A.A. ELSAYED*, ***#, M.W. SHAFAA**, R.A. RIZK**, A.M. IBRAHIM**
*Solar Research Laboratory, Solar and Space Research Department, National Research Institute of Astronomy and Geophysics (NRIAG), Helwan, Cairo, Egypt
**Biophysics Branch, Physics Department, Faculty of Science, Helwan University, Cairo, Egypt
*** Department of Physics, Faculty of Science and Humanitarian Studies, Shaqra University, Al-Quwaiyayah, Kingdom of Saudi Arabia
Continuous depletion of the stratospheric ozone layer, mainly due to anthropogenically released atmospheric pollutants such as chlorofluoro carbons (CFCs) has resulted in an increase in Ultraviolet-B (UV-B; 280 – 315 nm) radiation on the Earth’s surface which inhibits photochemical and photobiological processes in cyanobacteria. However, these organisms have developed several lines of defense mechanisms such as screening to counteract the damaging effects of UVR. The cyanobacterium used in this study was Nostoc commune. It was isolated from Egyptian rice fields. The organisms were grown in 1 L glass flasks containing 500 mL of BG11 medium. A locally manufactured ultraviolet source was used for exposure experiments. It consisted of mercury (Hg) arc lamp (HBO, Philips, 200 W/2) with a continuous spectrum of wavelengths from 220 nm to 900 nm. A coloured cut-off glass filter was used [WG280 nm (2mm thick), with a dimension of 100 mm x 100 mm]. MAAs were extracted using 100% HPLC grade methanol and separated through high-performance liquid chromatography. Absorption spectroscopic analyses of the methanolic extracts of samples revealed a typical MAA peak at 335nm in Nostoc commune. The high performance liquid chromatographic (HPLC) analysis of water-soluble compounds revealed the biosynthesis of only one type of MAA, shinorine (retention time = 2.25 min and absorption maximum at 334 nm). MAA content was highly increased by UV exposure. 16 h exposure of UV-light induced the largest amount of MAA for a sample covered with WG280 nm cut-off filter as compared to UV non-exposed sample as control. Liquid chromatography/mass spectrometry (LC/MS) analysis of MAA reveals the molecular weight of shinorine (m/z = 333.1 Da). Nostoc commune was able to synthesize MAA in response to UV radiation. It was found to possess a very effective mechanism for adapting to deleterious doses of UV-B radiation. MAA formation was photo-induced and the final MAA concentration was controlled by irradiance and duration of exposure. The present strain could act as a model organism for studying the biosynthetic route of MAAs in cyanobacteria and could be used for the industrial production of MAAs.
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