Silica Nanoparticle Size Determines the Mechanisms Underlying the Inhibition of Iron Oxide Nanoparticle Uptake by Daphnia magna .

Aquatic environments are complicated systems that contain different types of nanoparticles (NPs). Nevertheless, recent studies of NP toxicity, and especially those that have focused on bioaccumulation have mostly investigated only a single type of NPs. Assessments of the environmental risks of NPs that do not consider co-exposure regimes may lead to inaccurate conclusions and ineffective environmental regulation. Thus, the present study examined the effects of differently sized silica NPs (SiO 2 NPs) on the uptake of iron oxide NPs (Fe 2 O 3 NPs) by the zooplankton Daphnia magna . Both SiO 2 NPs and Fe 2 O 3 NPs were well dispersed in the experimental medium without significant heteroaggregation. Although all three sizes of SiO 2 NPs inhibited the uptake of Fe 2 O 3 NPs, the underlying mechanisms differed. SiO 2 NPs smaller than the average mesh size (∼200 nm) of the filtering apparatus of D. magna reduced the accumulation of Fe 2 O 3 NPs through uptake competition, whereas larger SiO 2 NPs inhibited the uptake of Fe 2 O 3 NPs mainly by reducing the water filtration rate of the daphnids. Overall, in evaluations of the risks of NPs in the natural environment, the different mechanisms underlying the effects of NPs of different sizes on the uptake of dissimilar NPs should be considered.