The role of AHLs-mediated quorum sensing in inducing the formation and stability of aerobic granular sludge under quinoline stress

Aerobic granular sludge (AGS) technology exhibits unique advantages in removing refractory pollutants, yet it remains constrained by the bottlenecks of a long start-up period and insufficient structural stability. Quorum sensing (QS) modulates bacterial community behavior and exerts a crucial regulatory role in promoting the formation and stability of AGS. Taking quinoline as the target pollutant, this study successfully cultivated AGS. Meanwhile, the potential role of QS in inducing AGS formation via regulating extracellular polymeric substances (EPS) was systematically investigated, and the feasibility of maintaining AGS stability through an exogenous addition strategy was evaluated. The results showed that AGS with excellent settling performance was cultivated under quinoline stress with a concentration gradient of 30-100 mg/L, with a 30-minute sludge volume index (SVI30) of 38.46 mL/g. The quinoline-degrading bacterium Rhodococcus, as well as EPS-producing bacteria Nakamurella and Micropruina were enriched in this AGS. During granule formation, the concentration of N-acyl homoserine lactones (AHLs) (with C4-HSL and C8-HSL as the main components) surged, concomitantly with a significant upregulation of AHL-associated synthesis genes (mainly rhlI) and sensing genes. QS activated by quinoline stress altered the composition and structure of EPS. EPS content significantly increased from 12.5 mg/gVSS to 97.5 mg/gVSS, and the PN/PS ratio reached 2.3. Elevated proportions of hydrophobic amino acids and monosaccharides in EPS resulted in enhanced relative hydrophobicity of AGS, reduced its negative surface charge, and caused the energy barrier to vanish, ultimately strengthening microbial aggregation. Furthermore, exogenous addition of 1 uM C8-HSL effectively improved granule stability by regulating EPS biosynthesis. This work elucidates the regulatory mechanism of QS in mediating AGS formation and stability under toxic stress, and lays a theoretical foundation for enhancing AGS-based wastewater treatment to resist toxic interference and maintain AGS stability.