Impacts of ultraviolet and photosynthetically active radiations on photosynthetic efficiency and antioxidant systems of the cyanobacterium Spirulina subsalsa HKAR-19.

This study summarizes the response of cyanobacterium Spirulina subsalsa HKAR-19 under simulated light conditions of photosynthetically active radiation (PAR), PAR+UV-A (PA), and PAR+UV-A+UV-B (PAB). Exposure to UV radiation caused a significant (P < 0.05) decrease in chlorophyll a, phycocyanin, and total protein. In contrast, total carotene content increased significantly (P < 0.05) under PA and PAB with increasing irradiation time. The photosynthetic efficiency of photosystem II also decreased significantly in PA and PAB radiation. We have also recorded a decrease in the fluorescence emission intensity of phycocyanin under PA and PAB exposure. The phycocyanin fluorescence shifted towards shorter wavelengths (blue-shift) after 72 h of PA and PAB exposure. Intracellular reactive oxygen species (ROS) levels increased significantly in PA and PAB. Fluorescence microscopic images showed an increase in green fluorescence, indicating ROS generation in UV radiation. We have also quantified ROS generation using green and red fluorescence ratio represented as G/R ratio. A 2-6-fold increase in antioxidative enzymes activity was observed to overcome the damaging effects caused by UV stress as compared to untreated control cultures. The lipid peroxidation was assessed in terms of malondialdehyde content which increases significantly (P < 0.05) as the duration of exposure increases. These results suggest that a combined effect of PAR, UV-A, and UV-B was more deleterious than an individual one.