Cerium (Ce) and cerium oxide nanoparticles (CeO₂ NP) are increasingly used in different applications. Upon their release into the aquatic environment, the exposure of aquatic organisms becomes likely. In this study, the uptake of CeO₂ NP and Ce³⁺ into the wild type and cell wall free mutant of Chlamydomonas reinhardtii was examined upon short term exposure. Separation of CeO₂ NP and Ce³⁺ not taken up or loosely bound to the cells was performed by washing algae with EDTA.

Despite a concentration and time dependent increase of cellular Ce upon exposure to CeO₂ NP with the maximal calculated Ce concentration corresponding to 1.1 CeO₂ NP per cell, an internalization of CeO₂ NP with a mean size of 140 nm in C. reinhardtii was excluded. In contrast, dissolved Ce³⁺ (1 and 10 μM) was taken up both in the wild type and cell wall free mutant of C. reinhardtii, with a linear increase of cellular Ce within 1–2 h and maximal cellular Ce of 6.04 × 10⁻⁴ mol L_cell⁻¹ (wild type) and 9.0 × 10⁻⁵ mol L_cell⁻¹ (cell wall free mutant). Based on competition with Ca²⁺ for Ce³⁺ uptake, on the comparison of the wild type and the cell wall free mutant and on inhibition of photosynthetic yield, we suggest that no efficient uptake routes for Ce³⁺ are available in C. reinhardtii and that a fraction of the cellular Ce in the wild type strongly sorbs to the algal cell wall.