Metabolomics reveals the mechanism of persistent toxicity of AgNPs at environmentally relevant concentrations to Daphnia magna†

Although the ecotoxicity of silver nanoparticles (AgNPs) has been of great concern, the persistence and underlying mechanisms of AgNP toxicity remain understudied. This study explored the persistent mechanisms of AgNP toxicity at two sizes (AgNP-10 nm and AgNP-70 nm at 2 μg L⁻¹) to Daphnia magna using traditional toxicological methods alongside metabolomics analyses during exposure and recovery phases. After 24 h, both AgNP-10 and -70 nm exposures resulted in high silver accumulation levels in D. magna, leading to reduced heart rate and paddling frequency. Despite a significant decrease in silver content after 24 h of recovery, the heart rate reduction persisted in AgNP-exposed D. magna. Metabolomics analysis revealed differential expression of 53 and 54 metabolites induced by AgNP-10 and -70 nm exposures, respectively, primarily enriched in lipid metabolism pathways. Following the recovery period, AgNP-10 and -70 nm induced differential expression of 71 and 110 metabolites, respectively, mainly enriched in lipid metabolism and protein digestion and uptake pathways. These findings indicate that the persistence of toxicity of D. magna induced by AgNPs at physiological and metabolomic levels, predominantly attributed to silver retention and damage to D. magna's digestive system. Overall, this study provides novel insights into the mechanism underlying the persistence of AgNP toxicity to aquatic organisms.