Response of the cyanobacterium Microcystis flos-aquae to levofloxacin.

The effects of levofloxacin (LEV) on Microcystis flos-aquae and its mechanism were investigated by determining the responses of some parameters of M. flos-aquae to LEV stress, including growth inhibition ratio, chlorophyll a content, superoxide dismutase (SOD) and catalase (CAT) activities, malondialdehyde (MDA) content, F v/F 0 and F v/F m, etc. The results indicated that LEV at 0.001-0.1 μg L(-1) could stimulate the growth of M. flos-aquae and increase the chlorophyll a content but did not induce a significant increase in the activity of antioxidant enzymes (SOD and CAT) and the content of MDA. When the LEV concentration exceeds 10 μg L(-1), the growth of M. flos-aquae could be significantly inhibited (the highest inhibition ratio can be up to 88.38 % at 100 μg L(-1)) and chlorophyll a content, SOD and CAT activities, and MDA content also significantly decreased in a concentration-dependent manner, indicating that high concentrations of LEV caused a severe oxidative stress on algal cells, resulting in a large number of reactive oxygen species produced in algal cells and thereby inhibiting the growth of algae. At the same time, the F v/F m and F v/F 0 values of M. flos-aquae decreased significantly with both exposure time and increasing test concentration of LEV, showing that the process of photosynthesis was inhibited.