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

Exposure of C. reinhardtii to Ce(NO3)3 decreased the photosynthetic yield in a concentration dependent manner with EC50 of 7.5 ± 0.84 μM for wild type and EC50 of 6.3 ± 0.53 μM 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 (100 μM), while no effect was observed for dispersed CeO2 NP. The low toxicity of agglomerated CeO2 NP was attributed quantitatively to Ce3+ ions co-occurring in the nanoparticle suspension whereas for dispersed CeO2 NP, dissolved Ce3+ 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.