Silver Ion Release Accelerated in the Gastrovascular Cavity of Hydra vulgaris Increases the Toxicity of Silver Sulfide Nanoparticles

Silver nanoparticles (Ag-NPs) streamed into aquatic environments are chemically transformed into various forms, and one of the predominant forms is silver sulfide NPs (Ag2S-NPs). Because of the lower dissolution rate of silver ions (Ag+), the toxicity of Ag2S-NPs could be lower than that of Ag-NPs. However, the toxicity of Ag2S-NPs has been observed to be restored under oxidative or acidic conditions. In the present study, 4 aquatic organisms, Pseudokirchneriella subcapitata (algae), Daphnia magna (crustacean), Danio rerio (fish), and Hydra vulgaris (cnidarian), were exposed to Ag2S-NPs transformed from Ag-NPs using Na2S under anoxic conditions; and acute toxicity was evaluated. The acute toxicity of Ag2S-NPs was rarely observed in algae, crustaceans, and fish, whereas it was significantly restored in cnidarians. Although the dissolution rate was low in the medium exposed to Ag2S-NPs, high Ag+ was detected in H. vulgaris. To understand the mechanisms of Ag2S-NP toxicity in cnidarians, transcriptional profiles of H. vulgaris exposed to Ag-NPs, Ag2S-NPs, and AgNO3 were analyzed. As a result, most of the genes that were significantly changed in the Ag2S-NPs group were also found to be significantly changed in the AgNO3 group, indicating that the toxicity of Ag2S-NPs was caused by Ag+ dissolved by the acidic condition in the gastrovascular cavity of H. vulgaris. This finding is the first in an aquatic organism and suggests the need to reconsider the stability and safety of Ag2S-NPs in the aquatic environment. Environ Toxicol Chem 2021;00:1-11. (c) 2021 SETAC

Automated summary

The acute toxicity of Ag2S-NPs was low in algae, crustaceans, and fish, but significantly restored in cnidarians. This restoration of toxicity in cnidarians may be caused by Ag+ dissolved in the gastrovascular cavity under acidic conditions.