This paper investigated toxicity of three engineered nanoparticles (ENP), namely, Al2O3, SiO2, and TiO2to the unicellular green algae, exemplified byPseudokirchneriella subcapitatawith an emphasis on particle size. The changes in pH, cell counts, chlorophyll a, and lipid peroxidation were used to measure the responses of the algal species to ENP. The most toxic particle size was TiO2at 42 nm with an EC20 of 5.2 mg/L and Al2O3at 14–18 nm with an EC20 of 5.1 mg/L. SiO2was the least toxic with an EC20 of 318 mg/L. Toxicity was positively related to the surface charge of both ENP and algae. The chlorophyll content of the algal cells was influenced by the presence of ENP, which resulted in limited light and availability of nutrients due to increase in turbidity and nutrient adsorption onto the ENP surface, separately. Lipid peroxidation was attributed to reactive oxygen species (ROS). Fast reaction between algal cells and ROS due to direct contact between TiO2and algal cells is an important factor for lipid peroxidation.

Nano-sized titanium dioxide (nano-TiO 2 ) has wide industrial applications and therefore considerable chances of exposure are created for human beings and ecosystems. To better understand the interactions between nano-TiO 2 and aquatic organisms, we first studied TiO 2 uptake by algae exemplified by Pseudokirchneriella subcapitata. P. subcapitata were exposed to nano-TiO 2 in a series of concentrations and at various pH. TiO 2 uptake was quantified using a sedimentation curve analysis technique. After exposure of algae to TiO 2 , the variation of zeta potential was measured and the morphology of algae-TiO 2 aggregate was observed with scanning electron microscopy and the optical microscopy. The steady-state TiO 2 uptake was found to be pH-dependent and the isotherms can be described well by Freundlich model. TiO 2 deposited on algal surfaces causes the shift of pH zpc of TiO 2 -covered algae from that of algae toward that of TiO 2 . The attraction between TiO 2 -covered algal cells induces the agglomeration of algae and TiO 2 and thus the formation of algae-TiO 2 aggregates in the size of 12 to 50 µm. The 2-D fractal dimension of the aggregates is pHdependent and ranges from 1.31 to 1.67. The theoretical analysis of the Gibbs energy of interaction indicates that both TiO 2 uptake by algae and the formation of algae-TiO 2 aggregate are influenced by the interaction between TiO 2 particles.