulated the growth of syntrophic acetate oxidizing bacteria and hydrogenotrophic methanogen. Moreover, the concentration of acetate decreased to 224.50 mg‧L-1, and the COD removal increased to 75.10% after application of 1 mg L-1 boric acid. The activated AI-2 may induce multiple quorum-sensing circuits enhance the level of AI-2 and AHLs in parallel, and in turn assisted in anaerobic digestion recovery from VFAs accumulation.

Roles of quorum-sensing molecules in methane production from anaerobic digestion aided by biochar

Biochar has been used to enhance methane generation from anaerobic digestion through establishing direct interspecific electron transfer between microorganisms. However, the microbial communication is still inadequate, thereby limiting further methane production improvement contributed by biochar. This study investigated the roles of quorum-sensing molecules, acylated homoserine lactone (AHL), in anaerobic digestion of waste activated sludge aided by biochar. Results showed that the co-addition of separated biochar and AHL achieved best methane production performance, with the maximal methane yield of 154.7 mL/g volatile suspended solids, which increased by 51.9%, 47.2%, 17.9%, and 39.4% respectively compared to that of control, AHL-loaded biochar, sole AHL, and sole biochar groups. The reason was that the co-addition of separated biochar and AHL promoted the stages of hydrolysis and acidification, promoting the conversion of organic matters and short-chain fatty acids, and optimizing the accumulation of acetate acid. Moreover, the methanogenesis stage also performed best among experimental groups. Correspondingly, the highest activities of electron transfer and coenzyme F420 were obtained, with increase ratios of 33.2% and 27.2% respectively compared to that of control. Furthermore, biochar did more significant effects on the evolution of microbial communities than AHL, and the direct interspecific electron transfer between fermentative bacteria and methanogens were possibly promoted.

N-acyl homoserine lactones (AHLs) enhanced removal of cadmium and other pollutants by algae-bacteria consortia

Signal transduction is an important mode of algae-bacteria interaction, in which bacterial quorum sensing (QS) may affect microalgal growth and metabolism. Currently, little is known whether acyl homoserine lactones (AHLs) released by bacteria can affect the pollutant removal by algae-bacteria consortia (ABC). In this study, we constructed ABC using Chlorella vulgaris (Cv) with two AHLs-producing bacteria and investigated their performance in the removal of multiple pollutants, including chemical oxygen demand (COD), total nitrogen (TN), phosphorus (P), and cadmium (Cd). The AHLs-producing bacteria, namely Agrobacterium sp. (Ap) and Ensifer adherens (Ea), were capable of forming a symbiosis with C. vulgaris. Consortia of Cv and Ap with ratio of 2:1 (Cv2-Ap1) showed the optimal growth promotion and higher removal of Cd, COD, TN, and P compared to the C. vulgaris monoculture. Cv2-Ap1 ABC removed 36.1-47.5% of Cd, 94.5%-94.6% COD, 37.1%-56.0% TN, and 90.4%-93.5% P from the culture medium. In addition, increase of intracellular neutral lipids and extracellular protein, as well as the types of functional groups on cell surface contributed to Cd removal and tolerance in the Cv2-Ap1 ABC. Six AHLs were detected in the Cv2-Ap1 culture. Among these, 3OC8-HSL and 3OC12-HSL additions promoted the ABC growth and enhanced their Cd accumulation. These findings may contribute to further understanding of AHL-mediated communication between algae and bacteria and provide support bioremediation efforts of metal-containing wastewater.

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.

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.

Unravelling the resilience of magnetite assisted granules to starvation and oxytetracycline stress

Starvation and antibiotics pollution are two frequent perturbations during breeding wastewater treatment process. Supplying magnetite into anaerobic system has been proved efficient to accelerate microbial aggregates and alleviate the adverse effect caused by process disturbance. Nevertheless, whether these magnetite-based granules are still superior over normal granules after a long-term starvation period remains unknown, the responsiveness of these granules to antibiotics stress is also ambiguous. In current study, we investigated the resilience of magnetite-based anaerobic granular sludge (AnGS) to starvation and oxytetracycline (OTC) stress, by unravelling the variations of reactor performance, sludge properties, ARGs dissemination and microbial community. Compared with the AnGS formed without magnetite, the magnetite assisted AnGS appeared more robust defense to starvation and OTC stress. With magnetite supplement, the average methane yield after starvation recovery, 50 mg/L and 200 mg/L OTC stress was enhanced by 48.95%, 115.87% and 488.41%, respectively, accompanied with less VFAs accumulation, improved tetracycline removal rate (76.3-86.6% vs. 51.0-53.5%) and higher ARGs reduction. Meanwhile, magnetite supplement effectively ameliorated the potential sludge breakage by triggering more large granules formation. Trichococcus was considered an important impetus in maintaining the stability of magnetite-based AnGS process. By inducing more syntrophic methanogenesis partnerships, especially for hydrogenotrophic methanogenesis, magnetite ensured the improved reactor performance and stronger resilience at stress conditions.

