Diversity of Acyl Homoserine Lactone Molecules in Anaerobic Membrane Bioreactors Treating Sewage at Psychrophilic Temperatures

Anaerobic membrane bioreactors for municipal wastewater: Progress in resource and energy recovery improvement approaches

Anaerobic membrane bioreactor (AnMBR) offer promise in municipal wastewater treatment, with potential benefits including high-quality effluent, energy recovery, sludge reduction, and mitigating greenhouse gas emissions. However, AnMBR face hurdles like membrane fouling, low energy recovery, etc. In light of net-zero carbon target and circular economy strategy, this work sought to evaluate novel AnMBR configurations, focusing on performance, fouling mitigation, net-energy generation, and nutrients-enhancing integrated configurations, such as forward osmosis (FO), membrane distillation (MD), bioelectrochemical systems (BES), membrane photobioreactor (MPBR), and partial nitrification-anammox (PN/A). In addition, we highlight the essential role of AnMBR in advancing the circular economy and propose ideas for the water-energy-climate nexus. While AnMBR has made significant progress, challenges, such as fouling and cost-effectiveness persist. Overall, the use of novel configurations and energy recovery strategies can further improve the sustainability and efficiency of AnMBR systems, making them a promising technology for future sustainable municipal wastewater treatment.

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.

Effect of thermal hydrolysis pretreatment on the occurrence of N-acyl homoserine lactone during anaerobic digestion of waste activated sludge

This study sought to investigate the relationship between N-acyl homoserine lactones (AHLs) and methanogenic microorganisms, focusing on endogenous AHLs in the anaerobic digestion (AD) process. By analyzing waste activated sludge (WAS) samples, we examine the changes in microbial communities and the AHLs-methanogens connection. The Mantel test and Spearman correlation analysis were conducted to gain novel insights into the AD process. Our findings demonstrate that thermal hydrolysis pretreatment (THP) modifies AHL concentrations during AD, thereby enhancing methanogenic bacteria activity and regulating social interactions among microorganisms. In the Eth group (AD of THP samples labeled Eth), Methanobacterium and Methanosarcina accounted for over 80% of the methanogenic bacteria, with correlation coefficients greater than 0.5 between these bacterial taxa and N-hexyl-l-homoserine lactone (C6-HSL) and N-enanthyl-l-homoserine lactone (C7-HSL).

Identification of AHL Synthase in Desulfovibrio vulgaris Hildenborough Using an In-Silico Methodology

Sulfate-reducing bacteria (SRB) are anaerobic bacteria that form biofilm and induce corrosion on various material surfaces. The quorum sensing (QS) system that employs acyl homoserine lactone (AHL)-type QS molecules primarily govern biofilm formation. Studies on SRB have reported the presence of AHL, but no AHL synthase have been annotated in SRB so far. In this computational study, we used a combination of data mining, multiple sequence alignment (MSA), homology modeling and docking to decode a putative AHL synthase in the model SRB, Desulfovibrio vulgaris Hildenborough (DvH). Through data mining, we shortlisted 111 AHL synthase genes. Conserved domain analysis of 111 AHL synthase genes generated a consensus sequence. Subsequent MSA of the consensus sequence with DvH genome indicated that DVU_2486 (previously uncharacterized protein from acetyltransferase family) is the gene encoding for AHL synthase. Homology modeling revealed the existence of seven α-helices and six β sheets in the DvH AHL synthase. The amalgamated study of hydrophobicity, binding energy, and tunnels and cavities revealed that Leu99, Trp104, Arg139, Trp97, and Tyr36 are the crucial amino acids that govern the catalytic center of this putative synthase. Identifying AHL synthase in DvH would provide more comprehensive knowledge on QS mechanism and help design strategies to control biofilm formation.

The relationship between quorum sensing dynamics and biological performances during anaerobic membrane bioreactor treatment

Anaerobic membrane bioreactors (AnMBRs) enhance carbon neutrality with biomethane recovery from wastewater; however, microbial signaling, which may affect biological performances, was poorly understood. Here, we thus evaluate quorum sensing (QS) dynamics while monitoring acyl-homoserine lactones (AHLs) and autoinducer-2 (AI-2) levels during long-term AnMBR operations after sludge inoculation. Significant organic removal and methane production were achieved with the reactor startup. Signal molecule levels varied with transient organic loading rates, depending on their types. A starving condition may cause an increase in short- and medium-chain AHLs and AI-2. Biopolymers, biosolids, volatile fatty acids, and alkalinity levels had positive correlations with short- and medium-chain AHLs and AI-2, whereas methane production had positive correlations with long-chain AHLs. The principal component analysis of QS signal composition and biological performance data explains their interconnectivity. The findings of this study help to understand that QS signals regulate metabolic pathways in addition to microbial group behaviors.

