Magnesium Sulfate Salt Solutions and Ices Fail to Protect Serratia liquefaciens from the Biocidal Effects of UV Irradiation under Martian Conditions.

The growth of Serratia liquefaciens has been demonstrated under martian conditions of 0.7 kPa (7 mbar), 0°C, and CO-enriched anoxic atmospheres (Schuerger et al., 2013, Astrobiology 13:115-131), but studies into the survivability of cells under hypersaline conditions that are likely to be encountered on Mars are lacking. Serratia liquefaciens cells were suspended in aqueous MgSO solutions, or frozen brines, and exposed to terrestrial (i.e., 101.3 kPa, 24°C, O/N-normal atmosphere) or martian (i.e., 0.7 kPa, -25°C, CO-anoxic atmosphere) conditions to assess the roles of MgSO and UV irradiation on the survival of S. liquefaciens. Four solutions were tested for their capability to attenuate martian UV irradiation in both liquid and frozen forms: sterile deionized water (SDIW), 10 mM PO buffer, 5% MgSO, and 10% MgSO. None of the solutions in either liquid or frozen forms provided enhanced protection against martian UV irradiation. Sixty minutes of UV irradiation reduced cell densities from 2.0 × 10 cells/mL to less than 10 cells/mL for both liquid and frozen solutions. In contrast, 3-4 mm of a Mars analog soil were sufficient to attenuate 100% of UV irradiation. Results suggest that terrestrial microorganisms may not survive on Sun-exposed surfaces on Mars, even if the cells are embedded in frozen martian brines composed of MgSO. However, if dispersed microorganisms can be covered by only a few millimeters of dust or regolith, long-term survival is probable. Key Words: Hypobaria-Mars-Planetary protection-Brines. Astrobiology 17, 401-412.