Indole inhibits anaerobic digestion by disrupting AHLs-mediated quorum sensing

The emerging understanding highlights indole as a disruptive factor to quorum sensing (QS) mechanisms, prompting further investigation into its role in anaerobic digestion (AD) system inhibition. However, relevant studies are still scarce and the potential mechanism linking indole and AD inhibition remains unclear. This study showed that indole (1, 2, and 3 mM) significantly reduced cumulative methane production by 8.47-51.89% and extended the lag phase by 1.34-6.68 days. Time-series AHLs quantification, metagenomics, and circular clustering heatmaps analysis revealed that indole might disrupt microbial communication between hydrolysis-acidification bacteria and acetoclastic methanogens by reducing the AHLs level (C6-HSL, 3-oxo-C8-HSL, C10-HSL, C12-HSL, 3-oxo-C10-HSL, and C18-HSL). Notably, indole degradation alleviated the inhibition of C10-HSL, C18-HSL, and 3-oxo-C10-HSL, which might restore hydrolysis and acidification and mitigate AD inhibition. Exogenous AHLs (1 and 5 µM) restored methane production by 48.44-55.59% in 3 mM indole-inhibited reactors (p < 0.05), while the quorum quenching agent vanillin further reduced methane production by 72.55%, suggesting that AHLs play an important role in helping microorganisms resist indole stress. These findings highlight the importance of AHLs-mediated inter-microbial communication in counteracting indole-inhibited methanogenesis inhibition, suggesting potential practical strategies to enhance AD stability and efficiency in challenging conditions.