Effects of undissociated SiO and TiO nano-particles on molting of Daphnia pulex: Comparing with dissociated ZnO nano particles.

The toxic effects of different nanoparticles (NPs) have been reported to be quite different. The present study exposed Daphnia pulex to undissociated TiO NPs and SiO NPs, and dissociated ZnO NPs. The acute toxicity of the three oxide NPs and their influence on D. pulex molting, as well as the expressions of genes related to molting, energy metabolism and genetic material expression were compared. The results showed that the toxicities of TiO NPs and SiO NPs to D. pulex were weaker than ZnO NPs. During the exposure period, agglomerates of undissociated TiO NPs and SiO NPs influenced movements of D. pulex, and induced their molting after attaching to the body surface. Meanwhile, gene expressions of molting (eip) and energy metabolism (scot and idh) were up-regulated. Therefore, we inferred that the adhering to the surface of daphnids, promoting their molting and improving their energy metabolism may be parts of the toxicity mechanisms of undissociated NPs to D. pulex. On the contrary, dissociated ZnO NPs inhibited molting and gene expressions of eip, scot and idh, which showed a similar trend as bulk ZnO and ZnSO·7HO under the low-dose exposure condition. This indicates that the toxic effects of dissociated ZnO NPs were primarily caused by released Zn ions. The results provided direct evidence about the effect of nanoparticles on molting and revealed that the toxicity mechanisms of dissociated NPs were different from undissociated NPs.