There is continuous depletion of the stratospheric ozone layer, due to anthropogenically released atmospheric pollutants such as chlorofluorocarbons (CFCs), and, consequently an increase in solar ultraviolet-B (UV-B; 280 - 315 nm) radiation reaching the Earth`s surface. Cyanobacteria with a cosmopolitan distribution are the most common photosynthetic prokaryotes on Earth. Members of cyanobacteria play an important role both in aquatic as well as terrestrial ecosystems, particularly in rice paddy fields, in improving the fertility of soil as a natural biofertilizer by their inherent capacity to fix atmospheric nitrogen. UV- B radiation has been reported to suppress a number of photochemical and photobiological events in cyanobacteria. Motility, growth, survival, pigmentation, nitrogen and carbon fixation has been shown to be susceptible to UV-B. Because of its high energy, UV-B easily destroys proteins since aromatic amino acids such as tryptophane, tyrosine, phenylalanine and histidine strongly absorb in the UV-B range. Another major target of UV-B is DNA where it mainly induces thymine dimers. However, certain cyanobacteria produce photo-protective compounds such as mycosporine-like amino acids (MAAs) and scytonemin to protect themselves from deleterious short wavelength solar radiation. This review deals with the targets of UV-B radiation in cyanobacteria and the possible defense mechanisms against the negative effects of UV-B.
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