The response mechanisms of microalgal mutant Chlorella PY-ZU1 cells were investigated in their removal of antibiotic tilmicosin from wastewater under 15% CO2. Low concentrations (0.01-2 mg L-1) of tilmicosin in wastewater stimulated the growth of microalgal cells, whereas high concentrations (5-50 mg L-1) of tilmicosin significantly inhibited cell growth. When initial tilmicosin concentration increased from 0 to 50 mg L-1, fractal dimension of microalgal cells monotonically increased from 1.36 to 1.62 and cell size monotonically decreased from 4.86 to 3.75 mu m. In parallel, malondialdehyde content, which represented the degree of cellular oxidative damage, monotonically increased from 1.92 x 10(-7) to 7.07 x 10(-7) nmol cell(-1). Superoxide dismutase activity that represented cellular antioxidant capacity first increased from 2.59 x 10(-4) to the peak of 6.60 x 10(-4) U cell(-1), then gradually decreased to 2.39 x 10(-4) U cell(-1). The maximum tilmicosin removal efficiency of 99.8% by Chlorella PY-ZU1 was obtained at the initial tilmicosin concentration of 50 mg L-1. (C) 2016 Elsevier B.V. All rights reserved.