| Summary: | Cerium oxide engineered nanoparticles (nCeO<sub>2</sub>) are widely used in various applications and are, also, increasingly being detected in different environmental matrixes. However, their impacts on the aquatic environment remain poorly quantified. Hence, there is a need to investigate their effects on non-target aquatic organisms. Here, we evaluated the cytotoxic and genotoxic effects of <25 nm uncoated-nCeO<sub>2</sub> on algae <i>Pseudokirchneriella subcapitata</i>. Apical (growth and chlorophyll <i>a</i> (Chl <i>a</i>) content) and genotoxic effects were investigated at 62.5–1000 µg/L after 72 and 168 h. Results demonstrated that nCeO<sub>2</sub> induced significant growth inhibition after 72 h and promotion post 96–168 h. Conversely, nCeO<sub>2</sub> induced enhanced Chl <i>a</i> content post 72 h, but no significant changes were observed between nCeO<sub>2</sub>–exposed and control samples after 168 h. Hence, the results indicate <i>P. subcapitata</i> photosynthetic system recovery ability to nCeO<sub>2</sub> effects under chronic-exposure conditions. RAPD-PCR profiles showed the appearance and/or disappearance of normal bands relative to controls; indicative of DNA damage and/or DNA mutation. Unlike cell recovery observed post 96 h, DNA damage persisted over 168 h. Thus, sub-lethal nCeO<sub>2</sub>-induced toxicological effects may pose a more serious threat to algae than at present anticipated.
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