Unraveling the impact of coexistent of oxytetracycline and nanomaterials on algal-bacterial granular sludge: Performance, microbial community variations and antibiotic resistance genes

The algal-bacterial granular sludge (ABGS) system is an emerging wastewater treatment technology. With widespread coexistence of nanomaterials and antibiotics in the environment, their combined impact on wastewater treatment has attracted significant attention. In this study, oxytetracycline (OTC), ZnO nanoparticles (ZnO-NPs) and polystyrene nanoparticles (PS-NPs) were selected as model pollutants. The performance, microbial community, and variation in antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) of ABGS systems were explored under three long-term exposure (0.5 mg/L OTC, 0.5 mg/L OTC and 1 mg/L ZnO-NPs, 0.5 mg/L OTC and 1 mg/L PS-NPs). The results revealed that the coexistence of PS-NPs and OTC inhibited denitrification and phosphorus removal. ZnO-NPs addition promoted phosphorus removal efficiency (89.57 %±5.42 %), attributed to the hormesis of microalgae, low abundance of Competibacteraceae (0.2 %), and precipitation. The coexistence of nanomaterials and OTC increased reactive oxygen species and lactate dehydrogenase levels by 34.08–50.55 % and 3.56–60.82 % compared to that of single OTC stress (248.09 and 7.36 U/g·MLSS), indicating that nanomaterials enhanced OTC toxicity by disrupting cell membranes. The extracellular polymeric substance and adenosine triphosphatase overproduction of ABGS served as the defensive response to combined stress. The presence of nanomaterials, particularly PS-NPs, facilitated ARGs spread and proliferation, through significant MGE amplification (2.01 times) and increased abundance of potential hosts. This study underscores the importance of considering the role of nanomaterials in developing strategies to mitigate the ARGs spread and provides valuable insights into the effects of nanomaterial-antibiotic coexistence on ABGS systems.