Sensitivity of two Nostoc species harbouring diverse habitats to ultraviolet-B radiation

Abstract

The effects of ultraviolet-B (UV-B; 280–315 nm) radiation on certain key physiological and biochemical processes were studied in two Nostoc species harbouring diverse habitats. Nostoc sp. strain HKAR-2, a thermophilic cyanobacterium, was isolated from the hot-spring, Rajgir, whereas, Nostoc sp. HKAR-6 was a rice-field isolate from Banaras Hindu University, Varanasi. Complete killing of the cells occurred after 48 h of UV-B exposure in Nostoc sp. strain HKAR-6, whereas in case of Nostoc sp. strain HKAR-2 death occurred only after 72 h. Chlorophyll a and phycocyanin content were found to be adversely affected by UV-B irradiation in both the test organisms. However, a progressive increase in carotenoids content was observed upto 10 h of UV-B exposure but subsequently declined after 12 h in both the Nostoc sp. The antioxidative enzymes such as superoxide dismutase, catalase, ascorbate peroxidase and peroxidase that scavenge the UV-B generated harmful reactive oxygen species were found to have multifold induction in their content in both the strains following UV-B exposure in comparison to non-irradiated control cultures. In addition, these organisms also synthesize mycosporine-like amino acids (MAAs); able to carry out UV-screening. Porphyra-334 and shinorine were found to be the common MAAs in both the Nostoc sp., while an unknown MAAs with a retention time of 6.9 min (λmax—334 nm) was found in Nostoc sp. strain HKAR-2. Findings from this study suggest that both enzymatic and non-enzymatic defense mechanisms are being employed by Nostoc sp. strain HKAR-2 and Nostoc sp. strain HKAR-6 to counteract the damaging effects of UV-B radiation.