Insights into azithromycin biodegradation by freshwater microalgae Chlorella thermophila: Assessing removal kinetics and co-metabolic activity

• Toxicological impact of azithromycin on Chlorella thermophila was studied. • 96 h EC 50 of azithromycin for Chlorella thermophila was 63.6 μg/L. • Azithromycin influenced the biochemical characteristics of microalgal cells. • MDA and SOD of Chlorella thermophila were significantly increased by azithromycin. • Sodium acetate acts as an electron donor and enhanced azithromycin removal. Azithromycin, a widely utilized macrolide antibiotic, presents considerable environmental challenges due to its persistence in aquatic ecosystems, which contributes to ecological imbalances and the emergence of antibiotic resistance. This study aims to investigate the biodegradation potential of Chlorella thermophila , a freshwater microalga, specifically focusing on its capacity for azithromycin removal under controlled experimental conditions. The research evaluates critical parameters, including degradation efficiency, biochemical stress responses, and the impact of co-metabolic substrates. To assess cellular stress responses, growth inhibition, chlorophyll and carotenoid levels, and malondialdehyde activity were meticulously analyzed. The findings reveal that Chlorella thermophila achieved a removal efficiency of 63 ± 1% after 12 days at an initial azithromycin concentration of 100 μg L⁻¹, primarily through biodegradation with minimal alterations to cell surface integrity. Additionally, the study assessed the role of co-metabolic substrates in enhancing degradation, with sodium acetate significantly improving removal efficiency to 81%. This research provides novel insights into the mechanisms underlying microalgal antibiotic degradation and highlights the potential of Chlorella thermophila as a bioremediation agent for emerging contaminants in aquatic environments. By addressing existing gaps in antibiotic removal strategies and demonstrating the viability of microalgal-based remediation, this study contributes to the advancement of sustainable wastewater treatment solutions.