Organic matter excreted by the protozoan Tetrahymena thermophila and its effects on the bioaccumulation of nanoparticles

Although organic matter (exudate) excreted by aquatic organisms is an important component of dissolved organic matter (DOM) in the natural environment, its potential effects on the bioaccumulation of nanoparticles (NPs) remain unclear. In the present study, we examined the effects of the exudates from the protozoan Tetrahymena thermophila on the bioaccumulation (including uptake and cell surface adsorption) of iron oxide (Fe₂O₃, polyacrylate coated) and silica (SiO₂) NPs in T. thermophila. The exudates were mostly (93.6 %, in carbon) composed of < 1-kDa molecules (e.g., lipids). When the exudates were mixed with the NPs, significant adsorption occurred on SiO₂ NPs but not on Fe₂O₃ NPs. Independent of their adsorption by the NPs, the exudates significantly inhibited the bioaccumulation of both SiO₂ NPs and Fe₂O₃ NPs by T. thermophila. This inhibitory effect was shown to be mainly due to their inhibition of NP adsorption on the cell surface. By contrast, the exudates had negligible effects on the uptake of either NP type, most likely due to their low molecular weight. Since DOM in the aquatic environment is dominated by molecules < 1 kDa, the potential effects of low-molecular-weight DOM, such as exudates from aquatic organisms, on the bioaccumulation of NPs merits greater attention. ENVIRONMENTAL IMPLICATION: Nanoparticles (NPs) are hazardous materials widespread in the natural environment. Previous studies showed that dissolved organic matter (DOM) in aquatic environments determine the environmental behavior and ecological effects of NPs. Although organic matter (exudate) excreted by aquatic organisms is an important component of DOM, its potential effects on the bioaccumulation of NPs remain unclear. In the present study, we found that the exudates inhibited the cell-surface adsorption of NPs but had no effects on NP uptake, as different from the well-known effects of DOM on NP bioaccumulation. This finding merits attention during evaluations of the environmental risks of NPs.