The significance of nanomaterial post-exposure responses in Daphnia magna standard acute immobilisation assay: Example with testing TiO2 nanoparticles

One of the most widely used aquatic standarized tests for the toxicity screening of chemicals is the acute toxicity test with the freshwater crustacean Daphnia magna, which has also been applied in the toxicity screening of manufactured nanoparticles (NPs). However, in the case of non-soluble NPs most of the results of this test have showed no effect. The aim of the work presented here was to modify the standardized test by the least possible extent to make it more sensitive for non-soluble particles. The standard acute immobilisation assay with daphnids was modified by prolonging the exposure period and by measuring additional endpoints. Daphnids were exposed to TiO2 NPs in a standard acute test (48 h of exposure), a standard acute test (48 h of exposure) followed by 24 h recovery period in clean medium or a prolonged exposure in the NPs solutions totaling 72 h. Together with immobility, the adsorption of NPs to body surfaces was also observed as an alternative measure of the NPs effects. Our results showed almost no effect of TiO2 NPs on D. magna after the 48 h standard acute test, while immobility was increased when the exposure period to TiO2 NPs was prolonged from 48 h to 72 h. Even when daphnids were transferred to clean medium for additional 24h after 48h of exposure to TiO2 NPs the immobility increased. We conclude that by transferring the daphnids to clean medium at the end of the 48 h exposure to TiO2 NPs, the delayed effects of the tested material can be seen. This methodological step could improve the sensitivity of D. magna test as a model in nanomaterial environmental risk assessment.