Mars-relevant microorganisms in simulated subsurface environments under hydration/dehydration conditions
2010
The ability of certain microbial species to survive adverse conditions such as radiation and
desiccation has been an important research subject for astrobiology. The reason for this is
perhaps the traditional view that life at the surface of planets, which generally have thin
atmospheres, low amounts of water and intense radiation, might be possible. Interestingly, many
highly resistant organisms such as Deinococcus geothermalis or D. radiodurans have not been
isolated from environments resembling in any respect conditions which are known to exist on
Mars or other terrestrial planets. Many of these organisms are heterotrophs relying on the
availability of complex organic molecules which are scarce -if existent at all- on planets such
as Mars. Although these organisms are important to understand the tenacity of certain forms of
life, their role in potential Martian ecosystems might not be entirely realistic. If present, life on
Mars could be located at either the shallow or deep subsurface were conditions are “less
extreme” as compared to the surface of the planet. The subsurface’s porous matrix would offer
increased physical protection to environmental stresses as well as higher water availability.
Potential life forms thriving in these environments, would obtain their energy from redox
disequilibria caused by chemical species at different oxidation states. In the case of Mars, the presence of iron in both oxidized and reduced forms as well as the presence of several forms of
sulfur might allow life forms which could utilize these compounds in a hypothetical energy
conserving process. Furthermore, the presence of sulfates suggests that water has been
abundant at intermittent stages during Mars history although it is known to be scarce at
present. Based on these assumptions, we are studying several species of iron-sulfur bacteria
including Acidithiobacillus sp. and Sulfobacillus sp. These bacteria are chemolithoautotrophs,
able to utilize iron and sulfur compounds in various forms for their energy requirements and
therefore might be more relevant for Mars. The resistant model organism Deinococcus
geothermalis is used for comparative purposes. Microbial colonization as a biofilm is
simulated in porous materials consisting of Mars-relevant minerals. Key physiological
responses to hydration/dehydration cycles and later simulated Martian environmental
conditions are studied using both microbiological and physicochemical techniques. These
techniques include: fluorescence microscopy, confocal laser scanning microscopy, colorimetric
assays, ion chromatography and microcalorimetry. Results with iron- sulfur biofilms grown in
absence of a porous environment suggests low resistance to de-hydration, however, the role of
minerals and high specific surfaces of porous environments on microbial activity during or
after hydration stress remains an open question which is currently under investigation.
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