Toxicity of cerium and thorium on Daphnia magna

Cerium (Ce) and thorium (Th) are always thought to be chemically similar and have comparable toxic properties on living organisms. In the present study, the acute and chronic toxicity of these two elements to freshwater crustacean Daphnia magna were investigated in the modified reconstituted water (6mg/L KCl, 123mg/L MgSO₄·7H₂O, and 294mg/L CaCl₂·2H₂O in Milli-Q water, pH 7.8). It seemed that Ce and Th had comparable acute toxicity on Daphnia: 24/48h EC₅₀ for Th and Ce were 7.3/4.7μM and 16.4/10.7μM, respectively. However, Ce was present as soluble ions while all of Th was present as particulate ThO₂ in the exposure medium. Considering their different chemical forms and bioavailability, the toxic mechanisms of Ce³⁺ and ThO₂ on Daphnia would be totally different. To our knowledge, this is the first time to investigate the aquatic toxicity of thorium and cerium based on their actual chemical speciation in the exposure medium. The results also suggest that more attention should be paid on the detrimental effect of Th in the form of particulate ThO₂.