Changes of the acute and chronic toxicity of three antimicrobial agents to Daphnia magna in the presence/absence of micro-polystyrene

The effects of microplastics on aquatic organisms are drawing growing attention, but little has been focused on their effects on the toxicity of other chemicals. In this study, we examined the acute and chronic toxicity of micro-polystyrene (5.8 μm dia.), and its effects on the toxicity of three antimicrobial agents (triclosan, triclocarban and methyl-triclosan) to Daphnia magna. Results indicated that polystyrene had a low acute toxicity with an EC50 of 36.5 mg/L. The presence of polystyrene (1 mg/L) did not produce significant effect on the acute toxicity of three chemicals, because the 95% confidence intervals of their EC50 values had a large overlap of 11.3%–48.3%. For the 21 day chronic toxicity, polystyrene alone had significant toxicity with concentrations of at least 2 mg/L, which prolonged the time of the first brood, limited the number of broods, and reduced the total number of neonates. Compared with the chemicals alone, the addition of polystyrene enhanced their reproduction toxicity. Based on the various reproduction indicators, an intrinsic rate of natural increase (rm) was calculated to assess the rate of population growth. Results suggested that the rm values of three chemicals decreased in the presence of PS, and further decreased with increasing PS concentrations. Among the three chemicals, methyl-triclosan was the most affected. These results suggested that the presence of microplastics would exacerbate the detrimental influence of pollutants on Daphnia magna.