| Autor: |
Horne WH; School of Medicine, Uniformed Services University of the Health Sciences (USUHS), Bethesda, Maryland, USA.; Department of Chemistry and Life Science, United States Military Academy, West Point, New York, USA., Volpe RP; School of Medicine, Uniformed Services University of the Health Sciences (USUHS), Bethesda, Maryland, USA.; Henry M. Jackson Foundation for the Advancement of Military Medicine, Bethesda, Maryland, USA., Korza G; Department of Molecular Biology and Biophysics, UConn Health, Farmington, Connecticut, USA., DePratti S; Department of Molecular Biology and Biophysics, UConn Health, Farmington, Connecticut, USA., Conze IH; School of Medicine, Uniformed Services University of the Health Sciences (USUHS), Bethesda, Maryland, USA.; Institute for Genetics and Cologne Excellence Cluster on Cellular Stress Responses in Aging-Associated Diseases (CECAD), Cologne, Germany., Shuryak I; Center for Radiological Research, Columbia University Irving Medical Center (CUIMC), New York, New York, USA., Grebenc T; Slovenian Forestry Institute, Ljubljana, Slovenia., Matrosova VY; School of Medicine, Uniformed Services University of the Health Sciences (USUHS), Bethesda, Maryland, USA.; Henry M. Jackson Foundation for the Advancement of Military Medicine, Bethesda, Maryland, USA., Gaidamakova EK; School of Medicine, Uniformed Services University of the Health Sciences (USUHS), Bethesda, Maryland, USA.; Henry M. Jackson Foundation for the Advancement of Military Medicine, Bethesda, Maryland, USA., Tkavc R; School of Medicine, Uniformed Services University of the Health Sciences (USUHS), Bethesda, Maryland, USA.; Henry M. Jackson Foundation for the Advancement of Military Medicine, Bethesda, Maryland, USA., Sharma A; Department of Chemistry, Northwestern University, Evanston, Illinois, USA., Gostinčar C; Biotechnical Faculty, University of Ljubljana, Ljubljana, Slovenia., Gunde-Cimerman N; Biotechnical Faculty, University of Ljubljana, Ljubljana, Slovenia., Hoffman BM; Department of Chemistry, Northwestern University, Evanston, Illinois, USA., Setlow P; Department of Molecular Biology and Biophysics, UConn Health, Farmington, Connecticut, USA., Daly MJ; School of Medicine, Uniformed Services University of the Health Sciences (USUHS), Bethesda, Maryland, USA.; Member, Committee on Planetary Protection (CoPP), National Academies of Sciences, Washington, DC, USA. |
| Abstrakt: |
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. |
