We provide experimental evidence that changes in the membrane fatty acid profile of Tetrahymena mophila incubated with nano- or bulk TiO2 particle are not accompanied by ROS generation or lipid peroxidation. Consequently these changes are interpreted as acclimation to unfavorable conditions and not as toxic effects. T. thermophila cells were exposed to TiO2 particles at different concentrations for 24 h at 32 degrees C. Treatment of cultures with nano- and bulk TiO2 particles resulted in changes of membrane fatty acid profile, indicating increased membrane rigidity, but no lipid peroxidation or ROS generation was detected. There were no differences in membrane composition when T. thermophila was exposed to nanosized or bulk-TiO2 particles. We also observed reversible filling of food vacuoles, but this was different in case of nano- or bulk TiO2 exposure. Our results suggest that interactions of particles and cell membranes are independent of oxidative stress. (C) 2012 Elsevier B.V. All rights reserved.