The influence of zinc chloride and zinc oxide nanoparticles on air-time survival in freshwater mussels

The purpose of this study was to determine the cumulative effects of exposure to either dissolved zinc or nanozinc oxide (nanoZnO) and air-time survival in freshwater mussels. Mussels were exposed to each forms of zinc for 96 h then placed in air to determine survival time. A sub-group of mussels before and after 7 days of exposure to air were kept aside for the determination of the following biomarkers: arachidonate-dependent cyclooxygenase (COX) and peroxidase (inflammation and oxidative stress), lipid metabolism (total lipids, esterases activity, HO-glycerol, acetyl CoA and phospholipase A2) and lipid damage (lipid peroxidation [LPO]). The results showed that air-time survival was decreased from a mean value of 18.5 days to a mean value of 12 days in mussels exposed to 2.5 mg/L of nanoZnO although it was not lethal based on shell opening at concentrations below 50 mg/L after 96 h. In mussels exposed to zinc only, the median lethal concentration was estimated at 16 mg/L (10-25 95% CI). The air-time survival did not significantly change in mussels exposed to the same concentration of dissolved Zn. Significant weight losses were observed at 0.5 mg/L of nanoZnO and at 2.5 mg/L for dissolved zinc chloride, and were also significantly correlated with air-time survival (r = 0.53; p < 0.01). Air exposure significantly increased COX activity in control mussels and in mussels exposed to 0.5 mg/L of nanoZnO and zinc chloride. The data also suggested fatty acid breakdown and β-oxidation. Mussels exposed to contaminants are more susceptible to prolonged exposure to air during low water levels.