Uptake and effects of cerium(III) and cerium oxide nanoparticles to Chlamydomonas reinhardtii.

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 Lcell-1 (wild type) and 9.0 × 10-5 mol Lcell-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.

Influence of agglomeration of cerium oxide nanoparticles and speciation of cerium(III) on short term effects to the green algae Chlamydomonas reinhardtii.

Cerium oxide nanoparticles (CeO2 NP) are increasingly used in industrial applications and may be released to the aquatic environment. The fate of CeO2 NP and effects on algae are largely unknown. In this study, the short term effects of CeO2 NP in two different agglomeration states on the green algae Chlamydomonas reinhardtii were examined. The role of dissolved cerium(III) on toxicity, its speciation and the dissolution of CeO2 NP were considered. The role of cell wall of C. reinhardtii as a barrier and its influence on the sensitivity to CeO2 NP and cerium(III) was evaluated by testing both, the wild type and the cell wall free mutant of C. reinhardtii. Characterization showed that CeO2 NP had a surface charge of ∼0mV at physiological pH and agglomerated in exposure media. Phosphate stabilized CeO2 NP at pH 7.5 over 24h. This effect was exploited to test CeO2 NP dispersed in phosphate with a mean size of 140nm and agglomerated in absence of phosphate with a mean size of 2000nm. The level of dissolved cerium(III) in CeO2 NP suspensions was very low and between 0.1 and 27nM in all tested media. Exposure of C. reinhardtii to Ce(NO3)3 decreased the photosynthetic yield in a concentration dependent manner with EC50 of 7.5±0.84µM for wild type and EC50 of 6.3±0.53µM for the cell wall free mutant. The intracellular level of reactive oxygen species (ROS) increased upon exposure to Ce(NO3)3 with effective concentrations similar to those inhibiting photosynthesis. The agglomerated CeO2 NP caused a slight decrease of photosynthetic yield at the highest concentrations (100µM), while no effect was observed for dispersed CeO2 NP. The low toxicity of agglomerated CeO2 NP was attributed quantitatively to Ce(3+) ions co-occurring in the nanoparticle suspension whereas for dispersed CeO2 NP, dissolved Ce(3+) was precipitated with phosphate and not bioavailable. Furthermore CeO2 NP did not affect the intracellular ROS level. The cell wall free mutant and wild type of C. reinhardtii showed the same sensitivity to CeO2 NP and Ce(NO3)3, indicating a minor role of the cell wall on toxicity. For both algae strains, a flocculation of cells was observed upon exposure to agglomerated CeO2 NP and Ce(NO3)3, only algae exposed to agglomerated CeO2 NP were tightly packed in exopolymeric substances.

Chemical Aspects of Nanoparticle Ecotoxicology.

Nanoecotoxicology strives to understand the processes and mechanisms by which engineered nanoparticles (ENP) may exert toxic effects on aquatic organisms. Detailed knowledge of the chemical reactions of nanoparticles in the media and of their interactions with organisms is required to understand these effects. The processes of agglomeration of nanoparticles, of dissolution and release of toxic metal ions, and of production of reactive oxygen species (ROS) are considered in this article. Important questions concern the role of uptake of nanoparticles in various organisms, in contrast to uptake of ions released from nanoparticles and to nanoparticle attachment to organism surfaces. These interactions are illustrated for effects of silver nanoparticles (AgNP), cerium oxide (CeO2 NP) and titanium dioxide (TiO2 NP), on aquatic organisms, including algae, biofilms, fish cells and fish embryos.