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

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.

Changes in membrane fatty acids of a halo-psychrophile exposed to magnesium perchlorate and low temperatures: Implications for Mars habitability

“…Our results support the need of studying polyextreme environments (i.e., those that harbor two or more harsh conditions for life, e.g., Horne et al, 2022) and polyextremophilic microorganisms to understand life's adaptation to extreme environments. This is key in the research of habitability, understand limits of life and search for biosignatures on Mars and icy moons of the outer Solar System, where high content of salts and cold temperatures are present simultaneously.…”

Medical Astro-Microbiology: Current Role and Future Challenges

“…Simulating ionising radiation conditions encountered on Mars, a recent study estimated that the extremophilic Deinococcus radiodurans could survive over 100 million years buried 10 m below Mars’s surface. 183 This finding was quantified using cell manganese antioxidant accumulation as a sign of tolerance to different forms of radiation. This raises the possibility that if microbial life ever existed on Mars, it could still be possibly viable with unchartered potential effects and interactions with Earth’s biosphere.…”

NASA must protect Earth's biosphere even if Mars samples hold mirror life – motivating planetary protection with new scenarios like mirror life - survey of recent research relevant to a Mars sample return - and how to keep Earth's biosphere 100% safe with a miniature telerobotic lab above GEO, and protect other biospheres 100% with sterile robotic explorers

“…0.3 megagrays is enough for a trillion fold reduction of radiodurans based on a million fold reduction at 0.14 megagrays (Horne et al, 2022).…”

“…That might be more than enough since the number of viable Radiodurans microbes is reduced a million fold at 0.14 megagrays when it is at its most resilient, desiccated and frozen (Horne et al, 2022). This works out as an approximately 1.02 fold reduction in amino acids, destroying around 2% of the amino acids.…”