Toxicity of silver nanoparticles and ionic silver: Comparison of adverse effects and potential toxicity mechanisms in the freshwater clam Sphaerium corneum.

A range of studies has addressed possible environmental impacts of nanosilver, but most focused on acute effects in few species. Moreover, it remains unclear if toxic effects are particle-specific or mediated by released silver ions. We investigated chronic effects of nanosilver and soluble silver (AgNO3) on the freshwater bivalve Sphaerium corneum. Animals were exposed to nanosilver (0-500 μg Ag L(-1)) and AgNO3 (0-318 μg Ag L(-1)) over 28 days, and effects on reproduction and behavioral changes were assessed. To explore toxic mechanisms, we evaluated the effects on intracellular levels of reactive oxygen species (ROS) and the activity of antioxidant enzymes (superoxide dismutase, catalase, glutathione-S-transferase, glutathione peroxidase). We further explored the activity of the sodium-potassium adenosine triphosphatase (Na(+)/K(+)-ATPase). Chronic exposure to nanosilver and AgNO3 resulted in negative effects on reproduction at concentrations of 5 and 3.18 µg Ag L(-1) (LOEC), respectively. ROS levels significantly increased after exposure to nanosilver at 10 µg Ag L(-1) and AgNO3 at 63.5 µg Ag L(-1). Both forms of silver altered the activities of antioxidant enzymes. Nanosilver (500 μg Ag L(-1)) and AgNO3 (318 μg Ag L(-1)) inhibited Na(+)/K(+)-ATPase activity by 82.6 and 78.9%, respectively. Nanoparticulate and soluble silver produced similar effects in S. corneum suggesting that toxicity of nanosilver is mainly mediated by dissolution of nanoparticles in the test media or after uptake by the test organisms.