Endogenous N-acyl homoserine lactones improve the efficiency of swine wastewater treatment by regulating biological activity and functional microbiota in microalgae-bacteria system

The influence and the underlying mechanism of endogenous N-acyl-homoserine lactones (AHLs) on microalgae-bacteria (MB) system treating swine wastewater remain unclear. In this study, Scenedesmus quadricauda BLMCX5 and Brevundimonas intermedia BMMCX3 were utilized to construct a MB system, and then vanillin was added to establish a microalgae-bacteria-vanillin (MBV) system as control. The results indicated C6-HSL and 3-oxo-C10-HSL as the main endogenous AHLs. Endogenous AHLs alleviated oxidative stress by enhancing antioxidant enzyme activity and promoting microalgal growth and photosynthetic efficiency. The microalgal biomass and chlorophyll a concentration in MB system (4.51 g/L and 7.79 mg/L, respectively) were significantly higher than MBV system (3.43 g/L and 6.66 mg/L). In addition, MB system demonstrated higher removal efficiencies of nitrogen and phosphorus (>99.6 %) and chemical oxygen demand (87.46 %) than MBV system (87.60 %–91.46 %, >99.3 % and 81.12 %, respectively). Microbial community analysis determined the reduction of antibiotic resistance genes loads and migration. Nitrogen metabolism pathway analysis revealed ammonia assimilation as the primary nitrogen removal pathway. The expression of nitrogen transporters and ammonia assimilation-related genes in microalgae significantly increased in MB system. Furthermore, the dissimilatory nitrate reduction and organic nitrogen hydrolysis in MB system, and the denitrification in MBV system, were also important. AHLs mediated by Brevundimonas and Acinetobacter directly affected nitrogen metabolism or indirectly promoted the relative abundance of genes associated with nitrogen metabolism by enriching key functional bacteria. This study revealed role of bacteria-mediated AHLs in improving the wastewater treatment efficiency of MB systems, and provided a theoretical foundation for application of microalgal bacterial technology.