Antioxidant responses of different microalgal species to nonylphenol-induced oxidative stress

The antioxidant responses of four green microalgae, i.e., Chlorella vulgaris, Chlorella sp., Selenastrum capricornutum and Scenedesmus quadricauda, under control, low (0.1 mg L^?1) and high (1.0 mg L^?1) nonylphenol (NP) concentration were studied. The antioxidant responses of microalgae to NP depended on both NP concentrations and exposure time. The effects of NP on antioxidant responses were most obvious on the first day of exposure and the effects decreased with prolonged exposure time. At low NP concentration, there were no significant changes in activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), or in glutathione (GSH) content, in all four species, while high concentration of NP led to different changes in these parameters. In NP-tolerant species, i.e., C. vulgaris and Chlorella sp., activities of SOD, CAT and POD increased remarkably when exposed to high NP concentration, while the increase was less evident or insignificant in Se. capricornutum and Sc. quadricauda, the two NP-sensitive species. On the other hand, the malondialdehyde (MDA) content declined gradually with increase in NP concentrations, particularly in C. vulgaris and Chlorella sp. Similarly, NP exposure caused an inhibition of glutathione peroxidase (GPX) activity in all four species. However, the changes of glutathione reductase (GR) and glutathione S-transferase (GST) activity did not seem to correlate with the NP tolerance of microalgae. These results suggested that various antioxidant mechanisms were involved in microalgae when exposed to NP, and the NP-tolerant species displayed more evident and rapid changes in some antioxidant responses than the NP-sensitive ones.