Combined toxicity of nano-TiO2 and Cd2+ to Scenedesmus obliquus: Effects at different concentration ratios.

The continuous release of manufactured nanomaterials (MNMs) to environments raised concerns on their combined toxicological risks with co-existing contaminants, since MNMs might severely alter the environmental behavior and fate of the contaminants. In this study, the combined toxicity of nano-sized titanium dioxide (nTiO2) and cadmium (Cd2+) to the green alga Scenedesmus obliquus and the underlying physicochemical mechanisms were investigated for the first time at different concentration ratios of Cd2+ to nTiO2 to closely mimic the realistic environment scenarios where the concentration ratios of nTiO2 to other contaminants are constantly changing. Our results suggested that under the co-exposure to different concentration ratios of Cd2+ to nTiO2, the co-exposure contaminants exhibited three different combined toxicity modes (antagonistic, partially additive, and synergistic). Specifically, antagonistic combined toxicity was observed under co-exposure to a low concentration ratio of nTiO2 to Cd2+ as the absorption by nTiO2 decreased the bioavailability of Cd2+. However, the partially additive and synergistic combined toxicity occurred when the proportion of nTiO2 in the co-exposure system was relatively high, which would mechanically and/or oxidatively damage the alga cell structures. Even worse, as a carrier of Cd2+, nTiO2 enhanced the amount of Cd2+ entering cells, which significantly enhanced the toxicity of Cd2+ to algae. Overall, we demonstrated that concentration ratios of nTiO2 to Cd2+ play an important role in determining the combined toxicity mode, which would provide a novel reference to environmental and health risk assessment of co-exposure to conventional pollutants and MNMs.