Antioxidant defenses and histological changes in Carassius auratus after combined exposure to zinc and three multi-walled carbon nanotubes

• The toxicity of different MWCNTs, Zn, and their mixtures in fish was evaluated.
• Oxidative stress biomarkers in fish liver were determined.
• Histopathological assessments were measured.
• Toxicity order was proposed via the IBR index.

With the increasing applications of carbon nanotubes (CNTs) worldwide, considerable concerns have been raised regarding their inevitable releases into natural waters and ecotoxicity. It was supposed that CNTs may interact with some existing pollutants like zinc in aquatic systems and exhibit different effects when compared with their single treatments. However, data on their possible combined toxicity on aquatic species are still lacking. Moreover, the interactions of Zn with different functionalized CNTs may be distinct and thereby lead to diverse results. It is like that functional groups play a vital role in illustrating the differences in toxicity among various CNTs. In this study, the single and joint effects of multi-walled carbon nanotubes (MWCNTs) and two MWCNTs functionalized with carboxylation (COOH-MWCNTs) or hydroxylation (OH-MWCNTs) in the absence or presence of zinc (Zn) on antioxidant status and histopathological changes in Carassius auratus were evaluated. Synergistic effect was tentatively proposed for joint-toxicity action, which was supported by apparently observed oxidative stress and histopathological changes in joint exposure groups. The integrated biomarker response index was calculated to rank the toxicity order, from which the conclusion of synergistic effect was strengthened. Regarding differences among various CNTs, our data showed that OH-MWCNTs and COOH-MWCNTs were more stressful to fish than raw MWCNTs. This finding sustained that functionalization is an important factor in nanotoxicity, which may serve as clues for future design and application of CNTs. Overall, these results provided some valuable toxicological data on the joint effects of CNTs and heavy metals on aquatic species, which can facilitate further understanding on the potential impacts of other coexisting pollutants in the culture of freshwater fish.