Comparison on the effects of water-borne and dietary-borne accumulated ZnO nanoparticles on Daphnia magna

Nanoparticles (NPs) can exert toxicity to aquatic animals through water and dietary exposure. However, little is known about the contribution of the two exposure pathways to the total toxicity of NPs to aquatic animals. In the present study, we fed Daphnia magna with ZnO NPs-accumulated-algae or directly exposed them to ZnO NPs suspension at the same Zn concentration. The body Zn concentrations were the same in both treatments, but the surface elastic modulus in the water-borne group was lower than that in the dietary-borne group, indicating more severe surface damage in the water-borne treatment. Total egg numbers of the first brood in both Zn treatments were significantly lower than the control, but they were not significantly different between the two treatments. However, the first brood in the water-borne treatment was distinctly delayed than that in the dietary-borne group. The dietary-borne treatment suppressed the expression of Hem and SOD genes. The water-borne treatment significantly suppressed the expression of Pro, SOD, VTG, but increased the GST expression. In particular, the expression of Pro, a gene functions in modulation of cell multiplication, was lower in the water-borne treatment, which might be related to the suppression of body growth in D. magna. The expression of SOD in the water-borne treatment decreased by about four times in comparison with the dietary-borne treatment, and the SOD enzyme activity of the water-borne treatment was significantly lower than that of the dietary-borne treatment. These results indicate that the two exposure routes have different targets and responding toxic mechanisms.