Ecotoxicity of ultraviolet radiation on ciliates: Population growth, DNA damage and photooxidative stress.

Global change drivers like solar ultraviolet radiation (UVR) are a central focus in ecology. Ciliates are ideal environmental indicators, but key gaps persist in assessing UVR ecotoxicity in these unicellular eukaryotes. To address this, we determined the 2-h UV-B LD50 (median lethal dose) for two model ciliates: Euplotes vannus and Paramecium multimicronucleatum. We exposed them to four UVR combinations: photosynthetically active radiation (PAR), PA (PAR + UV-A), PAB (PAR + UV-A + UV-B), and PB (UV-B only), applied at three intensities to clarify toxic mechanisms. E. vannus had significantly higher UV-B LD50 than P. multimicronucleatum. UVR inhibited population relative growth rate in the order PAR > PA > PB > PAB, with a significant interaction between irradiation intensity and light component. UVR increased reactive oxygen species (ROS); UV-A + UV-B synergism was more pronounced under high exposure and was stronger in P. multimicronucleatum than in E. vannus. The trends of cyclobutane pyrimidine dimer content mirrored those of ROS, varying with exposure intensity. High intensity UV-B enhanced caspase-3 activation, indicating stronger apoptosis under higher UVR. In conclusion, UVR is toxic to ciliates, with toxicity dependent on light composition, intensity, and species traits. DNA damage and photooxidative stress are core mechanisms, and apoptosis contributes to growth inhibition.