Dual role of titanium dioxide nanoparticles in the accumulation of inorganic and methyl mercury by crustacean Daphnia magna through waterborne and dietary exposure.

Titanium dioxide nanoparticles (nTiO) are widely used in numerous products, yet their role in the accumulation and transfer of other contaminants in the aquatic food webs is not well understood. The influence of nTiO on inorganic (IHg) and monomethyl mercury (MeHg) accumulation in invertebrate Daphnia magna through waterborne and dietary exposure was thus thoroughly investigated. The results showed that nTiO led to a substantial decrease of the total mercury body burden (THg) in D. magna in direct waterborne exposure to IHg/MeHg. However, exposure to nTiO pre-treated with IHg/MeHg resulted in an increase of the THg body burden in daphnids. The presence of nTiO led to a substantial decrease of the THg in D. magna when exposed to IHg/MeHg via algal food. These effects were more pronounced for IHg than that for MeHg due to the higher adsorption capabilities of nTiO for IHg. In addition, high concentrations of nTiO favored the trophic transfer of IHg/MeHg through feeding on nTiO pre-treated with Hg, however lessened it when D. magna were fed on alga pre-treated with IHg/MeHg. Comparable assimilation efficiency (AE), determined as Hg retained in daphnids after depuration, was observed in D. magna when exposed to IHg/MeHg via algal food regardless the absence or presence of 20 mgL nTiO. By contrast, an increase of the AE of MeHg through feeding on nTiO and alga was found in the presence of higher concentration of 200 mgL nTiO. The present results will help to better understand the role of nTiO on bioavailability and trophic transfer of global contaminants, such as mercury, known to bioaccumulate and biomagnify in the aquatic environment.