1 Space Exposure Experiment of Microbes at the International Space Station By Shin-ichi YOKOBORI 1) , Yuko KAWAGUCHI 1) , Yinjie YANG 1) , Hirofumi HASHIMOTO 2) , Kazumichi NAKAGAWA 3) , Issay NARUMI 4) , Satoshi YOSHIDA 5) , Kensei KOBAYASHI 6) , Akihiko YAMAGISHI 1) , and TANPOPO WG 2) 1) Laboratory of Extremophiles, Department of Applied Life Sciences, School of Life Sciences, Tokyo University of Pharmacy and Life Sciences, Japan 2) Institute of Space and Astronautical Science, JAXA, Japan 3) Graduate School of Human Development and Environment, Kobe University, Japan 4) Ion Beam Mutagenesis Research Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency, Japan 5) National Institute of Radiological Sciences, Japan 6) Graduate School of Engineering, Yokohama National University, Japan Survival rates of candidate microbes for the space exposure experiments at the Exposure Facility of KIBO in ISS (International Space Station) proposed by Tanpopo Mission were tested under the various environmental factors (vacuum, temperature cycle, gamma ray irradiation, heavy ion irradiation, and UV irradiation). The candidate microbes were various deinococcal species, cyanobaceteria, and fission yeast (spore). The UV irradiation decreased the survival rates of microbes. However, the microbes survived for long period, when the microbes were shaded by other cells at the surface of cell aggregate. The periodical change in temperature of ISS will also affect the survival of microbes seriously. However, most microbes we tested are expected to survive in space for long period, more than one year, if UV is efficiently shielded as an aggregated form of cells. We selected the microbes suitable for the space exposure experiments in Tanpopo Mission. Key Words: ISS (International Space Station), Space exposure, Microbes 1. Introduction Microbes have been collected from high altitude using balloons, aircraft and meteorological rockets since 1936, even it is not clear how could those microbes be ejected up to such high altitude 1) . Spore-forming fungi, spore-forming Bacilli, and Micrococci (probably Deinococci) have been isolated in these experiments. We have also collected microbes at high altitude by using airplanes and balloons. Spore forming fungi and Bacilli, and Deinococci have been isolated in these experiments. Our two high-altitude isolates of Deinococci were novel species by molecular phylogenetic analyses and other microbiological characterizations (Deinococcus aerius TR0125 and Deinococcus aetherius ST0316) 2-4) . Spores and Deinococci are known by their extremely high resistance against UV, gamma ray, and other radiations. D. aerius and D. aetherius showed high resistance comparable with D. radiodurans R1 to the UV and ionizing radiation such as gamma rays. If microbes could be found present even at the higher altitude of low earth orbit (400 km), the fact would endorse the possible interplanetary migration of terrestrial life. The viability of life in space and the possibility of transfer of life between the Earth and extraterrestrial bodies have been studied in various areas such as astronomy, cosmology, planetary sciences, and biology. This transfer process is called “panspermia”, which was proposed by Arrhenius 5) . He proposed that the interplanetary transfer of single spores is propelled by radiation pressure. To evaluate the panspermia hypothesis, feasibility of natural interplanetary exchange of microbes inside rocks as the result of natural impacts, a process which is known as “lithopanspermia” or “transpamia”, has been tested both theoretically and experimentally 6–12) . The scenario of lithopanspermia consists of several phases: rocks ejected from a certain planet (e. g. the Earth or Mars), transport of microbes through the space for a long period, and finally, landing on another planet 6,7,13,14) . Since 1960’s, various space exposure experiments have been conducted to determine the survivability of microbes and fungi during the transfer in space, outside the Earth’s magnetic field (Apollo 16) or in low Earth orbit (LEO). These space exposure experiments have been reviewed in Horneck et al. 15) . EXPOSE-E, EXPOSE-R, BIORISK and BIOMEX have been performed on the International Space Station (ISS)) 16–19) . Environmental parameters in LEO such as high vacuum (10 –4 ~10 –5 Pa), intense solar ultraviolet (UV) radiation, various components of cosmic radiation, and high and low temperatures affect the survivability of microbes 20) . For example, spores of Bacillus subtilis in multi-layer survived under solar UV radiation for about 6 years though all the spores of B. subtilis in monolayer were killed 21, 22) . The result suggested that microbe spores might survive for long period if the spores are shielded from intense solar radiation 23) . We proposed the ”Tanpopo” mission to examine possible interplanetary migration of microbes, and organic compounds on the Exposure Facility (EF) of Japan Experimental Module (JEM) of the International Space Station (ISS) 24) . Tanpopo consists of six subthemes. Two of them are on the possible interplanetary migration of microbes: the capture experiment of microbes at the ISS orbit and space exposure experiment of microbes. In this paper, we focus on the space exposure 2013-p-12p