► Tetrahymena thermophila was exposed to TiO2 particles at sub-toxic concentrations. ► Exposure to TiO2 particles resulted in changed cell membrane in fatty acid profile resulting in increased membrane rigidity. ► Altered cell membrane composition is not accompanied by lipid peroxidation or ROS elevation. ► Particles were detected in food vacuoles where filling and subsequent release was followed. We provide experimental evidence that changes in the membrane fatty acid profile of Tetrahymena thermophila 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 24h at 32°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.