Combined toxicity of erythromycin and roxithromycin and their removal by Chlorella pyrenoidosa

The ecological effects of antibiotics in surface water have attracted increasing research attention. In this study, we investigated the combined ecotoxicity of erythromycin (ERY) and roxithromycin (ROX) on the microalgae, Chlorella pyrenoidosa, and the removal of ERY and ROX during the exposure. The calculated 96-h median effect concentration (EC₅₀) values of ERY, ROX, and their mixture (2:1 w/w) were 7.37, 3.54, and 7.91 mg∙L^-1, respectively. However, the predicted EC₅₀ values of ERY+ROX mixture were 5.42 and 1.51 mg∙L^-1, based on the concentration addition and independent action models, respectively. This demonstrated the combined toxicity of ERY+ ROX mixture showed an antagonistic effect on Chlorella pyrenoidosa. During the 14-d culture, low-concentration (EC₁₀) treatments with ERY, ROX, and their mixture caused the growth inhibition rate to decrease during the first 12 d and increase slightly at 14 d. In contrast, high-concentration (EC₅₀) treatments significantly inhibited microalgae growth (p < 0.05). Changes in the total chlorophyll contents, SOD and CAT activities, and MDA contents of microalgae suggested that individual treatments with ERY and ROX induced higher oxidative stress than combined treatments. After the 14-d culture time, residual Ery in low and high concentration Ery treatments were 17.75% and 74.43%, and the residual Rox were 76.54% and 87.99%, but the residuals were 8.03% and 73.53% in ERY+ ROX combined treatment. These indicated that antibiotic removal efficiency was higher in combined treatments than that in individual treatments, especially at low concentrations (EC₁₀). Correlation analysis suggested that there was a significant negative correlation between the antibiotic removal efficiency of C. pyrenoidosa and their SOD activity and MDA content, and the enhanced antibiotic removal ability of microalgae benefited from increased cell growth and chlorophyll content. Findings in this study contribute to predicting ecological risk of coexisting antibiotics in aquatic environment, and to improving biological treatment technology of antibiotics in wastewater.