Effects of mercury on antioxidant mechanisms in the marine phanerogam Posidonia oceanica.

Biochemical markers of oxidative stress such as catalase activity, glutathione S-transferase (GST) activity and levels of lipid peroxidation evaluated in terms of thiobarbituric acid reactive substances (TBARS) were measured in the sheaths of the marine phanerogam Posidonia oceanica (L.) Delile experimentally exposed to 0.01, 0.1 and 1 microgHg l(-1) for 48 h. Up to a threshold concentration of 0.1 microg Hg l(-1), an increase in catalase and GST activities and TBARS levels was observed, indicating that the antioxidant mechanisms were overtaxed and could not prevent membrane lipid peroxidation. Paradoxically, at 1 microg Hg l(-1), the damage seemed to decrease, as the lipid peroxidation levels of exposed sheaths were lower than those of controls and as catalase and GST activities were not different from those of controls. A possible rapid induction of phytochelatins detoxifying mercury could occur at this high level of mercury.