Abstract: In this study, we examined the photosynthetic response of diatom Thalassiosira pseudonana to UV radiation under different nitrogen concentrations. Results showed that the maximum photochemical yield (F_v/F_m) of PSII was significantly inhibited by UV radiation, which was aggravated by nitrogen limitation. Under nitrogen limitation, the net closing rate of the reaction center (M_o) increased, the electron transfer efficiency from Q_A^- to Q_B (ψ_o) decreased, the absorption efficiency of light energy (α) was depressed, which together reduced the tolerance of T. pseudonana cells to UV radiation. UV radiation, especially when under nitrogen limitation, significantly decreased the ratio of PSII repair rate to damage rate (r/k), indicating that the photoinhibition of PSII was intensified by UV radiation through the damage of repair process. Our results suggest that the combination of nitrogen limitation and UV radiation could decrease photosynthetic capacity of T. pseudonana, with consequences on the primary production.

Key words: diatom, photoinhibition, nitrogen limitation, UV radiation.