A study on the combined toxicity assessment of sulfamethoxazole and tetracycline hydrochloride in actual aquaculture wastewater using Chlorella vulgaris and the mechanisms of their removal.

The combined load of tetracycline and sulfonamide antibiotics in aquaculture wastewater poses a potential ecological risk to aquatic ecosystems. Based on actual aquaculture wastewater, this study investigated the combined toxicity of sulfamethoxazole (SMX) and tetracycline hydrochloride (TC-HCl) to Chlorella vulgaris, as well as the degradation mechanisms and ecological risks of their metabolites. The results indicate that TC-HCl is significantly more toxic to C. vulgaris than SMX, and that the combined use of these two substances produces a synergistic toxic effect. At low concentrations, SMX acts as a stimulant, promoting algal growth; in contrast, TC-HCl significantly inhibits algal growth, reduces chlorophyll content and the Fv/Fm ratio, and induces oxidative stress through membrane lipid peroxidation. Under stress conditions involving high concentrations of TC-HCl, the pH of the wastewater drops significantly, causing TC-HCl to convert to its cationic form, thereby increasing its bioavailability and toxicity. The degradation kinetics conform to a first-order model (R2 > 0.988). C. vulgaris demonstrated higher degradation efficiency for TC-HCl than for SMX, with biodegradation being the primary degradation mechanism. SMX is primarily degraded through the cleavage of the sulfonamide bond, whereas TC-HCl is mainly transformed through dehydration, demethylation, and ring-opening reactions. The ECOSAR assessment indicates that the acute toxicity of most metabolites is lower than that of the parent compound; however, some metabolites exhibit higher toxicity. This study confirms that C. vulgaris possesses a strong degradation capacity for mixed antibiotic loads; however, attention must be paid to the chronic ecological risks posed by the transformation products.