The toxicity of coated silver nanoparticles to the alga Raphidocelis subcapitata

The use of silver nanoparticles (AgNPs) is growing exponentially, especially in consumer products due to their excellent antimicrobial properties. However, concerns are growing on their possible negative effects on environmental and human health. AgNPs from consumer products enter aquatic ecosystems where their physicochemical properties including surface functionalization are critical to their impact on aquatic organisms. The effects of AgNPs coated with three different ligands; tyrosine (T-AgNP), epigallocatechin gallate (E-AgNP) and curcumin (C-AgNP) and -Ag+ ions on the freshwater green alga Raphidocelis subcapitata were investigated. Stability tests of AgNPs revealed that the coating significantly affects the fate and behaviour of AgNPs. All types of AgNPs tested and ionic silver were found to be toxic to the alga and differential growth inhibition of algae were observed from differently coated AgNPs, with the 48 h EC50 of C-AgNPs, T-AgNPs and E-AgNPs being 0.155, 0.163 and 0.243 mg L-1, respectively, in comparison with 0.051 mg L-1 for -Ag+ ions. Associated Ag in the algae increased with increased concentrations of all AgNPs and -Ag+ ions and the toxicity positively correlated to the associated Ag content in algae. The antioxidant enzymes glutathione S-transferase and catalase were activated in algal cells by the AgNPs and -Ag+ ions, but a consistent difference in response was not identified with different concentrations of NPs. This study shows the effects of the surrounding environment and surface functionalization of AgNPs on algae highlighting the importance of considering them in environmental risk assessment of AgNPs.