Chronic Exposure to Environmentally Relevant Palladium Nanoparticles Reprograms Oxidative, Reproductive, and Genomic Stress Pathways in Zebrafish ( <i>Danio rerio</i> )

Palladium nanoparticles (Pd NPs), extensively used in automobile catalytic converters, are increasingly released into the environment and represent an emerging nanopollution concern for aquatic ecosystems. This study examined the chronic effects of environmentally relevant Pd NP exposure on the freshwater vertebrate model Danio rerio, integrating bioaccumulation analysis, oxidative stress profiling, histopathology, and bulk RNA-seq transcriptomics with computational cell-type inference analyses. Adult zebrafish were exposed for 42 days to low (0.4 ng/L) and high (22 ng/L) Pd NP concentrations. Inductively coupled plasma-mass spectrometry confirmed dose-dependent Pd bioaccumulation in whole-body tissues. Biochemical analyses indicated a disruption of gonadal redox homeostasis, characterized by altered activities of superoxide dismutase, catalase, glutathione S-transferase, glutathione reductase, and lipid peroxidation, indicating sustained oxidative stress. Histological examination of ovaries and testes demonstrated progressive structural damage, including follicular atresia, delayed oocyte maturation, and impaired spermatogenesis, highlighting reproductive vulnerability. Transcriptomic profiling showed concentration-dependent transcriptional changes under Pd NP exposure, including reduced expression of mitochondrial energy metabolism genes and increased expression of DNA repair, cell cycle regulation, steroid biosynthesis, and stress-response pathways. High-dose Pd exposure strongly increased the expression of cell cycle and stress-response genes, including ccnb1 (41 to 8296 TPM), cdc25b (41 to 1937 TPM), and tp53 (110 to 604 TPM), while mitochondrial energy metabolism genes were consistently suppressed. Notably, PI3K – AKT – mTOR, p53, and cell cycle signaling axes exhibited biphasic regulation, reflecting compensatory and maladaptive stress responses. This study identifies potential ecological and human health risks associated with palladium nanoparticle dispersal and emphasizes the need for safer catalyst design and stricter environmental management of platinum group nanoparticles.