TiO₂ Nanoparticles Act As a Carrier of Cd Bioaccumulation in the Ciliate Tetrahymena thermophila

When nanoparticles can enter a unicellular organism directly, how may they affect the bioaccumulation and toxicity of other pollutants already present in the environment? To answer this question, we conducted experiments with a protozoan Tetrahymena thermophila. The well-dispersed polyacrylate-coated TiO₂ nanoparticles (PAA-TiO₂–NPs) were used as a representative nanomaterial, and Cd as a conventional pollutant. We found that PAA-TiO₂–NPs could get into Tetrahymena cells directly. Such internalization was first induced by low concentrations of Cd, but later suppressed when Cd concentrations were higher than 1 μg/L. Considering its significant adsorption on PAA-TiO₂–NPs, Cd could be taken up by T. thermophila in the form of free ion or metal-nanoparticle complexes. The latter route accounted for 46.3% of Cd internalization. During the 5 h depuration period, 4.34–22.1% of Cd was excreted out, which was independent of the concentrations of intracellular Cd and PAA-TiO₂–NPs. On the other hand, both free and intracellular Cd concentrations only partly predicted its toxicity at different levels of PAA-TiO₂–NPs. This may have resulted from PAA-TiO₂–NPs’ synergistic effects and the distinct subcellular distribution of Cd taken up via the two routes above. Overall, we should pay attention to the carrier effects of nanoparticles when assessing their environmental risks.