This study evaluated the toxicity and cellular stresses of ciprofloxacin (CIP) and its co-metabolic removal in a freshwater microalga Chiamydomonas mexicana. The toxicological effects of CIP on C. mexicana were assessed by studying the growth and biochemical characteristics of the microalga including total chlorophyll, carotenoid content, malondialdehyde (MDA) and superoxide dismutase (SOD) activity. The calculated effective concentration (EC50) of CIP on C. mexicana was 65 +/- 4 mg L-1 at 96 h. The growth of C mexicana was significantly inhibited at increased concentrations of CIP, showing 36 +/- 1, 75 +/- 13. and 88 +/- 13% inhibition at 40, 60 and 100 mg L-1 CIP, respectively, compared to the control after 11 days of cultivation. The total chlorophyll, carotenoid, MDA and SOD activity were significantly increased as a result of relatively high concentrations of CIP stress. C. mexicana showed 13 +/- 1% removal of CIP (2 mg L-1) after 11 days of cultivation; however, the addition of an electron donor (sodium acetate, 4g L-1) highly enhanced the removal of CIP (2 mg L-1) by >3-fold after 11 days. Kinetic studies showed that removal of CIP followed a first-order model (R-2 0.94-0.97) with the apparent rate constants (k) ranging from 0.0121 to 0.079 d(-1). (C) 2016 Elsevier B.V. All rights reserved.