Photoprotective role of UV-screening pigment scytonemin against UV-B-induced damages in the heterocyst-forming cyanobacterium Nostoc sp. strain HKAR-2

Scytonemin acts as a natural photoprotectant against high-intensity solar photosynthetically active radiation and harmful ultraviolet radiation, which has been reported in several cyanobacteria, and gets accumulated in their extracellular polysaccharide sheath. UV-B radiation (280-315 nm) has detrimental effect on physiological and biochemical processes of living organisms. In this study, scytonemin was extracted from dried cyanobacterial mats collected from the bark of mango trees and was partially characterized using high-performance liquid chromatography and electrospray ionization-mass spectrometry (retention time: 1.7 min; UV lambda(max): 386 nm; [M + H](+)m/z: 545.1). Thereafter, photoprotective capabilities of scytonemin against UV-B radiation in a non-sheathed cyanobacterium Nostoc sp. strain HKAR-2 were assessed followed by studying its role in the recovery process. We found that UV-B radiation inhibited growth, survival, Chl a content and total protein concentration and caused an increase in total carotenoid content. The activities of nitrogen-assimilating enzymes, glutamine synthetase and nitrate reductase, were also affected. In vivo nitrate reductase activity exhibited a stimulatory response, while in vivo glutamine synthetase activity was adversely inhibited after UV-B exposure. Scytonemin with different concentrations exhibited efficient photoprotective ability by nullifying the deleterious effects of UV-B and also enhanced the recovery process. UV-screening effects of scytonemin in terms of growth, pigmentation, survival and nitrogen metabolism enzymes in the cyanobacterial strain have been reported for the first time in the present study. Our results suggest that scytonemin aids in the better survival and adaptability of cyanobacteria in stressed habitats facing harsh environmental conditions.