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, [rmickol@uark.edu], 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.