Bioaccumulation of Nanosilver by Chlamydomonas reinhardtii-nanoparticle or the free ion?

The bioavailability of a small silver nanoparticle (nAg; nominal size of 5 nm with a polyacrylate coating) by the green alga C. reinhardtii was investigated in order to assess the contributions of Ag(+) and nAg to cellular internalization. Upon exposure to nAg, Ag biouptake exceeded what was predicted based upon measured Ag(+) concentrations. Indeed, although Ag biouptake was greatly reduced when excess cysteine was added to the nAg, it was nonetheless significantly above control levels. For both exposures to nAg and Ag(+), expression levels of the Copper Transport Protein 2 (CTR2) indicated that Ag biouptake could be attributed to the internalization of Ag(+). Exposure to Ag(+) or nAg increased CTR2 expression, even when cysteine was present with the nAg. Darkfield microscopy coupled with hyperspectral imagery showed that the presence of silver nanoparticles inside the cells was more likely due to the rereduction of Ag(+) than to the internalization of nAg. The weight of evidence indicated that nAg increased Ag biouptake by locally increasing the surface concentrations of Ag(+).