Cyanobacterial responses to UV radiation

Summary: The influence of ultraviolet radiation (UVR) on populations of microorganisms has been the subject of serious investigation for at least the past 20-25 years. UVR that is applicable to the Earth's surface (past or present) is arbitrarily divided into UVA (400-320 or 315 nm), UVB (280-320 or 315 nm), UVC (∼180-280 nm). Although essentially all organisms are affected by UVR, microorganisms show more rapid, immediate and measurable effects than macro-organisms. This chapter is mainly relegated to UVR and cyanobacteria, although UV effects on other phototrophs and microorganisms, when relevant, will be included. Some ancestors of living cyanobacteria, the oldest oxygenic organisms, may have evolved in the Archean or early Proterozoic Eons, from 3.5 to 2.5 Gyr, respectively, in a time when UV radiation fluxes reaching the surface, particularly UVB and UVC, were much higher than at present. The latter wavelength region (UVC) does not reach the Earth's surface at present. Thus, cyanobacteria and other microorganisms in that distant age had to have evolved a strategy to tolerate these greater levels of UV radiation, and at present this strategy may demonstrably involve multiple devices, even within one organism. The best understood in the past several years for numerous organisms has been the active metabolic strategies that compensate for the destruction of vital genetic components, such as the development of efficient metabolic DNA repair systems. The implementation of gliding motility system for escaping the effects of high visible and UV radiation has been better described and understood. Some of the most revealing results in the last 10 years have been an almost complete understanding of the regulation of the UV-protective compounds, scytonemin and mycosporine-like compounds, that partially or completely avoid the damage caused by UV radiation.