N-Acyl-Homoserine Lactone (AHL)-Mediated Microalgal–Bacterial Communication Driving Chlorella-Activated Sludge Bacterial Biofloc Formation

N-acyl-homoserine lactones (AHLs) as autoinducers of Gram-negative bacteria for quorum sensing regulation have shown positive effects on the production of aromatic proteins in extracellular polymeric substances (EPSs) during bioflocculation. To investigate the role of AHLs in aromatic protein production, a Chlorella–bacteria system with great bioflocculation was established via fed-batch cultivation. Tryptophan and aromatic proteins as the main compounds in the EPS of bioflocs showed an increasing trend during fed-batch cultivation. The Chlorella cells only secreted tryptophan rather than aromatic proteins during axenic cultivation. N-dodecanoyl-l-homoserine lactone (C12-HSL) was correlated with the flocculation activity and extracellular protein content of bioflocs during fed-batch cultivation. The addition of exogenous C12-HSL enhanced the flocculation activity of the Chlorella–bacteria system and aromatic protein production in the EPS. Chlorella cells sensed exogenous C12-HSL and significantly upregulated the aromatic protein synthesis pathway during axenic cultivation. In addition, vanillin as a quorum-sensing inhibitor suppressed the positive effect of C12-HSL on flocculation activity and aromatic protein production and synthesis. This result indicated that vanillin intercepts the response of Chlorella cells to C12-HSL. Overall, C12-HSL is supposed to be an important signal molecule to achieve communication between Chlorella and Gram-negative bacteria and subsequently induce Chlorella cells to produce aromatic proteins for biofloc formation.