The use of silver nanoparticles (AgNPs) in agriculture and many consumer products has led to a significant release of Ag in the environment. Although Ag toxicity in terrestrial organisms has been studied extensively, very little is known about the accumulation capacity and coping mechanisms of organisms in Ag-contaminated soil. In this context, we exposed Eisenia fetida earthworms to artificial OECD soil spiked with a range of concentrations of Ag (AgNPs or AgNO3). The main aims were to (1) identify the location and form of accumulation of Ag in the exposed earthworms and (2) better understand the physiological mechanisms involved in Ag detoxification. The results showed that similar doses of AgNPs or AgNO3 did not have the same effect on E. fetida survival. The two forms of Ag added to soil exhibited substantial differences in speciation at the end of exposure, but the Ag speciation and content of Ag in earthworms were similar, suggesting that biotransformation of Ag occurred. Finally, 3D images of intact earthworms obtained by X-ray micro-computed tomography revealed that Ag accumulated preferentially in the chloragogen tissue, coelomocytes, and nephridial epithelium. Thus, E. fetida bioaccumulates Ag, but a regulation mechanism limits its impact in a very efficient manner. The location of Ag in the organism, the competition between Ag and Cu, and the speciation of internal Ag suggest a link between Ag and the thiol-rich proteins that are widely present in these tissues, most probably metallothioneins, which are key proteins in the sequestration and detoxification of metals.
Keywords: Accumulation; Earthworm; Nanomaterials; Silver; Speciation; X-ray absorption spectroscopy; X-ray micro-computed tomography.