Toxicity of silver nanoparticles (AgNPs) in the model ciliate Paramecium multimicronucleatum: Molecular mechanisms of activation are dose- and particle size-dependent

Current understanding of toxicity mechanisms of nanoparticles is still far from comprehensive, partly because of the neglect of control factors such as the dependence of mechanism activation on the exposure dosage and particle size. To reveal molecular mechanisms of silver nanoparticle (AgNP) toxicity, the model ciliate Paramecium multimicronucleatum was exposed for 12 h to different concentrations of AgNPs with particle size of 20 nm (0.08, 0.12, and 0.30 mg/l) and 40 nm (0.08 and 0.30 mg/l). Transcriptomes of the tested ciliates were then analyzed based on dendrograms of gene expression, Gene Ontology (GO) terms, KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways, and up- and down-regulated genes. Results showed that: (1) toxicity mechanisms of AgNP revealed by analyses of GO and KEEG were significantly involved in the metabolic pathways of nutrients and the biosynthesis of macromolecules; (2) the top five up-regulated genes were mainly related to biological oxidation, biosynthesis, and oxidative stress, while top five down-regulated genes were mainly related to glycolysis; (3) activated mechanisms varied both in quantity and in type with dosages and particle sizes of AgNPs; (4) AgNP-treatments with different exposure dosages and particle sizes can produce the same toxicity in terms of 12 h-EC₅₀, but the underlying molecular mechanisms differed significantly. In brief, this study provides insights into the molecular mechanisms of AgNP toxicity through transcriptome analyses and confirmed their dependence of activation on the exposure dosage and particle size of AgNPs.