Deciphering responses of sulfur autotrophic denitrification system to the single and joint stress of Zn(II) and dialkyldimethyl ammonium compounds: Performance, microbial community and different fractions of resistance genes

Zn(II) and dialkyldimethyl ammonium compounds (DADMAC) are common heavy metal and disinfectant contaminants in wastewater treatment plants. The response of sulfur autotrophic denitrification (SAD) system under single and joint stress with different concentrations of Zn(II) (1–25 mg/L) and DADMAC (0.2–10 mg/L) was evaluated using sequencing batch fixed-bed biofilm reactors in this study. The joint stress of high concentrations of Zn(II) and DADMAC had stronger inhibitory effects on microbial activity and metabolic function genes compared to the single stress, thus significantly inhibiting the performance of the SAD system. Furthermore, Zn(II) stress contributed to the proliferation of resistance genes (RGs) in water (w-RGs), whereas stress of DADMAC was able to induce the proliferation of intracellular RGs (si-RGs) and extracellular RGs (se-RGs) in sludge. And under the joint stress of Zn(II) and DADAMC, the abundances of si/se-RGs were much higher than that under single stress, with a higher risk of RGs transmission. The α-Helix/(β-sheet + random coil) in tightly bound extracellular polymeric substances correlated positively with si- tnpA-04 and se- tnpA-04 . Horizontal gene transfer via mobile genetic elements was the main mode of RGs transmission. This work assessed the risk of DADAMC and Zn(II) in the SAD system and established new insights into the transmission of three-fraction RGs under DADMAC and Zn(II) stress.