Insights into aquatic toxicities of the antibiotics oxytetracycline and ciprofloxacin in the presence of metal: Complexation versus mixture

Co-contamination of ligand-like antibiotics (e.g., tetracyclines and quinolones) and heavy metals prevails in the environment, and thus the complexation between them is involved in environmental risks of antibiotics. To understand toxicological significance of the complex, effects of metal coordination on antibiotics' toxicity were investigated. The complexation of two antibiotics, oxytetracycline and ciprofloxacin, with three heavy metals, copper, zinc, and cadmium, was verified by spectroscopic techniques. The antibiotics bound metals via multiple coordination sites and rendered a mixture of various complexation speciations. Toxicity analysis indicated that metal coordination did modify the toxicity of the antibiotics and that antibiotic, metal, and their complex acted primarily as concentration addition. Comparison of EC50 values revealed that the complex commonly was highest toxic and predominately correlated in toxicity to the mixture. Finally, environmental scenario analysis demonstrated that ignoring complexation would improperly classify environmental risks of the antibiotics.