Removal of antibiotics metronidazole, sulfamethoxazole, and trimethoprim by the green microalga Chlorella sorokiniana, and assessment of their biotransformation products

Antibiotic consumption for human and animal health is increasing. This results in significant quantities of antibiotics remaining unmetabolized, entering wastewater and contaminating aquatic environments. Antibiotics disrupt microbial communities and pose environmental and human health risks. Microalgae-based bioremediation has emerged as an effective strategy for antibiotic removal. However, degradation pathways that produce transformation products (TPs) impact on microalgal growth, and nutrient dynamics remain underexplored when comparing single and mixed antibiotics treatments. In this study, antibiotics sulfamethoxazole (SMX), trimethoprim (TMP), and the less well investigated metronidazole (MTZ), were spiked individually or in combination into cultures of the green microalga Chlorella sorokiniana in BG11 medium, and the mechanisms associated with their degradation evaluated (in terms of % removal). After 21 days, the highest removal efficiency was observed for MTZ (98.8%), followed by TMP (52%) and SMX (36.2%) under the single antibiotic condition. In the presence of mixed antibiotics, the removal efficiency decreased to 31.6% for MTZ, 25.4% for TMP, and 34.5% for SMX. Biodegradation was the primary mechanism, with removal of 82.2% for MTZ, 42.7% for TMP and 31.6% for SMX in the presence of single antibiotics, and reduced to 2% for MTZ, 20.3% for TMP and 7.5% for SMX in the mixture. TPs analysis and identification for MTZ, TMP, and SMX suggested possible degradation pathways. C. sorokiniana growth significantly reduced chemical oxygen demand, and decreased nitrate, ammonium, and phosphate content in the growth media. These findings demonstrate the potential of C. sorokiniana for antibiotic and nutrient bioremediation and pave the way for future assessment of TPs environmental risk, in particular those associated with MTZ degradation.