Thermophiles and Psychrophiles as Analogs for Life on Mars and Europa

It is now known that microbial extremophiles thrive in a wide range of environments on Earth. The discovery of these hardy life forms greatly enhances the possibility that viable microbiota might have existed in the ancient oceans or rivers of Mars or the oceans that exist today beneath the ice crust of Europa. Psychrophiles and thermophiles might also survive today, living within the hydrothermal vents, fumaroles, and deep crustal rocks of Mars. Psychrophiles may live or be cryopreserved in the ice crust of Europa or in the permafrost, glaciers, and polar ice caps of Mars.

Sulfur- (S-) and sulfate-reducing bacteria and archaea are microbial extremophiles of significance to astrobiology. On Earth, they inhabit a wide variety of anaerobic and hypo-oxygenic eco-niches, such as geysers, volcanic fumaroles, deep-sea hydrothermal vents, deep crustal rocks, marine sediments, deep aquifers, animal intestines, soda lakes, high salinity lakes, soils, alkaline evaporates, and cryo-environments [10-12].

Sulfate or sulfur respiration is an anaerobic process with a combination of energy conservation by redox phosphorylation at the substrate level and electron flow on acceptors as sulfur or sulfur compounds. Hydrogen sulfide (H2S) is the end product of both catabolic processes (sulfur and sulfate respiration). Pikuta and Hoover [13] reviewed anaerobic sulfate- and sulfur-reducing bacteria on Earth and have suggested that they represent possible analogs for microbial life that might inhabit Jupiter's volcanic moon, Io. This frozen world

Figure 2. Viking 2 image of snow at the Lander site (48° N, 226° W) on May 18, 1979 (detector temperature: 18.1 °C) (Photo courtesy MSFC and NASA).

has active volcanoes driven by tidal heating from its proximity to the giant planet Jupiter. The temperature of Io ranges from -150 °C to over 2,000 °C. The surface of Io is covered with elemental sulfur and sulfur compounds, and the atmosphere of Io is primarily sulfur dioxide. Recent studies of sulfur- and sulfate-reducing bacteria at MSFC have resulted in the isolation and sequencing of two new species: (1) an alkaliphilic Sulfate-reducing bacterium from soda Mono Lake in northern California [14] (Desulfonatronum thiodismutans, sp. nov.) and (2) a thermophilic archaean (Thermococcus sulfurophilus, sp. nov.) [15] from the Rainbow deep-sea hydrothermal vent.

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