DNA Damage and Survival Time Course of Deinococcal Cell Pellets During 3 Years of Exposure to Outer Space.

Satoshi Kodaira,Kazumichi Nakagawa,Issay Narumi,Daisuke Fujiwara,Mio Shibuya,Akihiko Yamagishi,Yukio Uchihori,Yuka Murano,Iori Kinoshita,Hirofumi Hashimoto,Eiichi Imai,Jun Yatabe,Hajime Mita,Hiromi Shibata,Risako Hayashi,Yuko Kawaguchi,Shin-ichi Yokobori

doi:10.3389/fmicb.2020.02050

Abstract

The hypothesis called “panspermia” proposes an interplanetary transfer of life. Experiments have exposed extremophilic organisms to outer space to test microbe survivability and the panspermia hypothesis. Microbes inside shielding material with sufficient thickness to protect them from UV-irradiation can survive in space. This process has been called “lithopanspermia,” meaning rocky panspermia. We previously proposed sub-millimeter cell pellets (aggregates) could survive in the harsh space environment based on an on-ground laboratory experiment. To test our hypothesis, we placed dried cell pellets of the radioresistant bacteria Deinococcus spp. in aluminum plate wells in exposure panels attached to the outside of the International Space Station (ISS). We exposed microbial cell pellets with different thickness to space environments. The results indicated the importance of the aggregated form of cells for surviving in harsh space environment. We also analyzed the samples exposed to space from 1 to 3 years. The experimental design enabled us to get and extrapolate the survival time course to predict the survival time of Deinococcus radiodurans. Dried deinococcal cell pellets of 500 μm thickness were alive after 3 years of space exposure and repaired DNA damage at cultivation. Thus, cell pellets 1 mm in diameter have sufficient protection from UV and are estimated to endure the space environment for 2–8 years, extrapolating the survival curve and considering the illumination efficiency of the space experiment. Comparison of the survival of different DNA repair-deficient mutants suggested that cell aggregates exposed in space for 3 years suffered DNA damage, which is most efficiently repaired by the uvrA gene and uvdE gene products, which are responsible for nucleotide excision repair and UV-damage excision repair. Collectively, these results support the possibility of microbial cell aggregates (pellets) as an ark for interplanetary transfer of microbes within several years.

Highlights

Panspermia hypothesis postulates that microscopic forms of life, such as spores, can be dispersed in interplanetary space and thereby seed life from one planet to another (Arrhenius, 1908)
The slopes of the International Space Station (ISS) cabin controls were steeper than the ground controls for each strain
The ISS cabin controls of D. radiodurans R1 showed reduced survival compared to the ground controls after 3 years of exposure (Figure 2A)

Summary

Introduction

Panspermia hypothesis postulates that microscopic forms of life, such as spores, can be dispersed in interplanetary space and thereby seed life from one planet to another (Arrhenius, 1908). Microbes inside shielding material (e.g., small fragments of rock and mixtures of sugar or clay) with sufficient thickness to protect them from UV-irradiation can survive in space (Horneck et al, 2001; Onofri et al, 2012; Bryce et al, 2015; Panitz et al, 2015). This process has been called “lithopanspermia,” meaning rocky panspermia (Melosh, 1988; Mileikowsky et al, 2000; Horneck et al, 2002; Nicholson, 2009; Worth et al, 2013)

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Conclusion