Biological responses to solar UV radiation in space and on earth

Solar UV radiation is a dynamic driving force of organic chemical evolution. However, it has had also severe constraints in biological evolution. During the early history of life on Earth, before the formation of a protective ozone layer in the atmosphere, high intensities of solar UV radiation of short wavelengths could reach the surface of the Earth. Today the full spectrum of solar UV radiation is only experienced in space, where other important space parameters influence survival and genetic stability additionally, like vacuum, cosmic radiation, temperature extremes, microgravity. To reach a better understanding of the processes leading to the origin, evolution and distribution of life on Earth we have performed several space experiments with microorganisms. The ability of resistant life forms like bacterial spores to survive high doses of extraterrestrial solar UV alone or in combination with other space parameters, e.g. vacuum, was investigated. Extraterrestrial solar UV was found to have a thousand times higher biological effectiveness than UV radiation filtered by stratospheric ozone concentrations found today on Earth. Radiative transfer models predicting a strong correlation between the decrease in biologically effective UV radiation with increasing ozone concentrations during the history of life on Earth could be validated experimentally in space.