Effect and mechanism of TiO2 nanoparticles on the photosynthesis of Chlorella pyrenoidosa.

Titanium dioxide nanoparticles (n-TiO2) have been used in numerous applications, which results in their release into aquatic ecosystems and impact algal populations. A possible toxic mechanism of n-TiO2 on algae is via the disruption of the photosynthetic biochemical pathways, which yet remains to be demonstrated. In this study, Chlorella pyrenoidosa was exposed to different concentrations (0, 0.1, 1, 5, 10, and 20 mg/L) of a type of anatase n-TiO2, and the physiological, biochemical, and molecular responses involved in photosynthesis were investigated. The 96 h half growth inhibition concentration (IC50) of the n-TiO2 to algae was determined to be 9.1 mg/L. A variety of cellular and sub-cellular damages were observed, especially the blurry lamellar structure of thylakoids, indicating the n-TiO2 impaired the photosynthetic function of chloroplasts. Malondialdehyde (MDA) and glutathione disulfide (GSSG) significantly increased while the glutathione (GSH) content decreased. This implies the increased consumption of GSH by the increased intracellular oxidative stress upon n-TiO2 was insufficient to eliminate the lipid peroxidation. The contents of photosynthetic pigments, including chlorophyll a (Chl a) and phycobiliproteins (PBPs) in the exposed algal cells increased along with the up-regulation of genes encoding Chl a and photosystem II (PS II), which could be explained by a compensatory effect to overcome the toxicity induced by the n-TiO2. On the other hand, the photosynthetic activity was significantly inhibited, indicating the impairment on the photosynthesis via damaging the reaction center of PS II. In addition, lower productions of adenosine triphosphate (ATP) and glucose, together with the change of gene expressions suggested that the n-TiO2 disrupted the material and energy metabolisms in the photosynthesis. These findings support a paradigm shift of the toxic mechanism of n-TiO2 from physical and oxidative damages to metabolic disturbances, and emphasize the threat to the photosynthesis of algae in contaminated areas.