The impact of morphology and size of zinc oxide nanoparticles on its toxicity to the freshwater microalga, Raphidocelis subcapitata

Microalgae are key test organisms to assess the effects of chemicals on aquatic ecosystems. Zinc oxide nanoparticles (ZnO NPs) as a widely used metal oxide is considered a potential threat to these primary producers at the base of the food chain. This study investigates the toxicity of ZnO NPs, bulk ZnO, and Zn2+ to the representative of freshwater microalgae, Raphidocelis subcapitata. To examine the effect of shape and size of nanoparticles, two types of spherical ZnO NPs with different sizes (20 and 40nm) and two types of rod-shaped ZnO NPs with different lengths (100 and 500nm) were synthesized. Microalgal cells were exposed to eight concentrations of each ZnO NP type from 0.01 to 0.7mg/L for 96h. The results showed that 0.7mg/L of ZnO NP could completely inhibit algal growth. Size did not interfere with toxicity in spherical ZnO NPs, but the toxicity decreased by increasing the size of rod-shaped ZnO NPs. Spherical ZnO NPs acted more destructive to microalgal cells than nanorod shape. The addition of 0.7mg/L of ZnO nanorods to samples caused 30% cell death, while 50% cell death was observed by adding the same concentration of nanospherical ZnO. Nano ZnO revealed to be more toxic than bulk ZnO and Zn2+. The Zn2+ released from dissolution of ZnO NPs was one of the sources of toxicity, but the ZnO nanostructures were also an important factor in the toxicity.