Date of Graduation

8-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Space & Planetary Sciences (PhD)

Degree Level

Graduate

Department

Space & Planetary Sciences

Advisor/Mentor

Kral, Timothy A.

Committee Member

Lessner, Daniel J.

Second Committee Member

Kennefick, Daniel J.

Third Committee Member

Dixon, John C.

Fourth Committee Member

Schuerger, Andrew C.

Keywords

Astrobiology; Low Pressure; Mars; Methane; Methanogens; Microbiology

Abstract

The discovery of methane in the martian atmosphere via numerous ground- and space-based sources has prompted the study of methanogens as models for life on Mars. Methanogens are microorganisms within the domain Archaea, many of which utilize carbon dioxide (CO2) and hydrogen to produce methane. The non-photosynthetic nature of methanogens indicates that they could exist in sub-surface environments, protected from harmful UV and ionizing radiation on the surface of Mars. These organisms also do not require organics, which are sparse on the planet.

Additionally, the wide variety of environments we find life in on Earth, as well as evidence for liquid brines on the surface of Mars, suggest that habitable environments may still exist on the planet. However, there are a variety of conditions that any extant life on Mars would need to endure, including wide variations in temperature over one sol, a low-pressure atmosphere, and a limited availability of liquid water, among others.

This dissertation encompasses various experiments that examined the ability of four species of methanogens (Methanosarcina barkeri, Methanobacterium formicicum, Methanococcus maripaludis, and Methanothermobacter wolfeii) to survive and/or grow under 1) low-pressure conditions and 2) freeze/thaw cycles. Low pressure studies include both survival and active growth experiments conducted between 7 mbar (the average surface pressure on Mars) and 143 mbar. Freeze/thaw experiments utilized short- and long-term cycles varying in temperature between the organisms’ growth temperatures (22 C, M. maripaludis; 37 C, M. barkeri and M. formicicum; 55 C, M. wolfeii) and -80 C, encompassing Mars-relevant temperature changes. As a comparison to methanogen growth and survivability, additional experiments were conducted using a non-spore-forming bacterium, Serratia liquefaciens, previously shown capable of growth at 7 mbar, 0 C and within an anoxic CO2 atmosphere. The experiments described here assessed the survivability of S. liquefaciens exposed to martian UV irradiation within liquid brines and ices.

The experiments discussed here demonstrate the ability of Earth microorganisms to withstand certain extreme conditions on Mars and suggest that the planet may contain relatively habitable microenvironments within the near subsurface.

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