Evaluating the effect of exogenous phytohormones on bioremediation efficiency and physiological responses of Chlorella sorokiniana under ciprofloxacin exposure.

Treatment of antibiotic-contaminated wastewater using microalgal technology has recently attracted attention for its environmentally friendly nature and potential for resource recovery. However, antibiotic exposure may impact microalgal growth and physiological functions, potentially limiting the use of this technology. This study investigated the influence of phytohormones, specifically kinetin (KIN), indole-3-acetic acid (IAA), and gibberellic acid (GA), on enhancing the growth, physiology, and bioremediation efficiency of Chlorella sorokiniana in response to ciprofloxacin (CIP) exposure. IAA and GA significantly increased biomass accumulation by 67.7-139.1% and 74.1-151.4% during the mid-log phase, and by 43.5-54.8% and 44.1-69.4% during the late-log phase, respectively, compared to the control. The maximum CIP removal efficiency was observed under IAA supplementation (87.97 to 93.70%), followed by GA (84.96 to 88.76%), and KIN (48.75 to 76.05%), compared to the treatment without phytohormones (43.63 to 76.6%). Mechanistic analysis revealed that biodegradation was the dominant pathway for CIP removal. LC-MS analysis identified multiple transformation intermediates, indicating that CIP degradation proceeded via hydroxylation, oxidation, defluorination, dealkylation, and cleavage of the piperazine ring. Furthermore, IAA supplementation significantly increased the activities of superoxide dismutase (156.73%), catalase (175.2%), and ascorbate peroxidase (107%) at 10 mg L-1 CIP compared with the control. Metabolomic profiling revealed changes in lipid, hydrocarbon, and antioxidant metabolite metabolism, indicating an adaptive response to CIP and phytohormones. The findings highlighted the potential of phytohormones in enhancing microalgal bioremediation efficiency and biomass production under CIP exposure.