Review UV-protectants in cyanobacteria Rajeshwar P. Sinha a , Donat-P. Ha ¨der b, * a Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi 221005, India b Chair of Plant Ecophysiology, University of Erlangen-Nu ¨rnberg, Staudtstrasse 5, D-91058 Erlangen, Germany Received 14 September 2007; received in revised form 14 December 2007; accepted 14 December 2007 Available online 23 December 2007 Abstract Cyanobacteria are the largest group of Gram-negative photosynthetic prokaryotes on earth and have a cosmopolitan distribution. As cyanobacteria are believed to have originated in the Precambrian era at a time when the ozone shield was absent, they presumably faced high fluxes of UV radiation, which must have acted as an evolutionary pressure leading to the selection for efficient UV radiation protecting mechanisms. Tolerance of cyanobacteria to intense sunlight as well as UV radiation might have contributed to their success during early stages of colonization. The synthesis of UV-absorbing/screening compounds is an important mechanism to prevent UV-induced photodamage. In cyanobacteria photoprotectants such as mycosporine-like amino acids (MAAs) and scytonemin strongly absorb in the UV-A and/or UV-B region of the spectrum, and thus play an important role in allowing these organisms to grow and survive in habitats exposed to strong irradiation. # 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Cyanobacteria; Mycosporine-like amino acids (MAAs); Scytonemin; UV-screening compounds; UV radiation Contents 1. Introduction ................................................................................. 278 2. Structure and biosynthesis of MAAs ................................................................ 279 3. MAAs as photoprotectants ....................................................................... 283 4. MAAs in cyanobacteria ......................................................................... 283 5. Evolution of MAAs as UV-absorbing compounds ....................................................... 284 6. Structure and biosynthesis of scytonemin ............................................................. 284 7. Scytonemin as a photoprotectant ................................................................... 285 8. Role of scytonemin in evolution ................................................................... 286 9. Conclusions.................................................................................. 286 10. Future research needed .......................................................................... 287 Acknowledgement ............................................................................. 287 References .................................................................................. 287 1. Introduction Cyanobacteria are phylogenetically a group of Gram- negative photosynthetic prokaryotes having cosmopolitan distribution ranging from hot springs to the Antarctic and Arctic regions. The role of cyanobacteria in nitrogen fixation and in the maintenance of the fertility of rice is well documented [1–4]. They are also significant constituents of marine ecosystems and account for a high percentage of oceanic primary productivity representing an estimated 40% of the biomass. Absorption of solar energy to drive photosynthesis www.elsevier.com/locate/plantsci Available online at www.sciencedirect.com Plant Science 174 (2008) 278–289 Abbreviations: CFCs, chloroflurocarbons; HPLC, high performance liquid chromatography; MAAs, mycosporine-like amino acids; MALDI-TOF MS, matrix-associated laser desorption/ionization time-of-flight mass spectrometry; PAR, photosynthetically active radiation; UV-A, ultraviolet radiation in the 315–400 nm wavelength range; UV-B, ultraviolet radiation in the 280–315 nm wavelength range; UVR, UV radiation. * Corresponding author. Tel.: +49 9131 8528216; fax: +49 9131 8528215. E-mail address: dphaeder@biologie.uni-erlangen.de (D.-P. Ha ¨der). 0168-9452/$ – see front matter # 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2007.12.004