Distinguishing the effects of Ce nanoparticles from their dissolution products: identification of transcriptomic biomarkers that are specific for ionic Ce in Chlamydomonas reinhardtii.

Cerium (Ce) is a rare earth element that is incorporated in numerous consumer products, either in its cationic form or as engineered nanoparticles (ENPs). Given the propensity of small oxide particles to dissolve, it is unclear whether biological responses induced by ENPs will be due to the nanoparticles themselves or rather due to their dissolution. This study provides the foundation for the development of transcriptomic biomarkers that are specific for ionic Ce in the freshwater alga, Chlamydomonas reinhardtii, exposed either to ionic Ce or to two different types of small Ce ENPs (uncoated, ∼10 nm, or citrate-coated, ∼4 nm). Quantitative reverse transcription PCR was used to analyse mRNA levels of four ionic Ce-specific genes (Cre17g.737300, MMP6, GTR12, and HSP22E) that were previously identified by whole transcriptome analysis in addition to two oxidative stress biomarkers (APX1 and GPX5). Expression was characterized for exposures to 0.03-3 µM Ce, for 60-360 min and for pH 5.0-8.0. Near-linear concentration-response curves were obtained for the ionic Ce and as a function of exposure time. Some variability in the transcriptomic response was observed as a function of pH, which was attributed to the formation of metastable Ce species in solution. Oxidative stress biomarkers analysed at transcriptomic and cellular levels confirmed that different effects were induced for dissolved Ce in comparison to Ce ENPs. The measured expression levels confirmed that changes in Ce speciation and the dissolution of Ce ENPs greatly influence Ce bioavailability.