Effects and mechanisms of exogenous acyl-homoserine lactones on nitrogen removal from aquaculture wastewater.

To improve microbial nitrogen removal efficiency in aquaculture wastewater, we added different carbon-chain-length acyl-homoserine lactones (AHLs) to continuous-flow reactors, including N-butyryl-L-homoserine lactone (C4-HSL), N-hexanoyl-L-homoserine lactone (C6-HSL), N-octanoyl-L-homoserine lactone (C8-HSL), N-dodecanoyl-L-homoserine lactone (C12-HSL). We investigated their effects on nitrogen removal performance, sludge characteristics, and microbial community functions, and further analyzed the underlying mechanisms with metagenomics. The results showed that, compared with the nitrate concentration of (5.09±2.79) mg·L-1 in the control effluent, all AHLs improved nitrate removal, reducing the effluent nitrate concentrations during stable ope-ration to (2.60±2.20), (1.87±1.31), (2.55±1.92), and (2.51±2.12) mg·L-1, respectively. Among them, C6-HSL showed the best performance, which increased the nitrate removal rate by 8.0%, while maintained nitrite and total ammonia nitrogen at relatively low levels of (0.62±0.61) mg·L-1 and (0.13±0.13) mg·L-1, respectively. AHLs altered sludge surface morphology and extracellular polymeric substance composition, and promoted the enrichment of nitrogen-removing functional bacteria such as Denitratisoma, with the relative abundance of which being increased by 1.4%-3.4%. Metagenomic analysis indicated that AHLs differentially regulated functional genes related to quorum sensing, two-component systems, and nitrogen metabolism. Specifically, C4-HSL mainly enhanced genes associated with complete denitrification and biofilm formation. C6-HSL increased the expression of genes related to denitrification and dissimilatory nitrate reduction to ammonium. C8-HSL strengthened genes involved in phosphorus stress response and nitrogen fixation. C12-HSL promoted the expression of iron-acquisition-related functional genes. In conclusion, exogenous AHLs could enhance nitrogen removal by regulating microbial quorum sensing and metabolic processes, with C6-HSL showing the best perforemence.