Effect of Ultraviolet Radiation on Morphology, Physiology and Antioxidant Defense System of the Rice-Field Cyanobacterium Anabaena sp. Strain VKB01.

Anthropogenic-induced variation in ultraviolet-B radiation (UV-B) of the solar spectrum causes dramatic changes in agricultural crops and associated nitrogen-fixing cyanobacteria. A few or limited studies have been conducted to explore the adaptive strategies of cyanobacteria to elevated UV-B radiation. Despite their ecological importance, the adaptive strategy of rice-field cyanobacteria under elevated UV radiation remains poorly understood. In the present study, we investigated the morphological, physiological, and biochemical responses of the heterocystous cyanobacterium Anabaena sp. strain VKB01 under a combination of ultraviolet and photosynthetically active radiation [PAR (P), PAR + UV-A (PA), PAR + UV-A + UV-B (PAB)]. Cyanobacterial cultures exposed to PAR exhibited enhanced growth, pigment accumulation, and photosynthetic efficiency, whereas the same cultures irradiated with PA and PAB showed a marked reduction in these parameters. Structural analyses (SEM, TEM) revealed thylakoid disorganization and cytoplasmic damage, particularly under PAB treatment. Defense responses included elevated carotenoids, allophycocyanin, and carbohydrate levels, along with significant increases in antioxidant enzyme activity and radical scavenging capacity. Malondialdehyde content and fluorescence intensity also rose markedly under UV stress, indicating oxidative damage. Notably, mycosporine-like amino acids (MAAs) accumulated up to sixfold under prolonged PAB exposure, highlighting their role as photoprotective metabolites. These current findings demonstrate that Anabaena sp. strain VKB01 employs a multifaceted defense system, including pigment modulation, antioxidant regulation, and MAAs biosynthesis, to withstand UV-B-induced stress. This study enhances our understanding of cyanobacterial resilience in rice-field ecosystems and suggests potential applications in sustainable agriculture and the biotechnological production of natural photoprotective compounds.