Bioaccumulation and homeostatic alterations in trout exposed to a sublethal dose of polystyrene nanoplastics

• Short-term (96h) of PS-NPs exposure induce an anti-inflammatory state in trout • PS-NPs affect minimally the trout antioxidant and metabolic responses • PS-NPs accumulate mainly in the gut and blood, but not in the liver of trout The extensive use and improper disposal of plastic materials, along with the degradation processes undergone by plastic debris and the challenges in managing plastic waste, have increased the prevalence of nanoplastics (NPs) in aquatic environments and trophic webs. To refine the description of the effects of NPs in fish, this study evaluated the metabolic, immune, and oxidative stress responses of adult rainbow trout ( Oncorhynchus mykiss ) following oral exposure to polystyrene nanoplastics (PS-NPs, 44 nm, 100 µg/L). A sublethal concentration of PS-NPs was administered via oral intubation to quantify bioaccumulation and deviations from homeostasis in blood, intestine, liver, head kidney, and spleen, 96 h post-exposure. PS-NPs accumulated predominantly in the gut and blood but were absent in the liver. Haematological analyses revealed decreased heterophil counts and increased mononuclear cells, while other parameters remained unchanged. Plasma analyses showed reduced cortisol levels, elevated triglycerides, and increased alkaline phosphatase activity. Gene expression profiling indicated modulation of immune and lipid metabolism pathways. Oxidative stress markers were largely unaltered, except for changes in catalase activity in the gut and liver. No histopathological damage was observed in the intestinal tissue. Overall, sublethal oral exposure to PS-NPs leads to selective accumulation and subtle immune and metabolic effects in rainbow trout, highlighting the need for further ecological investigation.