Keywords: 
• antibiotics /
• heavy metals /
• Chlorella pyrenoidosa /
• joint toxicity /
• classical model /
• comparative evaluation /
• time-dependent microplate toxicity analysis

Abstract: Contaminants commonly present in the form of mixtures in the environment, and their cumulative toxicity and toxicity interaction have potential environmental risks. Therefore, aminoglycoside antibiotics (apramycin sulfate, dihydrostreptomycin, streptomycin sulfate) and heavy metal (Zn) commonly existing in aquatic environment were selected as target contaminants, Chlorella pyrenoidosa (C. pyrenoiosa) was taken as a test organism, and a direct equipartition ray method was used to design three binary mixture systems of three antibiotics and Zn in this study. The time-concentration-toxicity data of the three antibiotics, heavy metal Zn and their mixture systems toward C. pyrenoiosa in different exposure time was determined using the time-dependent microplate toxicity analysis method. Concentration addition (CA) and independent action (IA) were used as the standard additive reference models to analyze toxicity interaction within mixture systems and their changings with time. The results showed that toxicity of the three antibiotics and Zn toward C. pyrenoiosa gradually increased with the lengthening of exposure time. The two models, CA and IA, were basically consistent in the assessment of the toxicity interaction within three binary mixtures, i.e., the additive effect was always present in the low-concentration region, while the synergistic effect gradually changed to the additive effect in the high-concentration region with the extension of exposure time. For the same mixtrue system, deviations between the predicted toxicity by CA and IA models increased with the increasing of concentration. The predicted curves by IA were always above those predicted by CA, which indicated the toxicity predicted by IA was more accurate than that predicted by CA when assessing mixtures with different modes of action.