Contrasting effects of UV-A and UV-B on photosynthesis and photoprotection of β-carotene in two Dunaliella spp.

Photosynthetic and antioxidant responses following exposure to either ultraviolet-A or ultraviolet-B were contrasted in two species of the unicellular green alga, Dunaliella. Species selection was based on the ability of Dunaliella bardawil (UTEX 2538) to accumulate interthylakoid beta-carotene when subjected to environmental stress while Dunaliella salina (UTEX 200) lacks this ability. Cells were cultured in high and low levels of visible light (150 and 35 mumol photons m(-2) s(-1), respectively) and then either ultraviolet-A (320-400 nm) or ultraviolet-B (290320 nm) was added to visible light for 24-h exposure. A potassium chromate solution was found to be an ideal screen for removal of ultraviolet-A and ultraviolet-C from ultraviolet-B radiation. There were no significant changes in photosynthetic or antioxidant parameters following exposure to ultraviolet-B. Ultraviolet-A exposure significantly decreased photosynthetic parameters (>70% decrease in Fv/Fm and the ratio of light-limited to light-saturated photosynthesis in low beta-carotene cells) and resulted in 50% increases in ascorbate peroxidase activity and ascorbate concentrations. The results suggest exposure to ultraviolet-A (but not ultraviolet-B) directly affects photosynthesis, observed as a loss of photosystem II electron transport efficiency and increased radical formation. This research indicates that the accumulated beta-carotene in D. bardawil prevents UV-related photosynthetic damage through blue-light/ultraviolet-A absorption (supported by trends observed for antioxidant enzyme responses).