The SPORES experiment of the EXPOSE-R mission: Bacillus subtilis spores in artificial meteorites Corinna Panitz 1,2 , Gerda Horneck 1 , Elke Rabbow 1 , Petra Rettberg 1 , Ralf Moeller 1 , Jean Cadet 3 , Thierry Douki 3 and Guenther Reitz 1 1 Institute of Aerospace Medicine, Radiation Biology, DLR, D-51147 Cologne, Germany e-mail: cpanitz@ukaachen.de; corinna.panitz@dlr.de 2 Institute of Pharmacology and Toxicology, RWTH/Klinikum Aachen, D-52074 Aachen, Germany 3 Laboratoire Lésions des Acides Nucléiques, Institut Nanosciences et Cryogénie/SCIB UMR-E3 CEA-UJF/CEA Grenoble, 38054 Grenoble, France Abstract: The experiment SPORES ‘Spores in artificial meteorites’ was part of European Space Agency’s EXPOSE-R mission, which exposed chemical and biological samples for nearly 2 years (March 10, 2009 to February 21, 2011) to outer space, when attached to the outside of the Russian Zvezda module of the International Space Station. The overall objective of the SPORES experiment was to address the question whether the meteorite material offers enough protection against the harsh environment of space for spores to survive a long-term journey in space by experimentally mimicking the hypothetical scenario of Lithopanspermia, which assumes interplanetary transfer of life via impact-ejected rocks. For this purpose, spores of Bacillus subtilis 168 were exposed to selected parameters of outer space (solar ultraviolet (UV) radiation at λ >110 or >200 nm, space vacuum, galactic cosmic radiation and temperature fluctuations) either as a pure spore monolayer or mixed with different concentrations of artificial meteorite powder. Total fluence of solar UV radiation (100–400 nm) during the mission was 859 MJ m - 2 . After retrieval the viability of the samples was analysed. A Mission Ground Reference program was performed in parallel to the flight experiment. The results of SPORES demonstrate the high inactivating potential of extraterrestrial UV radiation as one of the most harmful factors of space, especially UV at λ > 110 nm. The UV-induced inactivation is mainly caused by photodamaging of the DNA, as documented by the identification of the spore photoproduct 5,6-dihydro-5(α-thyminyl)thymine. The data disclose the limits of Lithopanspermia for spores located in the upper layers of impact-ejected rocks due to access of harmful extraterrestrial solar UV radiation. Received 29 April 2014, accepted 13 June 2014, first published online 1 August 2014 Key words: Bacillus subtilis, bacterial spores, International Space Station, Lithopanspermia, space experiment. Introduction Since the discovery of Martian meteorites (Wasson & Wetherill 1979; Becker & Pepin 1984; Dreibus & Wänke 1984, 1985) it is a generally accepted supposition that rock fragments can escape from planetary bodies, e.g. ejected from very large impact craters, and that interplanetary transfer of matter has occurred several times during the history of our Solar System (O’Keefe & Ahrens 1986; Vickery & Melosh 1987). However, it is still an open question, whether living matter has been transported between the planets of our Solar System by the same mechanism, and, if so, whether resistant organisms can withstand the severe strain of a journey through the Solar System. During such a hypothetical interplanetary transfer, the organisms would have to cope with the following three major challenges: (1) the escape process, (2) the long-duration exposure to space and (3) the capture and entering process. Although it will be difficult to prove that resistant organisms could survive this cascade of strenuous attacks, estimates of the chances of the different steps of the process to occur can be obtained from measure- ments in space and laboratory simulation experiments, and from model calculations (Mileikowsky et al. 2000; Clark 2001; Horneck et al. 2008, 2010; Nicholson 2009; Onofri et al. 2012). In the SPORES (Spores in artificial meteorites) experiment of the EXPOSE-R (Exposure facility attached to the URM-D of the Zvezda Module of the ISS) mission on board of the International Space Station (ISS), we have addressed the question of the chances and limits of life to be transported from one body of our Solar System to another by natural processes by testing experimentally step 2, i.e. whether the meteorite material offers enough protection against the harsh environ- ment of space for spores to survive a long-term stay in space. For this purpose, spores of the bacterium Bacillus subtilis 168, which have proven their high resistance to outer space International Journal of Astrobiology 14 (1): 105–114 (2015) doi:10.1017/S1473550414000251 © Cambridge University Press 2014 . https://doi.org/10.1017/S1473550414000251 Downloaded from https://www.cambridge.org/core. IP address: 54.70.40.11, on 17 Dec 2018 at 03:43:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms