Photoprotective role of beta -carotene and antioxidant enzymes: A shield against ultraviolet damage in Dunaliella bardawil
Photosynthetic and antioxidant responses following exposure to an array of irradiances and frequencies were examined and contrasted in two species of the unicellular green alga Dunaliella. Species selection was based on the ability of Dunaliella bardawil (UTEX 2538) to accumulate massive quantities of inter-thylakoid beta-carotene during exposure to stress or high irradiances of photosynthetically active radiation (PAR: 400-700 nm) while Dunaliella salina (UTEX 200) lacks this ability. Cells were cultured in two irradiances (150 and 35 mumol photons ·m-2·s -1), and then exposed to high red (wavelengths > 590 nm) or blue light (390-540 nm; 150 mumol photons·m -2·s--1), ultraviolet-A (320-400 nm) or ultraviolet-B (290-320 nm), independently, for 24 hour test periods. Ascorbate and glutathione concentrations, ascorbate peroxidase activity, photosynthetic oxygen evolution and fluorescence parameters were monitored following exposure to UV and various light treatments to assess changes in photosynthetic processes. In addition, polyacrylamide gel electrophoresis was utilized to quantify the number of catalase and superoxide dismutase isoenzymes present in Dunaliella bardawil and Dunaliella salina , and to observe changes in catalase isozyme activity following exposure to different light regimes. There was no significant change in photosynthetic or antioxidant parameters following exposure to ultraviolet-B. Ultraviolet-A exposure significantly decreased photosynthetic capacity (>70% decrease in variable to maximum fluorescence and light limited to light saturated photosynthesis in low beta-carotene cells) and resulted in 50% increases in ascorbate peroxidase activity and both ascorbate and glutathione concentrations. Exposure to UV-A resulted in decreased catalase activity and inactivation of related isozymes. The results suggest exposure to ultraviolet-A (but not ultraviolet-B) directly affects photosynthesis, observed as a loss of photosynthetic electron transport efficiency (and increased radical formation). Dunaliella bardawil was shown to possess three isoforms of the enzyme catalase while two isozymes were visualized for Dunaliella salina. Isozymes found in Dunaliella bardawil exhibited varying degrees of photosensitivity. It has been determined that the accumulated beta-carotene in Dunaliella bardawil prevents UV-related photosynthetic damage through blue-light/ultraviolet-A absorption and is supported by trends observed for antioxidant enzyme responses.
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