Review Cyanobacteria and ultraviolet radiation (UVR) stress: Mitigation strategies Shailendra P. Singh a , Donat-P. Ha ¨der a, *, Rajeshwar P. Sinha b a Department of Biology, Friedrich-Alexander University Erlangen-Nuremberg, Staudtstrasse 5, D-91058 Erlangen, Germany b Laboratory of Photobiology and Molecular Microbiology, Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi 221005, India 1. Introduction Cyanobacteria are a phylogenetically primitive group of gram- negative prokaryotes having a cosmopolitan distribution ranging from hot springs to the Arctic and Antarctic regions (Stanier and Cohen-Bazire, 1977). They were probably the first photosynthetic oxygen-evolving organisms that appeared during the Precambrian era (between 2.8 and 3.5  10 9 years ago) and provided a favorable condition for the evolution of current aerobic life (Fischer, 2008). Cyanobacteria are major biomass producers both in aquatic as well as terrestrial ecosystems and represent more than 50% of the biomass in many aquatic ecosystems (Ha ¨der et al., 2007). These prokaryotes are valuable sources of various natural products of medicinal and industrial value (Cardozo et al., 2007). In addition, their inherent capacity to fix atmospheric nitrogen makes them ecologically important for rice-growing countries where they add to fertility of the rice fields as natural biofertilizer (Vaishampayan et al., 2001). They use the enzyme nitrogenase which directly converts the atmospheric nitrogen into ammonium (NH 4 ), a form through which nitrogen enters the food web. It has been calculated that cyanobacteria fix over 35 million tons of nitrogen annually and thus occupy a central position in nutrient cycling of ecosystems (Ha ¨der et al., 1989). Rapid industrialization in the past few decades has resulted in an increase in anthropogenically released chlorofluorocarbons, chlor- ocarbons and organobromides causing depletion of the strato- spheric ozone layer (Crutzen, 1992). Considerable amounts of reactive nitrogen species (RNS) such as nitric oxide (  NO), peroxynitrite (ONOO  ) and nitrous oxide (N 2 O), produced naturally from unpolluted terrestrial and aquatic ecosystems or from anthropogenic sources (biomass or fuel burnings and chemical fertilizers), also contribute to the depletion of the ozone layer (Kramlich and Linak, 1994). The process of ozone depletion has been reported at mid latitudes and especially in the Antarctic where ozone levels have been reported to decline by more than 70% during late winter and early spring in the polar vortex (Smith et al., 1992). Due to ozone depletion there is an increase in the amount of harmful ultraviolet radiation (UVR; 280–400 nm) on the Earth’s surface which is absorbed by biomolecules such as nucleic acids and proteins ultimately resulting in lethal effects on the biological systems (Ha ¨der et al., 2007). Both photosynthesis and nitrogen fixation are energy-dependent processes driven by solar energy. Harvesting of solar radiation exposes cyanobacteria simultaneously to harmful doses of UV-B (280–315 nm) and UV-A (315–400 nm) radiation in their natural habitats. Fig. 1 represents photographs of some common cyanobacterial habitats from India where they face intense solar radiation during summer. The high-energetic UV-B has Ageing Research Reviews 9 (2010) 79–90 ARTICLE INFO Article history: Received 20 March 2009 Received in revised form 22 May 2009 Accepted 27 May 2009 Keywords: Antioxidants Cyanobacteria DNA repair Mitigation strategies Mycosporine-like amino acids Programmed cell death Reactive oxygen species Scytonemin Signal transduction UV radiation ABSTRACT Cyanobacteria are primitive photosynthetic oxygen-evolving prokaryotes that appeared on the Earth when there was no ozone layer to protect them from damaging ultraviolet radiation (UVR). UVR has both direct and indirect effects on the cyanobacteria due to absorption by biomolecules and UVR-induced oxidative stress, respectively. However, these organisms have developed several lines of mitigation strategies/defense mechanisms such as avoidance, scavenging, screening, repair and programmed cell death to counteract the damaging effects of UVR. This review presents an update on the effects of UVR on cyanobacteria and the defense mechanisms employed by these prokaryotes to withstand UVR stress. In addition, recent developments in the field of molecular biology of UV-absorbing compounds such as mycosporine-like amino acids and scytonemin, are also added and the possible role of programmed cell death, signal perception as well their transduction under UVR stress is being discussed. ß 2009 Elsevier Ireland Ltd. All rights reserved. * Corresponding author. Tel.: +49 9131 8528216; fax: +49 9131 8528215. E-mail address: dphaeder@biologie.uni-erlangen.de (D.-P. Ha ¨ der). Contents lists available at ScienceDirect Ageing Research Reviews journal homepage: www.elsevier.com/locate/arr 1568-1637/$ – see front matter ß 2009 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.arr.2009.05.004