Abstract
The effects of UV-B radiation (UVBR) on photosynthetic activity (Fv/Fm) of aquatic Synechocystis sp. and desert Chroococcus minutus and effects on composition and fluorescence property of extracellular polymeric substances (EPSs) from Synechocystis sp. and C. minutus were comparatively investigated. The desert cyanobacterium species C. minutus showed higher tolerance of PSII activity (Fv/Fm) to UVBR than the aquatic Synechocystis sp., and the inhibited PSII activity of C. minutus could be fully recovered while that of Synechocystis sp. could be partly recovered. UVBR had significant effect on the yield and biochemical composition of EPS of both species. Protein-like and humic acid-like substances were detected in EPS from Synechocystis sp., and protein-like and phenol-like fluorescent compounds were detected in EPS from C. minutus. Proteins in EPS of desert and aquatic species were significantly decomposed under UVBR, and the latter was more easily decomposed. The polysaccharides were much more resistant to UVBR than the proteins for both species. Polysaccharides of Synechocystis sp. was degraded slightly but those of C. minutus was little decomposed. The higher tolerance to UVBR of the desert cyanobacterium can be attributed to the higher resistance of its EPS to photodegradation induced by UVBR in comparison with the aquatic species.
Highlights
Ultraviolet-B radiation (UVBR) at 280–315 nm is one of the major detrimental environmental factors for photosynthetic organisms in arid regions (Bowker et al, 2002)
In the case of C. minutus, Fv/Fm was reduced under all periods of UVBR in comparison with the control and its reduction correlated with radiation time
It was interesting that Fv/Fm increased to higher levels than their controls for both Synechocystis sp. and C. minutus irradiated with UV-B for 1 and 2 min, suggesting that short term UVBR can improve their PSII activities
Summary
Ultraviolet-B radiation (UVBR) at 280–315 nm is one of the major detrimental environmental factors for photosynthetic organisms in arid regions (Bowker et al, 2002). Depletion of the ozone layer leads to increasing levels of UVBR reaching the earth’s surface (Madronich et al, 1995). Cyanobacteria are important photosynthetic microorganisms that live widely in aquatic and terrestrial environments. Exposure to UVBR can be potentially harmful for cyanobacteria in case the provoked damage exceeds the capacity of defense and repair mechanism (Sutherland, 2001). Some studies have shown that UVBR could induce physiological changes, including inhibition and even damage to photosystem, and enhance the content of reactive oxygen species (ROS) in cyanobacterial cell (Kumar et al, 2004). The high level of ROS is potentially harmful to the normal
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