Effects of norfloxacin on the interaction between duckweed and its growth-promoting bacterial assemblages

The entry of antibiotics into aquatic environments can negatively affect various organisms. However, the specific effects of antibiotics on the interactions between aquatic plants and its growth-promoting bacterial assemblages (GPBA) remain inadequately explored. Thus, this study aimed to examine the performance of mono-culture and co-culture systems, involving duckweed and its GPBA, under varying norfloxacin concentrations (0, 0.05, 0.2, 0.5, and 1 mg L^-1), to elucidate the effects of norfloxacin on the interactions between duckweed and GPBA. The results revealed that norfloxacin concentrations of ≥ 0.5 mg L^-1 significantly inhibited duckweed growth and diminished the growth-promoting abilities of GPBA by reducing the relative abundances of plant growth-promoting bacteria (e.g., Pelomonas, Ensifer, Acidovorax and Sphingomonas). Furthermore, physiological analysis of duckweed suggests that GPBA may enhance the antioxidant capacity of duckweed, thereby alleviating membrane damage induced by norfloxacin. Additionally, norfloxacin concentrations of ≥ 0.2 mg L^-1 significantly reduced nitrogen and phosphorus removal efficiencies in the co-culture system. Overall, norfloxacin exerted a more pronounced negative impact on duckweed growth, microbial community and nutrient removal in the co-culture system compared to the mono-culture system, indicating that norfloxacin poses a heightened risk to the co-culture system by disrupting the interactions between duckweed and its GPBA.