Biological impacts of Ce nanoparticles with different surface coatings as revealed by RNA-Seq in Chlamydomonas reinhardtii

In order to better understand the risks of engineered nanoparticles (ENPs), it is necessary to determine their fate and biological effects under realistic exposure scenarios (e.g. low ENP concentrations). RNA-Seq was deployed to characterize the relative biological impacts of three small Ce ENPs (i.e. nominal size <20 nm, 70 μg L−1 Ce), with different coating properties (i.e. uncoated, citrate or poly-acrylic acid coated), towards a unicellular freshwater microalga, Chlamydomonas reinhardtii. After 2 h exposition at pH 7.0, distinct differences in transcriptomic effects were observed when comparing ionic Ce and Ce ENPs. Notably, Ce ENPs specifically modulated mRNA levels of genes related to the ubiquitin-proteasome system and to flagella structure. Compared to control conditions, transcriptomic effects induced by the citrate coated Ce ENPs were rather limited, as only 23 genes were differentially expressed by this treatment (Log2FC > |1.0|, padj < 0.001); compared to uncoated Ce ENPs (688); polyacrylic coated Ce ENPs (315) or a similar concentration of ionic Ce (138). Somewhat surprisingly, similar changes in the algal transcriptomes were observed for treatments with poly-acrylic acid coated Ce ENPs (mainly Ce(III), little dissolution) and uncoated Ce ENPs (mainly Ce(IV) atoms, largely agglomerated) (Log2FC > |1.0|, padj < 0.001). For the moderate exposure concentrations examined here, toxicity appeared to be minimal for both ionic Ce and Ce ENPs. Nonetheless, an important number of genes could not be assigned to a biological pathway. The study gives important insights with respect to the role of particle surface coatings on biological effects, the mechanisms of interaction of Ce ENP with a green alga, in addition to identifying several useful transcriptomic biomarkers of Ce ENP exposure.