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The Limits of Organic Life in Planetary Systems
other strategies for carbon-based life to thrive in nonaqueous solvents, such as exist on Titan or in the hot sulfuric acid atmosphere of Venus. Even more radical is the possibility of seriously “weird life” on extremely hot, cold, or gaseous giant planets.7,8 We know about only one kind of life, and it is carbon-based, requires liquid water, can evolve, and, given “optimal” environmental conditions, can evolve into complex organisms.
3.2 EXTREMOPHILES AND THE LIMITS OF LIFE
Extreme conditions that limit growth or prove lethal to most organisms can be ideal habitats for others. Extremes of high temperature, high and low pH, high salt concentration, concentrations of toxic metals and organic compounds, and high levels of radiation kill the overwhelming majority of Earth’s organisms. However, organisms in all three domains of life have adapted to many terrestrial extremes. High-temperature, low-pH, and high-salinity environments are probably ancient, as may be frozen environments. Those extreme environments are not rare: most of the ocean is cold and deep, and a vast portion of the subsurface is hot.
There are few natural environments on Earth where life is absent—life is the rule rather than the exception. Microbial life on Earth has proliferated in habitats that span nearly every imaginable physicochemical variable. Until recently, only the very highest temperatures or lowest water activities (desiccation) were thought to render terrestrial environments unsuitable for growth. Now there is evidence that environments that have MgCl2 at concentrations greater than 2.3 M, such as a high-brine lake on the Mediterranean Sea floor, may inhibit life and that this inhibition is due to the ability of MgCl2 to denature biological macromolecules.9 However, such conditions do not necessarily render the environments sterile; many organisms have adapted mechanisms for long-term survival at temperatures more than 100°C above their maximal growth temperature or in a desiccated state. Few of the supposedly sterile environments are actually free of surviving life. Viable microorganisms have been detected, albeit in low numbers, in Chile’s Atacama Desert, perhaps the driest environment on Earth and thought to be an analogue of sterile Martian soil.10 In contrast, although they are rare, some environments with liquid water do not appear to support life; they include water over 400°C at submarine hydrothermal vents that is kept liquid by hydrostatic pressure11 and the high-brine liquid water found in sea-ice inclusions at −30°C. Even in those extreme cases, there is evidence of viable microorganisms that apparently survive exposure to temperature extremes well outside their growth range.12
Several recent review articles discuss the limits of life, the characteristics of extremophiles, and implications for astrobiology.13-15 Most discussions of the limits of life focus on extremes of single physical or chemical conditions, such as temperature, salinity, heavy-metal concentrations, desiccation, and pH. There are also many excellent reviews of single classes of extremophiles that should be consulted for detailed information on their ecology, physiology, and biochemistry.16-20
Individual organisms are often highlighted for their ability to lead the pack in tolerance of or ability to grow under extreme conditions (Table 3.1). In some cases—such as high pH, high hydrostatic pressure, and high metal content—the stated limits for life merely reflect the limits found in natural environments. There appears to be no absolute maximal temperature or minimal concentration of water that will prevent cellular growth. Two distinct classes of extreme environmental conditions are based on how they affect cells. In one, the effects of extremes in pressure and temperature extend into the cytoplasm, and intracellular biosynthesis, metabolism, and macromolecular structures are adapted to function under such conditions. In the other, organisms capable of growing in extremes of pH, salinity, and irradiation and in the presence of high concentrations of organic solvents and toxic metals are adapted either to maintain intracellular conditions that are typical for nonextremophiles or compensate for the extreme conditions. There are some exceptions. A recently described acidophilic archaean, Picrophilustorridus, grows optimally at a pH of 0.7 and apparently maintains an intracellular pH of 4.6.21 Most acidophiles maintain an internal pH near neutrality, and P. torridus must have novel intrinsic factors for stabilizing proteins and nucleic acids at low pH. The extremely halophilic archaeans have an absolute requirement for salt and grow best at salt concentrations of 3.5-4.5 M but can also grow in saturated NaCl (5.2 M). The intracellular functional and structural components of haloarchaeans are adapted to high salt concentration (up to 5 M, mainly KCl), and their enzymes require high salt to maintain their active structure. Other moderately halophilic bacteria grow over