Effect of enhanced UV-B radiation on photosynthetic characteristics of marine microalgae Dunaliella salina (Chlorophyta, Chlorophyceae)

Ultraviolet B radiation (UV-B, 280-320 nm) is a natural stress factor that has the potential to negatively affect phytoplankton organisms. In the present study, the model marine microalga Dunaliella salina was exposed to UV-B enhancement (0.25-1.00 kJ m− 2 d− 1), and its photosynthetic processes, including light and carbon reactions, were analyzed. This study aimed to elucidate what occurs during microalgal photosynthesis under UV-B stress. The results revealed (1) the contents of both chlorophyll and carotenoids. The ratio between these was altered significantly with UV-B enhancement, particularly in the initial phase of exposure, and the ratios of chlorophyll a to b were markedly changed compared with the control. The results indicated that UV-B enhancement could interfere with the light absorption of D. salina, but the results suggest a well-developed strategy depending on the adjustment of photosynthetic pigment to cope with the stress. The damage to the chloroplast ultrastructure was analyzed by transmission electron microscopy. Their shape was irregular, and the lamellae thylakoids were fractured. The grana were disintegrated in the treated groups, and the extent of the damage increased with UVB enhancement. (2) The chlorophyll fluorescence parameters indicated that the maximal photochemical efficiency of PSII (Fv/Fm), the actual photochemical efficiency of PSII in the light (ΦPSII), the relative electron transport rate (rETR) and the photochemical quenching (qP) displayed few changes after initial exposure but were significantly decreased 48 h after UV-B enhancement compared with the control. The results indicate that UV-B stress can decrease the light energy utilization of photosystem II and thus damage the photosynthetic capacity in light reaction. (3) The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is the key carbon fixation enzyme, was observed to increase significantly in the treated groups compared with the control. This finding indicates that microalga have an adaptive mechanism to moderate UV-B radiation. The results of the present study suggest an alteration in energy distribution after exposure to different doses of UV-B radiation. Non-photochemical quenching may be the main pathway for the dissipation of excessive light energy in photosystem II exposed to low doses, whereas an improved photochemical quenching capacity may play an essential role in avoiding photoinhibition when exposed to higher doses. It appears that there is a dynamic balance between damage and adaptation in microalga that aids their coping with UV-B-induced alterations to the photosystem.