Anaerobic Acidogenesis Improvement and Fermentation-Type Regulation by Quorum Sensing

Anaerobic fermentation of organic wastewater is expected to produce short-chain fatty acids that are available as carbon sources for nitrogen and phosphorus removal in wastewater treatment. By long-term semicontinuous experiments, this study indicated that the introduction of exogenous N-acyl-homoserine lactone (AHL), a quorum sensing (QS) molecule, improved acidogenesis capacity and regulated fermentation type even at low pH. The product concentration increased from 13.5 gCOD/L in the control to 19.9 gCOD/L in the QS enhancement system. Moreover, the acidogenesis pathway related to acetyl-CoA, butyrate, and caproate production was also more highly expressed based on metagenomic sequencing accordingly. Notably, the introduction of exogenous AHL improved chain elongation (CE) during anaerobic fermentation and resulted in a 2.6-fold increase in caproate concentrations. Additionally, the abundance of caproate producers was also increased by 2.0–3.6 folds in the QS enhancement system. Metagenomic analysis results indicated that QS boosted the reverse β-oxidation pathways, and the higher acidogenesis pathway provided more lactate and butyrate available for CE. Importantly, QS enhancement upregulated genes associated with the detection of the typical acid stress signal. Concurrently, three typical acid stress resistance pathways, i.e., proton-consuming reactions, protons efflux, and extracellular polymeric substance production, were activated and highly expressed. Overall, this study proposes a novel strategy to improve microbial resistance to acidic conditions and to regulate the microbial community through QS enhancement and is potentially valuable to enhance resources and energy recovery by anaerobic fermentation.