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The Effect of Martian UV Radiation on the Survival of Serratia liquefaciens Within Martian Analog Brines
and Ices. R. L. Mickol1, J. L. Page
2, and A. C. Schuerger
3 1Arkansas Center for Space and Planetary Sciences, 346
½ N. Arkansas Ave., University of Arkansas, Fayetteville, Arkansas 72701, [[email protected]], 2Department of
Physics and Space Science, Florida Institute of Technology, Melbourne, Florida, 3Department of Plant Pathology,
University of Florida, Space Life Sciences Lab, Kennedy Space Center, Florida.
Introduction: Despite stringent procedures, nu-
merous bacterial and archaeal species have been isolat-
ed from spacecraft cleanrooms [1]. Bacteria able to
endure cleaning procedures may also have the ability to
survive interplanetary travel, and could possibly arrive
at Mars. A common spacecraft contaminant, Serratia
liquefaciens, was recently shown to grow under mar-
tian conditions of 7 mbar, 0 °C, and a CO2-dominated
atmosphere [2]. The experiments discussed here ex-
pand upon previous research by testing the
surviviability of S. liquefaciens exposed to martian UV
irradiation. Liquids, frozen salt solutions and regolith
simulants were tested for their attenuation properties.
Methods: Experiment 1: Cells of S. liquefaciens
were prepared at densities of 2 x 106 cells/mL in four
solutions: sterile deionized water (SDIW), 10 mM PO4
buffer, 5% MgSO4, or 10% MgSO4. The solutions
were dispensed into 6-cm petri dishes, equipped with
magnetic stir bars, and placed into the Mars Simulation
Chamber (MSC) atop magnetic stir pads. The dishes
were immediately assayed using a Most Probable
Number (MPN) procedure [3, 4]. The liquid solutions
were exposed to Mars UV irradiation for periods of 10,
30, 60, 300, 600, 1200, 1800, or 3600 seconds (cumu-
lative). A 1 mL sample was removed after each
timestep and assayed via MPN. The experiment was
conducted under Earth-normal lab conditions.
Experiment 2: Cell suspensions of S. liquefaciens
were prepared in four solutions as above. The dishes
were placed in the MSC atop an LN2 cold plate. The
MSC was equilibrated to 7 mbar, -25 °C, and a CO2-
enriched anoxic atmosphere. The frozen cell suspen-
sions were exposed to Mars UV irradiation for 0, 10,
60, 600, or 3600 sec. After 60 minutes, regardless of
the period of UV exposure, the MSC was equilibrated
to lab conditions and the cells were processed via
MPN. A second set of dishes were prepared similarly,
but were shielded from UV exposure.
Experiment 3: Cells of S. liquefaciens were pre-
pared in four solutions as above. For each solution, 12
mL were immediately processed via MPN, 12 mL were
incubated for 24 h at 24 °C, and 12 mL were incubated
for 24 h at -20 °C. All samples were maintained at a
lab-normal pressure of 1013 mbar, and processed after
24 h by the MPN procedure.
Experiment 4: Cells of S. liquefaciens at densities
of 2 x 106 cells/mL were added to solutions of 5%
MgSO4 within 6-cm petri dishes. The dishes were add-
ed to the MSC and the temperature was lowered to 0
°C at a pressure of 25 mbar. After the solutions froze,
the MSC was vented to lab conditions and 3-4 mm of
sterile analog regolith (basalt, Hawaiian palagonite)
were added atop the frozen solutions. The MSC was
equilibrated to -25 °C, 7 mbar, and Mars atmosphere.
The regolith and frozen solutions were exposed to
Mars UV radiation for 1 h. The MSC was vented to lab
conditions and the regolith was removed from the solu-
tions while still frozen. After melting, the solutions
were processed via MPN.
Results: Within all four liquids, survival of S.
liquefaciens cells decreased by 2.5-log after 10 s mar-
tian UV irradiation and by 6-log after 60 min exposure.
Frozen solutions offered no additional protection
against martian UV radiation, resulting in less than 10
viable cells/mL after only 10 min exposure. Survival in
frozen solutions (without exposure to martian condi-
tions) was similar to samples at 24 °C, except for 10
mM PO4 buffer (2-log decrease in the number of viable
cells). Complete attenuation of martian UV radiation
was achieved through the use of 3-4 mm of martian
analog regolith.
Discussion/Conclusions: Liquid solutions and fro-
zen ices offered no additional protection against Mars
equatorial UV radiation. Viable cell numbers of S.
liquefaciens were reduced by 6-log after 60 min expo-
sure of Mars UV radiation. Millimeter thicknesses of
martian analog regoliths proved capable of attenuating
the effects of radiation, suggesting that near-sub-
surface habitats may be more welcoming environments
for bacteria than the surface of the planet. Samples
exposed to 7 mbar, -25 °C, and a CO2 atmosphere, but
shielded from UV irradiation, confirm the ability of S.
liquefaciens to survive under multiple Mars conditions.
Acknowledgements: R. Mickol was supported by
the NASA Astrobiology: Exobiology and Evolutionary
Biology Program (grant NNX12AD90G), and worked
in A. Schuerger’s lab under a grant from NASA’s
Planetary Protection Office (#NNX12AJ84G). J. Page
was supported by Florida Space Grant Consortium and
Space Florida in A. Schuerger’s lab.
References: [1] Moissl-Eichinger, C., Rettberg, P.,
and Pukall, R. (2012) Astrobiology 12: 1024-1034.
[2] Schuerger, A.C., et al. (2013) Astrobiology 13:
115-31. [3] Koch, A. (1994) Methods for Gen. and
Molec. Bacter., 257-260. [4] Mancinelli, R.L., and
Klovstad, M. (2000) PSS 48: 1093-1097.