Fire and Ice: Life Under Extreme Temperature Conditions
The temperature range for microbes in cold surface habitats can be narrow, such as for psychrophiles that function only below ~20°C. Examples of these from Antarctic hypersaline lakes include the micro-algae Dunaliella sp. and Chlamydomonas sp. that are able to remain motile at temperatures as low as -14°C. The psychrophilic bacterial population of hypersaline Deep Lake (salinity 32% and mean temperature -15°C) is almost entirely composed of halophilic microorganism Archaea, the oldest root of the bacterial evolutionary tree. Alternatively, psychrotolerant organisms can metabolise at 0°C whilst continuing to function when the temperature exceeds ~20°C, as in maritime Antarctic soils.
Conversely, certain bacteria can withstand the exceptionally high temperatures that have been demonstrated in the habitable zone of hydrothermal oceanic vents on Earth where the current maximal growth temperature recorded is 113°C for Pyrolobus fumarolii. It is suggested that the maximum temperature for bacterial survival may even be as high as 150°C, and strains of the hyperthermophiles Pyrolobus and Pyrodictium have been shown to survive autoclaving at 121°C for one hour. There may therefore be a similar substantial subsurface geothermally-heated microbial ecosystem in groundwater on Mars.
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