Impacts of PAR and UV radiation on diurnal photosynthesis performance, pigment composition, and antioxidant function of the hot-spring cyanobacterium Nostoc sp. strain VKB02.

Diurnal oscillations are 24 h clock, that synchronize organisms biological functions based on the daily environmental fluctuation. Continuous increase in ultraviolet radiation have been shown to affect the biological clock of cyanobacteria. The present investigation deals with the 12/12 h of light/dark effects of PAR and PAB (PAR + UVA + UVB) irradiations on ecophysiology and defense management of a hot-spring cyanobacterium Nostoc sp. strain VKB02. The alternative L/D exposure of PAR showed increase in growth and pigment compositions. However, PAB radiation has significantly decreased within the same parameters after the L1 phase except for carotenoid and APC, while PE and PC recovered till the D2 phase corresponding to the counter light phase. The pigments destruction also resulted in Chl a fluorescence (F/F, F׳/F׳, Y(II), rETR) emission decline. In addition, PAB exposure accelerated free radicals generation with induced protein oxidation (RCG) and antioxidative enzymes (SOD, POD, CAT) as counteract defense during the light phase. The overall circadian regime facilitated the resynthesis fate of pigment-protein complexes and the mitigation of the high level of ROS production. This result suggests the unique survival strategy of the hot-spring cyanobacterium against ultraviolet radiation in a diurnal manner. This study also offers a deep understanding of the diurnal eco-physiological and biochemical responses of the cyanobacterium for the advancement in sustainable agricultural production with lower input in variable climate.