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; UVλmax: 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.

中文翻译：

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紫外线屏蔽色素 scytonemin 对形成杂囊的蓝细菌 Nostoc sp. 中紫外线-B 诱导的损伤的光保护作用。HKAR-2菌株

Scytonemin 作为一种天然光保护剂，可抵御高强度太阳光合有效辐射和有害紫外线辐射，这已在几种蓝藻中报道，并在其细胞外多糖鞘中积累。UV-B 辐射 (280–315 nm) 对生物体的生理和生化过程有不利影响。在这项研究中，从芒果树皮收集的干燥蓝藻垫中提取了 scytonemin，并使用高效液相色谱和电喷雾电离质谱进行了部分表征（保留时间：1.7 分钟；UVλmax：386 nm；[M + H ]+m/z：545.1)。此后，在无鞘蓝藻 Nostoc sp. 中，scytonemin 对 UV-B 辐射的光保护能力。对菌株 HKAR-2 进行评估，然后研究其在恢复过程中的作用。我们发现 UV-B 辐射会抑制生长、存活、Chl a 含量和总蛋白质浓度，并导致总类胡萝卜素含量增加。氮同化酶、谷氨酰胺合成酶和硝酸还原酶的活性也受到影响。体内硝酸还原酶活性表现出刺激反应，而体内谷氨酰胺合成酶活性在 UV-B 暴露后受到不利抑制。不同浓度的 Scytonemin 通过消除 UV-B 的有害影响表现出有效的光保护能力，并增强了恢复过程。scytonemin 在生长、色素沉着、本研究首次报道了蓝藻菌株中的存活和氮代谢酶。我们的结果表明，在面临恶劣环境条件的压力栖息地中，scytonemin 有助于蓝藻更好的生存和适应性。