Algal testing of titanium dioxide nanoparticles—Testing considerations, inhibitory effects and modification of cadmium bioavailability

The ecotoxicity of three different sizes of titanium dioxide (TiO2) particles (primary particles sizes: 10, 30, and 300 nm) to the freshwater green alga Pseudokirchneriella subcapitata was investigated in this study. Algal growth inhibition was found for all three particle types, but the physiological mode of action is not yet clear. It was possible to establish a concentration/dose–response relationship for the three particle sizes. Reproducibility, however, was affected by concentration-dependent aggregation of the nanoparticles, subsequent sedimentation, and possible attachment to vessel surfaces. It is also believed that heteroaggregation, driven by algal exopolymeric exudates, is occurring and could influence the concentration–response relationship. The ecotoxicity of cadmium to algae was investigated both in the presence and absence of 2 mg/L TiO2. The presence of TiO2 in algal tests reduced the observed toxicity due to decreased bioavailability of cadmium resulting from sorption/complexation of Cd2+ ions to the TiO2 surface. However, for the 30 nm TiO2 nanoparticles, the observed growth inhibition was greater than what could be explained by the concentration of dissolved Cd(II) species, indicating a possible carrier effect, or combined toxic effect of TiO2 nanoparticles and cadmium. These results emphasize the importance of systematic studies of nanoecotoxicological effects of different sizes of nanoparticles and underline the fact that, in addition to particle toxicity, potential interactions with existing environmental contaminants are also of crucial importance in assessing the potential environmental risks of nanoparticles.