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Extreme Environments

Life on Earth is based on the chemistry of carbon in water. The temperature limits compatible with the existence of life are thus imposed by the intrinsic properties of chemical bonds involved in this type of chemistry at different temperatures. Microorganisms have invented several strategies to cope with or specifically adapt to environments, generally called "extremes" with regard to temperature, water activity, pressure, atmospheric composition, or radiation.

Liquid water is a necessary prerequisite for life. At an elevated boiling point of water, e.g. by high hydrostatic pressure, the deep-sea Archaea Pyrolobus fumarii grows at temperatures as high as 113°C and can tolerate 121°C for an hour. Important factors preventing life at temperatures well above 110°C are the thermal instability of some chemical bonds involved in biological molecules and membrane permeability.

Fig. 1: Haline.

Life is extremely diverse in the ocean at temperatures of 2°C. Living organisms, especially microorganisms, are also present in the frozen soils of arctic and alpine environments. Antarctica has a wide range of extreme habitats and the microbial ecosystems developing in dry valley rocks in particular. These extreme cold desert habitats are valuable analogues for postulated former habitats on Mars.

The lower limit for bacterial growth published in the literature is -20°C, the temperature at which intracellular ice is formed. To achieve such low habitat temperatures in nature, high concentrations of salts must accumulate. They can accumulate in water bodies such as Don Juan Pond, Antarctica, whose saturated CaCl2 solution does not freeze until the low temperature of -53°C which is rarely achieved, despite occasional minimum winter air temperatures of -62°C.

They also accumulate locally in soils where thin films of hypersaline brine can sustain liquid water down to -70°C. Salt-loving organisms, known as extreme halophiles, are well known in cold desert habitats and salt deposits such as salterns and salt mines. They tolerate a wide range of salt concentrations (1-20% NaCl) and some prokaryotes are so dependent on such high salt concentrations that they cannot grow at concentrations below 10% NaCl.

The chemistry of life on Earth is optimized for neutral pH value. Again, some micro-organisms have been able to adapt to extreme pH conditions, from extremely acidic at pH 0 to extremely alkaline at pH 12.5, albeit maintaining their intracellular pH between pH 4 and 9.
As with temperature, the intracellular machinery cannot escape the influence of pressure. However, there are organisms in the deepest parts of the ocean where they survive a pressure of about 1100 bar. The extreme pressure limit for life on Earth is unknown - environments of above 1100 bar have not been explored.

For a long time, it was believed that deep subterranean environments were sterile. It has now been recognized that bacteria actually thrive in the terrestrial crust. Subterranean microorganisms are usually detected in subterranean oil-fields or in the course of drilling experiments. For example, recent research has demonstrated that procaryotes are present much deeper in marine sediments than was previously thought Possible, extending to at least 750 meters below the sea floor, and probably much deeper.

To depths of at least 432 meter, microbes have been found altering volcanic glass. These data provide a preliminary, and probably conservative, estimate of the biomass in this important new ecosystem: about 10% of the surface biosphere. These discoveries have radically changed our perception of marine sediments and indicate the presence of a largely unexplored deep bacterial biosphere that may even rival the Earth's surface biosphere in size and diversity.

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