Influence of ultraviolet and photosynthetically active radiations on photosynthetic efficiency and antioxidant systems of the cyanobacterium Dolichospermum spiroides HKAR-23.

This study examines an in-depth analysis of the physiological and biochemical responses in the cyanobacterium Dolichospermum spiroides HKAR-23 under various treatment conditions of photosynthetically active radiation (PAR), PAR + UV-A (PA), and PAR + UV-A + UV-B (PAB). Exposure to UV radiation (UVR) significantly reduced the levels of Chl a and phycocyanin. In contrast, under PA and PAB irradiations, the total carotene level increased. We found that non-photochemical quenching (NPQ) is the primarily responsible pathway for excessive energy dissipation during UVR exposure, while photochemical quenching plays a crucial role in avoiding photoinhibition. The findings illustrate that the production of reactive oxygen species (ROS) may be a significant factor in UVR-induced damage. To mitigate oxidative stress, the cyanobacterium activates various antioxidant compounds like carotenoids and enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). Furthermore, in vivo ROS production has been determined by using 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) dye. The ROS generation has also been quantified using green and red fluorescence, represented as the G/R ratio. These findings indicate that increasing levels of PAB irradiation have more harmful effects on the cyanobacterium cells than other individual exposures. Concurrently, our study highlights the cumulative effects of increasing UVR to elucidate the underlying adaptive strategies, photoprotective mechanisms, and stress tolerance responses that enable the cyanobacterium Dolichospermum spiroides HKAR-23 to maintain cellular homeostasis and sustain survival in dynamic natural conditions.