Combined effects of sulfamethazine and sulfamethoxazole on a freshwater microalga, Scenedesmus obliquus: toxicity, biodegradation, and metabolic fate

Highlights: • Scenedesmus obliquus was tolerant to SMZ, SMX, and their mixture, and could withstand their high doses. • 96 h EC₅₀ of SMZ, SMX, and their mixture for S. obliquus was 1.23, 0.12, and 0.89 mg L^-1. • The risk quotients of SMZ, SMX, and their mixture were >1. • The removal of SMZ and SMX was more in the mixture than their individual medium. • Metabolic pathways of SMZ and SMX by S. obliquus were proposed. -- Abstract: This study investigated the environmental effects of two common emerging contaminants, sulfamethazine (SMZ) and sulfamethoxazole (SMX), and their mixture using a green microalga, Scenedesmus obliquus. The calculated EC₅₀ values of SMZ, SMX, and their mixture (11:1 wt/wt) after 96 h were 1.23, 0.12, and 0.89 mg L^-1, respectively. The toxicity of the mixture could be better predicted using a concentration addition model than an independent action model. The risk quotients of SMZ, SMX, and their mixture were >1 during the experiment, indicating their high potential risks on aquatic microorganisms. Despite their toxicity, S. obliquus exhibited 17.3% and 29.3% removal of 0.1 mg L^-1 and 0.2 mg L^-1 after 11 days of cultivation. The changes of SMZ and SMX removal were observed when combined, which showed a significantly improved removal of SMZ (up to 3.4 folds) with addition of SMX (0.2 mg L^-1). The metabolic pathways of SMZ and SMX were proposed according to mass spectroscopic analysis, which showed six metabolites of SMX and seven intermediates of SMZ, formed as a result of ring cleavage, hydroxylation, methylation, nitrosation, and deamination.