Self-assembly of algal-bacterial granules induced by bacterial N-acyl-homoserine lactone variation in response to high-strength pyridine

Algal-bacterial granules (ABGs) system represents a promising technology for organic wastewater treatment due to its high settleability, efficient oxygen transfer, and low-energy consumption. However, the secretion of extracellular polymeric substances (EPS) in algae, which played a key role in self-assembly of ABGs, would be inhibited by concentrated organic wastewater. This study proposed a novel strategy for developing ABGs by inducing bacterial N-acyl-homoserine lactone (AHL) variation through high-strength pyridine application. Results showed that bacterial long-chain AHL concentrations significantly increased in response to high-strength pyridine at 550 mg L^-1, inducing the secretion of algal extracellular aromatic proteins and facilitating ABGs construction. The ABGs system achieved over 99% pyridine removal efficiency and 82% settleability. Moreover, the proportions of β-sheet and α-helix structures in the extracellular aromatic proteins of ABGs increased, while the random coil structures decreased. This shift in protein structure lowered the surface free energy and energy barriers, which in turn enhanced the surface hydrophobicity and promoted cell adhesion. Furthermore, based on metatranscriptomic analysis, the mechanism for AHL-regulated physiological and behavioral responses between algae and bacteria in ABGs was proposed. This study provides an economically feasible approach to develop efficient and sustainable ABGs systems for industrial wastewater treatment.