The Effects of Freeze-Thaw and UVC Radiation on Microbial Survivability in a Selected Mars-like Environment.

The purpose of this study was to determine survivability of , and under Mars-simulated conditions for freeze-thawing (-80 °C to +30 °C) and UV exposure alone and in combination. ATCC 25922, and remained viable following 20 successive freeze-thaw cycles, exhibiting viabilities of 2.3%, 96% and 72.6%, respectively. ATCC 9079 was non-recoverable by cycle 9. When exposed to UV irradiation, cells withstood doses of 870 J/m ( ATCC 25922), 200 J/m ( ATCC 9079), 50,760 J/m () and 44,415 J/m (). Data suggests is highly UV-resistant. Combined freeze-thawing with UV irradiation showed freezing increased UV resistance in ATCC 25922, DSM 9079 and by 6-fold, 30-fold and 1.2-fold, respectively. Conversely, freezing caused to exhibit a 1.75-fold increase in UV susceptibility. Strain-dependent experimentation demonstrated that freezing increases UV resistance and prolongs survival. These findings suggest that exposure to short wavelength UV rays (254 nm) and temperature cycles resembling the daily fluctuating conditions on Mars do not significantly affect survival of , and ATCC 25922 following 20 days of exposure.