Unraveling stress responses of microalgal-bacterial granular sludge when treating ciprofloxacin-laden wastewater.

Unraveling the potential of microalgal-bacterial granular sludge (MBGS) technology for sustainable treatment of ciprofloxacin (CIP)-laden wastewater and mitigation of antibiotic resistance genes (ARGs) remains limited. This study evaluated the performance of bacterial granular sludge (BGS) and MBGS systems in terms of nutrient and CIP removal, granular stability, and ARG attenuation under long-term exposure to CIP for the first time. While both systems achieved effective pollutant removal at low CIP concentrations (0.1 and 0.5 mg/L), MBGS demonstrated superior resilience and efficiency under high CIP loads (10 mg/L). Notably, MBGS improved phosphorus removal by 32.71 %, achieved a 70.42 μg/(g-SS)/d greater CIP removal and maintained structural integrity, unlike BGS, which disintegrated under oxidative stress. The microalgae species (Pseudoneochloris and Chlamydopodium) could effectively resist various concentrations of CIP. Additionally, the relative abundance of ARGs in MBGS was 30.91 % lower than that in BGS, suggesting that microalgae in MBGS system could reduce ARG production. Overall, these findings improve our understanding of the role of microalgae in enhancing CIP remediation and controlling ARG propagation in MBGS systems.