The impact of silver nanoparticles (AgNPs) on aquatic algae has largely been studied with model species that possess a rigid cell wall. Here, we explored the interactions of AgNPs with Euglena gracilis, a green alga having no cell wall but a pellicle. The toxicity and silver uptake upon 1-2 h of exposure to various concentrations of AgNO3 and AgNPs, having a mean size of 47 nm measured in the exposure medium, were examined. The photosynthetic yield decreased in a concentration-dependent manner and AgNPs were less toxic than AgNO3 based on the total silver added. The cell morphology was significantly altered by AgNPs and AgNO3. The damaging effects of AgNPs on the photosynthesis and morphology were completely prevented by cysteine, suggesting that the toxicity of AgNPs was mediated by dissolved Ag. Indeed, the maximal quantity of cell-associated silver was higher upon exposure to AgNPs compared to that upon AgNO3 exposure, amounting to 5.1 x 10(-4) mol L-cell(-1) and 1.4 x 10(-4) mol L-cell(-1) for AgNPs and AgNO3, respectively. However, the difference was not caused by the cellular uptake of AgNPs, but by the strong sorption of AgNPs onto the pellicle.