The toxic effects of norfloxacin (NOR) and butylated hydroxyanisole (BHA) on the microalga Scenedesmus obliquus were assessed in terms of growth rate, concentration of chlorophyll a, activities of 7-ethoxyresorufin-o-dealkylases (EROD), glutathione s-transferase (GST), catalase (CAT) and total malondialdehyde content (MDA). The 96 h EC50 was 38.49 and 11.12 mg/l for NOR and BHA, respectively. Growth of S. obliquus was affected slightly under low concentrations of BHA (<4 mg/l) and NOR (<15 mg/l) over the 96 h exposure period. With the increasing concentrations of these two compounds, growth of S. oblique decreased significantly. Growth inhibition was 82.4% at 60.0 mg/l for NOR and 60.6% at 16.0 mg/l for BHA after 96 h. A similar trend was also observed for chlorophyll a. NOR and BHA affected Phase I and Phase II enzyme activities differently. Upon exposure to NOR, EROD was induced at concentration <15.0 mg/l and depressed at concentrations >30 mg/l significantly. CAT and GST exhibited similar trends during the exposure period. Compared to controls, MDA content only showed high induction at high concentrations of NOR (>30 mg/l). However, EROD activity did not display any change compared to control responses during BHA exposure, whereas GST showed significant induction for all concentrations over the exposure period. CAT activity showed induction at low concentration and depression at high concentration. MDA content increased with the rise of BHA during the exposure period. These types of assays, revealing toxic effects of NOR and BHA to phototrophs, could be employed to assess the potential risks of these xenobiotics to aquatic ecological systems.
