SOLAR ULTRAVIOLET AND THE EVOLUTIONARY HISTORY OF CYANOBACTERIA

On the basis of photobiological, evolutionary, paleontological, paleoe nvironmental and physiological arguments, a time course for the role o f solar ultraviolet radiation (UVR, wavelengths below 400 nm) in the e cology and evolution of cyanobacteria is proposed in which three main periods can be distinguished. An initial stage, before the advent of o xygenic photosynthesis, when high environmental fluxes of UVC (wavelen gths below 280 nm) and UVB (280-320 nm) may have depressed the ability of protocyanobacteria to develop large populations or restricted them to UVR refuges. A second stage lasting between 500 and 1500 Ma (milli on years), started with the appearance of true oxygen-evolving cyanoba cteria and the concomitant formation of oxygenated (micro)environments under an oxygen free-atmosphere. In this second stage, the age of UV, the overall importance of UVR must have increased substantially, sinc e the incident fluxes of UVC and UVB remained virtually unchanged, but additionally the UVA portion of the spectrum (320-400 nm) suddenly be came biologically injurious and extremely reactive oxygen species must have formed wherever oxygen and UVR spatially coincided. The last per iod began with the gradual oxygenation of the atmosphere and the forma tion of the stratospheric ozone shield. The physiological stress due t o UVC all but disappeared and the effects of UVB were reduced to a lar ge extent. Evidence in support of this dynamics is drawn from the phyl ogenetic distribution of biochemical UV-defense mechanisms among cyano bacteria and other microorganisms. The specific physical characteristi cs of UVR and oxygen exposure in planktonic, sedimentary and terrestri al habitats are used to explore the plausible impact of UVR in each of the periods on the ecological distribution of cyanobacteria.