Survival and DNA damage of cell-aggregate of Deinococcus spp. exposed to space for two-years in Tanpopo mission
Abstract
The concept of panspermia hypothesis proposes the interplanetary transfer of life propelled by solar radiation-pressure (Arrhenius, 1903). Japanese space mission named Tanpopo, which means dandelion in Japanese, has performed capture and exposure experiments of microbes using outside of International Space Station (ISS) (Yamagishi et al., 2007; Kawaguchi et al., 2016). Previous exposure experiment of microbes in space revealed that microbes inside shielding (e.g. small fragments of rock, mixture of sugar or clay) with efficient thickness to protect from UV irradiation survive in space for a long period. On the other hand, we proposed the possible interplanetary transfer of cell-aggregate in sub-millimeter to survive in harsh space environment (Kawaguchi et al., 2013). We named the hypothesis massapanspermia. For the investigation of microbial survival and DNA damage induced in space, dried cells of radioresistant bacteria, Deinococcus radiodurans, Deinococcus aerius and Deinococcus aetherius, as well as D. radiodurans DNA repair-deficient mutants, were put in wells of aluminum plates in Exposure Panels (EPs) and exposed in space at the outside of Exposure Facility, Japanese Experimental Module of ISS since May 2015. After one-year exposure, the first set of EPs were retrieved into the ISS pressurized area in June 2016, and returned to the ground laboratory in September 2016. Cell-aggregate of dried-deinococcal cells with 100 _m-thickness irradiated with sunlight corresponding to 3.4 _ 103 kJ/m2 (110-315 nm) UV dose were dead. Quantitative-PCR analysis revealed that intact DNA (%) in the 100 _m-thick sample was less than 1%. On the other hand, cell-aggregates with over 500 _m-thickness were alive. We tested the survival of DNA repair-deficient mutants, D. radiodurans KH311, rec30 and UVS78. From comparison of their surviving fractions between ground control and space samples, we found that DNA damage such as single- and double-strand breaks as well as base damage were induced by space factors. Intriguingly, base damage was only detected from cell-aggregate exposed to space. We concluded that the cell-aggregates with 100 _m-thickness were dead by UV-irradiation, while the 500 _m-thick cell layer is sufficient to protect subsurface cells from UV-radiation in space. In addition, UV-induced pyrimidine dimers and ionizing radiation- and space vacuum-induced double-strand breaks were also detected in space exposed samples. These results highlight the importance of microbial cell-aggregates as an ark for interplanetary transfer of microbes as we hypothesized in massapanspermia (Kawaguchi et al., 2013). We also report survival and DNA damage detected from cell-aggregates exposed to space for two-years exposure.ReferencesArrhenius, 1908, Worlds in the Making: the Evolution of the Universe, Harper & Brothers.Onofri et al., 2012, Astrobiology, 12, 508-516.Kawaguchi et al., 2016, Astrobiology, 16, 363-376.Kawaguchi et al., 2013, Orig. Life Evol. Biosph., 43, 411-428.Yamagishi et al., 2007, Biol. Sci. Space, 21, 67-75.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E1714K