Enriched autoinducer-2 (AI-2)-based quorum quenching consortium in a ceramic anaerobic membrane bioreactor (AnMBR) for biofouling retardation

This study is the first to enrich a facultative QQ consortium for AI-2-based quorum sensing (QS) disruption (FQQ2) and discover its quorum quenching (QQ) performance in an anaerobic membrane bioreactor (AnMBR) for membrane fouling retardation. Herein, FQQ2 was enriched by the enrichment culture using 4,5-dihydroxy-2,3-pentanedione (DPD) followed by anaerobic screening. FQQ2 was composed of various facultative AI-2-based QQ microorganisms including Acinetobacter, Comamonas, Stenotrophomonas, and FQQ2 was capable to degrade 96.96% of DPD in 9 h. More importantly, FQQ2 prolonged membrane filtration operation by an average of 3.72 times via reduction of DPD in the AnMBR treating domestic wastewater (p ≤ 0.05). QQ was implicated to reduce the content of proteins and carbohydrates of the extracellular polymeric substances (EPS) of suspended biomass by 24.16% and 10.39%, respectively, and concentration of proteins of the soluble microbial products (SMP) by 18.77%. Parallel factor (PARAFAC) modelling of excitation-emission matrix (EEM) demonstrated that QQ could reduce the content of fulvic acid-like and humic acid-like substances, aromatic proteins and soluble-microbial-by-product-like proteins of the EPS (p ≤ 0.05) and abate the content of soluble-microbial-by-product-like proteins in the SMP (p ≤ 0.05). The lower EPS content of suspended biomass could be rendered with the reduced relative abundance of AI-2-regulated Christensenellaceae;g-, Hyphomicrobium, Leucobacter and Microbacterium by 48.48%, 76.56%, 64.78% and 59.26%, respectively, and QQ led to the reduction of the relative abundance of Christensenellaceae;g- and Leucobacter in the cake layer by 31.07% and 51.43%, respectively. Moreover, quantity of organics as well as planktonic microorganisms in the supernatant decreased in presence of FQQ2 (p ≤ 0.05). Of note, markedly lower relative abundance of AI-2-regulated Sulfurovum in supernatant by 97.74% resulted in its lower abundance of cake layer. Intriguingly, in the presence of QQ, methane production was statistically enhanced by 62.5% (p ≤ 0.05). It was closely linked to the decrease of sulfate reduction (p ≤ 0.05), which resulted from 37.93% lower abundance of sulfate-reduction Desulfomonile in the suspended biomass (p ≤ 0.05). Collectively, this study sheds lights on the development of AI-2-based QQ for biofouling control in AnMBRs.

Temperature and immigration effects on quorum sensing in the biofilms of anaerobic membrane bioreactors

Quorum sensing (QS), a microbial communication mechanism modulated by acyl homoserine lactone (AHL) molecules impacts biofilm formation in bioreactors. This study investigated the effects of temperature and immigration on AHL levels and biofouling in anaerobic membrane bioreactors. The hypothesis was that the immigrant microbial community would increase the AHL-mediated QS, thus stimulating biofouling and that low temperatures would exacerbate this. We observed that presence of immigrants, especially when exposed to low temperatures indeed increased AHL concentrations and fouling in the biofilms on the membranes. At low temperature, the concentrations of the main AHLs observed, N-dodecanoyl-L-homoserine lactone and N-decanoyl-L-homoserine lactone, were significantly higher in the biofilms than in the sludge and correlated significantly with the abundance of immigrant bacteria. Apparently low temperature, immigration and denser community structure in the biofilm stressed the communities, triggering AHL production and excretion. These insights into the social behaviour of reactor communities responding to low temperature and influx of immigrants have implications for biofouling control in bioreactors.

Mitigation of membrane biofouling in membrane bioreactor treating sewage by novel quorum quenching strain of Acinetobacter originating from a full-scale membrane bioreactor

A novel quorum quenching (QQ) strain, Acinetobacter guillouiae ST01, was isolated from a full-scale membrane bioreactor (MBR) and characterized for its QQ activities. Batch reactor studies at lab-scale showed that A. guillouiae ST01 exhibited higher QQ activity against acyl homoserine lactones (AHLs) with an oxo group compared to those without an oxo group. The organism was then inoculated (10%) in an MBR (Q-MBR) treating sewage over 48 days and was found to reduce quorum sensing (QS) activity by reducing AHL concentrations in the sludge and the biofilm of the Q-MBR. The concentration of polysaccharides was reduced up to 30% in both the biofilm and sludge relative to the control, whereas protein concentrations were reduced by 40% and 47% in the sludge and biofilm, respectively. The Q-MBR fouling rates were halved. These results indicate that A. guillouiae ST01 is a promising strain for biofouling reduction in MBR treating real wastewater.

A single point mutation converts a glutaryl-7-aminocephalosporanic acid acylase into an N-acyl-homoserine lactone acylase

Objective

To change the specificity of a glutaryl-7-aminocephalosporanic acid acylase (GCA) towards N-acyl homoserine lactones (AHLs; quorum sensing signalling molecules) by site-directed mutagenesis.

Results

Seven residues were identified by analysis of existing crystal structures as potential determinants of substrate specificity. Site-saturation mutagenesis libraries were created for each of the seven selected positions. High-throughput activity screening of each library identified two variants—Arg255Ala, Arg255Gly—with new activities towards N-acyl homoserine lactone substrates. Structural modelling of the Arg255Gly mutation suggests that the smaller side-chain of glycine (as compared to arginine in the wild-type enzyme) avoids a key clash with the acyl group of the N-acyl homoserine lactone substrate.

Conclusions

Mutation of a single amino acid residue successfully converted a GCA (with no detectable activity against AHLs) into an AHL acylase. This approach may be useful for further engineering of ‘quorum quenching’ enzymes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10529-021-03135-9.