Impact of N-butyryl-l-homoserine lactone-mediated quorum sensing on acidogenic fermentation under saline conditions: Insights into volatile fatty acids production and microbial community

Anaerobic fermentation is often inhibited under high salinity conditions. This study discovered a strong, positive association between N-butyryl-l-homoserine lactone (C4-HSL)-mediated quorum sensing (QS) and the production of volatile fatty acids (VFAs) under saline conditions. N-acyl-homoserine lactones were identified during acidogenic fermentation for VFA production. Only C4-HSL was detected at all salt concentrations, and a maximum C4-HSL concentration of 0.49 μg/L was observed at a salt concentration of 15 g/L. C4-HSL secretion was closely related to salinity, and a strong correlation was observed between C4-HSL and VFAs (p < 0.01), especially butyrate. Further experiments with C4-HSL addition indicated that exogenous C4-HSL promoted substrate hydrolysis and increased butyrate production by 1.5 times at 15 g/L NaCl. Microbial community analysis indicated that unclassified_f__Enterobacteriaceae and Clostridium_sensu_stricto_1, associated with QS genes and butyrate production, were positively associated with C4-HSL. This study demonstrates the positive effect of C4-HSL-mediated QS on acidogenic fermentation.

Advances in AHLs-mediated quorum sensing system in wastewater biological nitrogen removal: mechanism, function, and application

Biological nitrogen removal process is to convert organic nitrogen and ammonia nitrogen into nitrogen via a series of reactions by microorganisms, and is widely used in wastewater treatment for its costless, high-effective, secondary pollution-free characteristics. Quorum sensing (QS) is a communication mode for microorganisms to regulate bacteria's physiological behaviors in response to environmental changes. N-acyl-homoserine lactones (AHLs)-mediated QS system is widespread in nitrogen removal-related functional bacteria and promotes biological nitrogen removal performance by regulating bacteria behavior. Recently, there has been an increasingly investigated AHLs-mediated QS system in wastewater biological nitrogen removal process. Consequently, the AHLs-mediated QS system is considered a promising regulatory strategy in the biological nitrogen removal process. This article reviewed the QS mechanism in various nitrogen removal-related functional bacteria and analyzed its effect on biological nitrogen removal performance. Combined with the application research of the QS system for enhanced biological nitrogen removal, it further put forward some prospects and suggestions which are of practical significance in practical application.

A novel biotechnology based on periphytic biofilms with N-acyl-homoserine-lactones stimulation and lanthanum loading for phosphorus recovery

This study presents an approach for developing periphytic biofilm with N-acyl-homoserine-lactones (AHLs) stimulation and lanthanum (La, a rare earth element) loading, to achieve highly efficient and stable phosphorus (P) recovery from wastewater. AHLs stimulated biofilm growth and formation, also improved stable P entrapment by enhancing extracellular polymeric substance (EPS) production and optimizing P-entrapment bacterial communities. Periphytic biofilms loading La is based on ligand exchanges, and La loading achieved initial rapid P entrapment by surface adsorption. The combination of AHLs stimulation and La loading achieved 99.0% P entrapment. Interestingly, the enhanced EPS production stimulated by AHLs protected biofilms against La. Moreover, a method for P and La separately recovery from biofilms was developed, achieving 89-96% of P and 88-93% of La recovery. This study offers a promising biotechnology to reuse La from La-rich wastewater and recover P by biofilm doped with La, which results in a win-win situation for resource sustainability.

Exogenous N-hexanoyl-L-homoserine lactone assists in upflow anaerobic sludge blanket recovery from acetate accumulation via aceticlastic methanogens enrichment

Volatile fatty acids (VFAs) accumulation caused by shock loading severely hampers the performance and stability of anaerobic digestion. An upflow anaerobic sludge blanket (UASB) was operated to examine its performance, sludge properties, and microbial community behavior during shock loading and recovery with exogenous N-hexanoyl-L-homoserine lactone (C₆-HSL). After shock loading, chemical oxygen demand (COD) removal was significantly reduced from 79.09% to 65.63%. The abundance of Methanosarcinales also significantly decreased, which resulted in acetate accumulation (1,163.55 mg/L). Sludge granules disintegrated along with the decrease in extracellular polymeric substances (EPS). After supplying 1 μg/L C₆-HSL, COD removal resumed to 75.10%. Furthermore, C₆-HSL enhanced the abundance and metabolic activity of aceticlastic methanogens, decreased acetate concentration to 146.87 mg/L, improved EPS secretion, and caused the re-assembly of disintegrated sludges to form large granules. These results advanced our understanding of microbial community behavior and provided an attractive strategy for restoration of UASB recovered from shock loading.

Study on the mechanism of inhibiting the calcification of anaerobic granular sludge induced by the addition of trace signal molecule (3O-C6-HSL)

Microbiota quorum sensing (QS) induced by 3O-C6-HSL (N-(β-ketocaproyl)-DL-homoserine lactone) inhibited the calcification of anaerobic granular sludge (AnGS), and the mechanism of promoting the activity recovery of calcified AnGS was studied in this paper. Through research, it was speculated that 3O-C6-HSL acted on calcified AnGS residual microorganisms to trigger QS. It enriched many functional microorganisms. For example, it promoted the growth of Methanospirillum. The CO₂ could be consumed quickly by Methanospirillum and reduced the calcium carbonate formation. The increase of microbial biomass would promote the activity of sludge. What's more, the pore size and porosity of sludge would increase, so the mass transfer channel will be broadened at same time. All those, could help the calcified AnGS quickly restore the activity and anaerobic system recover normal, which provided a new idea for the emergency rescue of anaerobic system in the future.