Mars-relevant Microorganisms in Simulated Subsurface Environments under Hydration/Dehydration Conditions

Abstract

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.