a1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh, Scotland, UK
a2 German Aerospace Center DLR, Institute of Aerospace Medicine, Koeln, Germany
a3 Centre for Astrobiology and Extremophiles Research, School of Life Sciences, University of Bradford, Bradford, UK
a4 Department of Physics and Astronomy, Space Science Research Centre, University of Leicester, Leicester, UK
On Earth, microorganisms living under intense ultraviolet (UV) radiation stress can adopt endolithic lifestyles, growing within cracks and pore spaces in rocks. Intense UV irradiation encountered by microbes leads to death and significant damage to biomolecules, which also severely diminishes the likelihood of detecting signatures of life. Here we show that porous rocks shocked by asteroid or comet impacts provide protection for phototrophs and their biomolecules during 22 months of UV radiation exposure outside the International Space Station. The UV spectrum used approximated the high-UV flux on the surface of planets lacking ozone shields such as the early Earth. These data provide a demonstration that endolithic habitats can provide a refugium from the worst-case UV radiation environments on young planets and an empirical refutation of the idea that early intense UV radiation fluxes would have prevented phototrophs without the ability to form microbial mats or produce UV protective pigments from colonizing the surface of early landmasses.
(Received February 12 2014)
(Accepted March 27 2014)
(Online publication May 14 2014)