Signaling molecule alleviates inhibitory impacts of surfactant on methane production during sludge and food waste co-digestion: Insights of electron bifurcation and quorum sensing.

Anaerobic co-digestion of food waste (FW) and waste-activated sludge (WAS) is increasingly recognized as a viable solution for managing organic wastes. However, emerging contaminants (ECs), such as surfactant like sodium dodecylbenzene sulfonate (SDBS), can severely inhibit methane production. This study explores the potential of C6-HSL, a quorum sensing (QS) signaling molecule, to mitigate inhibitory effects of SDBS during FW and WAS co-digestion. Results demonstrated that SDBS reduced methane yields from 122.2 mL/g VSS in the control to 18.5 mL/g VSS, but supplementation with C6-HSL alleviated this inhibition, increasing yields to 115.4 mL/g VSS. C6-HSL not only restored suppressed methanogen populations but also promoted bacteria-archaea mutualisms, enhancing system resilience and stability. Additionally, C6-HSL enhanced key electron bifurcation pathways critical for overcoming thermodynamic barriers in methane metabolism, increasing the relative abundance of functional genes involved in four methane metabolism modules. Moreover, C6-HSL enhanced QS system (e.g., SecY and trpE), prompting microorganisms to activate adaptive mechanisms, such as DNA replication (e.g., rfcL and rfcS), efflux pumps (e.g., mdlA and mdlB), and bacterial chemotaxis (e.g., cheB and cheD), to counter SDBS toxicity. Correspondingly, TCA cycle (e.g., fumA and fumB) was also upregulated to ensure sufficient energy and electrons for methane production and microbial adaptation.