Effects of Desiccation and Freezing on Microbial Ionizing Radiation Survivability: Considerations for Mars Sample Return

William H. Horne, Robert P. Volpe, George Korza, Sarah Depratti, Isabel H. Conze, Igor Shuryak, Tine Grebenc, Vera Y. Matrosova, Elena K. Gaidamakova, Rok Tkavc, Ajay Sharma, Cene Gostinčar, Nina Gunde-Cimerman, Brian M. Hoffman, Peter Setlow, Michael J. Daly*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

30 Scopus citations

Abstract

Increasingly, national space agencies are expanding their goals to include Mars exploration with sample return. To better protect Earth and its biosphere from potential extraterrestrial sources of contamination, as set forth in the Outer Space Treaty of 1967, international efforts to develop planetary protection measures strive to understand the danger of cross-contamination processes in Mars sample return missions. We aim to better understand the impact of the martian surface on microbial dormancy and survivability. Radiation resistance of microbes is a key parameter in considering survivability of microbes over geologic times on the frigid, arid surface of Mars that is bombarded by solar and galactic cosmic radiation. We tested the influence of desiccation and freezing on the ionizing radiation survival of six model microorganisms: vegetative cells of two bacteria (Deinococcus radiodurans, Escherichia coli) and a strain of budding yeast (Saccharomyces cerevisiae); and vegetative cells and endospores of three Bacillus bacteria (B. subtilis, B. megaterium, B. thuringiensis). Desiccation and freezing greatly increased radiation survival of vegetative polyploid microorganisms when applied separately, and when combined, desiccation and freezing increased radiation survival even more so. Thus, the radiation survival threshold of polyploid D. radiodurans cells can be extended from the already high value of 25 kGy in liquid culture to an astonishing 140 kGy when the cells are both desiccated and frozen. However, such synergistic radioprotective effects of desiccation and freezing were not observed in monogenomic or digenomic Bacillus cells and endospores, which are generally sterilized by 12 kGy. This difference is associated with a critical requirement for survivability under radiation, that is, repair of genome damage caused by radiation. Deinococcus radiodurans and S. cerevisiae accumulate similarly high levels of the Mn antioxidants that are required for extreme radiation resistance, as do endospores, though they greatly exceed spores in radioresistance because they contain multiple identical genome copies, which in D. radiodurans are joined by persistent Holliday junctions. We estimate ionizing radiation survival limits of polyploid DNA-based life-forms to be hundreds of millions of years of background radiation while buried in the martian subsurface. Our findings imply that forward contamination of Mars will essentially be permanent, and backward contamination is a possibility if life ever existed on Mars.

Original languageEnglish (US)
Pages (from-to)1337-1350
Number of pages14
JournalAstrobiology
Volume22
Issue number11
DOIs
StatePublished - Nov 2022

Funding

We thank Dr. Aaron Thompson and Mr. Michael Woolbert (USUHS) for assistance in irradiator maintenance and calibration. We also thank Dr. Cara Olsen and Dr. Sorana Raiciulescu of the USU Biostatistics Consulting Center for their guidance and recommendations for conducting a thorough statistical analysis of the cell survival data, and Mrs. Sofia C. Echelmeyer (USUHS) for graphical help. We are grateful to Dr. Stephanie A. Malfatti at the U.S. Department of Energy, Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, for sequencing, annotation, and analysis of pMD66. This study was supported by the Defense Threat Reduction Agency (DTRA) Grant HDTRA1620354 (to MJD), a Laboratory Gift Fund (GK, SD, PS), NIH Grant GM111097 (to BMH) and by funds received from the USUHS Intramural Program to the Deinococcus Group. NGC, CG, and TG acknowledge the financial support from the state budget of the Slovenian Research Agency (grants BI-US/18-20-032, BI-US/18-20-078, J4-2549, J4-3098, P1-0198, P4-0432, P4-0107, I0-0022 MRIC UL IC Mycosmo). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The opinions expressed herein are those of the authors and are not necessarily representative of those of the USUHS, HJF, DTRA, or NIH.

Keywords

  • Bacillus
  • DNA repair
  • Deinococcus
  • Desiccation
  • EPR
  • Escherichia
  • Freezing
  • Holliday junction
  • Ionizing radiation
  • Mars
  • Mn antioxidants
  • Planetary protection
  • ROS
  • Saccharomyces
  • Spores
  • Vegetative cells

ASJC Scopus subject areas

  • Agricultural and Biological Sciences (miscellaneous)
  • Space and Planetary Science

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