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

Abstract

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 β-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 μmol photons m-2 s-1, respectively) and then either ultraviolet-A (320-400 nm) or ultraviolet-B (290-320 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. UItraviolet-A exposure significantly decreased photosynthetic parameters (>70% decrease in Fv/Fm and the ratio of light-limited to light-saturated photosynthesis in low β-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 β-carotene in D. bardawil prevents UV-related photosynthetic damage through blue-light/ultraviolet-A absorption (supported by trends observed for antioxidant enzyme responses).