The aim of this study was comparing the toxic effects of zinc oxide nanoparticles (ZnO NPs) versus zinc ions (Zn2+) at a high non-lethal (500μg/L) and a low environmental relevant (0.05μg/L) concentrations on gills of rainbow trout (Oncorhynchus mykiss) following 14 days of waterborne exposure. Structural alterations, histopathological anomalies, and zinc bioaccumulation were investigated in the gills using field emission scanning electron microscopy (FESEM), hematoxylin and eosin staining (H&E), and graphite furnace atomic absorption spectrophotometry (GFAAS) respectively. Some damages such as shortening and fusion of secondary lamellae, surface epithelium hypertrophy, and hyperplasia of the primary lamellae were observed in the gill tissue. Histopathological alterations of gills were minimum in both none exposed (control) fish and fish exposed to 0.05μg/L Zn2+. The severity of gill damages were higher in fish exposed to 500μg/L ZnO NPs compared to 500μg/L Zn2+ and 0.05μg/L ZnO NPs. The Zn accumulation in the gills was concentration-dependent such that bioaccumulation order was as 500μg/L Zn2+> 500μg/L ZnO NPs ≈ 0.05μg/L Zn2+> 0.05μg/L ZnO NPs> control. In summary, the results of present study showed that although the accumulation capability of Zn2+ was higher than ZnO NPs, but NPs cause more structural damages to gills compare to ions.

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