Physiological response of marine centric diatoms to ultraviolet radiation, with special reference to cell size

Three centric diatoms, Thalassiosira pseudonana (diameter ~4 μm), Thalassiosira weissflogii (~11 μm), and Thalassiosira punctigera (~47 μm), were exposed to low and high levels of UV radiation. UV-induced inhibition on photosystem II was correlated with cell size under high light levels, though it was insignificant under low light levels (PAR<63 Wm(-2)). The highest inhibition (~15%) was observed for the smallest species. Several mechanisms may explain the observed relationship between cell size and response to UV. All three species counteracted UV-related photosystem damage via protein synthesis within the chloroplast. Non-photochemical quenching (NPQ) was induced when that process was blocked with an inhibitor in T. pseudonana and T. weissflogii, but not T. punctigera, as neither radiation nor the inhibitor had a significant effect on NPQ in this species. Moreover, UV-induced inhibition for cells treated with lincomycin was highest for T. weissflogii, which was in accordance with the highest UV exposure within the cell. The intracellular UV distribution was not associated with cell size, indicating that the package effect was not the only determinant of cell-size dependent UV sensitivity in phytoplankton.