Role of Photosynthetically Active Radiation and Dark Conditions on the Repair of Ultraviolet-B Radiation-induced Damages in the Cyanobacterium Nostoc sp. Strain HKAR-2

Introduction: We have investigated the effects of ultraviolet-B (UV-B; 280-315 nm) radiation on Chl a, phycocyanin, allophycocyanin, photosynthetic efficiency and reactive oxygen species (ROS) generation in Nostoc sp. strain HKAR-2 and the role of photosynthetically active radiation (PAR) and dark on the repair processes.Methods: Samples were exposed to UV-B for 14h followed by PAR and dark treatment for 10h. Chl a, phycocyanin and allophycocyanin concentration and photosynthetic efficiency i.e., the photosynthetic quantum yield of PS II (Fv/Fm) were estimated. In vivo detection of ROS by fluorescence microscopy and spectroscopy was done using the fluorescent probe 2′,7′-dichlorodihydrofluorescein diacetate (DCFH).Results: Progressive decrease of 50% and 41% in survival rates of cyanobacterium was observed after 12 and 14h of exposure respectively and a significant increase in survival was observed during the recovery process in PAR and dark. Chl a, phycocyanin and allophycocyanin concentration decreased after 12h of UV-B exposure. Significant recovery of phycocyanin and allophycocyanin was observed in the samples placed on PAR. Fv/Fm decreased with time and 42% decrease was observed after 14h of exposure. Pronounced fluorescence intensity, due to produced ROS was observed after 14h of UV-B exposure. The significant decrease in DCF fluorescence was observed in the samples placed in PAR and dark.Conclusions: UV-B exposed cyanobacteria kept on PAR showed speedy recovery in comparison to those kept in dark. This indicates the possible role of repair processes such as photoreactivation in the repair of damage caused by UV-B to cyanobacterium. Hence, further studies are needed for the thorough understanding of repair processes in cyanobacteria in response to UVR.