The Limits of Organic Life in Planetary Systems (2007) / Chapter Skim
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3 Pushing the Boundaries of Life
3 Pushing the Boundaries of Life
Pages 29-42
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From page 29... ...
The limits of life on Earth may help to define the limits of life elsewhere even though different life forms may have different biochemistries and different origins. In addition, Earth life is possible outside the bounds of extreme conditions found on Earth (for example, the bacterium Deinococcus radiodurans can tolerate levels of radiation not found naturally on present-day Earth, and Escherichia coli apparently can tolerate hydrostatic pressures greater than 10 times the pressures in the deepest ocean trenches 1,2)
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From page 30... ...
However, we cannot answer the general question of whether there could be multiple settings for creating different carbonbased life forms. Would a separate origin of life under conditions different from the ones that produced life on Earth create a carbon-based life form capable of different evolutionary innovations, or do rules of organic chemistry limit carbon-based life to the physiological diversity represented by extant Earth life?
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From page 31... ...
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 230°C.
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From page 32... ...
Lost City hydrothermal vent (e.g., Mono Lake) environment High hydrostatic High diversity of bacteria High diversity of invertebrates 11,100 m (Challenger Deep, pressure and archaeans in deep ocean and fishes in ocean trenches Marianas Trench)
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From page 33... ...
that was only recently discovered is teeming with microorganisms.23 Some other combinations known to exist in natural environments -- such as high pressure, high salt, and low temperature -- have rarely been studied.24 There are also combinations of extreme conditions that are known to have a synchronous effect on the growth or survival of cells that are not adapted to them. That is the case for hydrostatic pressure and temperature and for salt and temperature.25 Low temperature and high hydrostatic pressure affect cell processes in the same way; the result is that the minimal growth temperature of nonpiezophilic microorganisms increases with increasing pressure.
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From page 34... ...
That strategy requires evolutionary adaptations of intracellular macromolecules and of metabolic and biosynthetic processes to operate despite high salt concentrations. Most other microorganisms and eukaryotes deal with desiccation by accumulating compatible organic compounds that include organophosphorus compounds, b-amino acids, and glycerol.
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From page 35... ...
lichens at 220°C.46 Water can remain liquid at temperatures lower than 230°C in the presence of salts or other solutes and at even lower temperatures in combination with soluble organic solvents. Enzyme activity has been measured at 2100°C in a mixture of methanol, ethylene glycol, and water.47 There is also evidence of electron transfer and enzyme activity at 280°C in a marine bacterium.48 The process of vitrification (liquid water moving directly into the glassy state without ice-crystal formation)
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From page 36... ...
) oceans Antarctic salt lakes Antarctic (and Europan?
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From page 37... ...
Panspermia is an important issue in the context of this report because the presence of life in any extraterrestrial setting would require that it have an origin -- de novo or by panspermia. Moreover, if panspermia is how life began on Earth, it is likely to be how life could arise elsewhere, and life forms found elsewhere would then resemble Earth life at the fundamental, biochemical level.
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From page 38... ...
It exploits the same DNA for genetics, the same amino acids in its proteins, and the same core metabolism. Furthermore, analysis of its chemical structure leaves no doubt that these exotic terran life forms are related to humans by common ancestry.
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From page 39... ...
It is clear that biological systems can manage the chemical reactivity of unstable species. For example, oxaloacetate -- a metabolic intermediate in terran metabolism that is a precursor of citric acid, malic acid, and the amino acid aspartic acid -- decarboxylates readily, with a half-life measured in minutes at room temperature at neutral pH.
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From page 40... ...
It is also probable that the limits of extant terran organisms might not be indicative of limits possible for organisms with terran biochemistry. Life occupies most known habitats and in some cases can tolerate extreme conditions, such as irradiation and hydrostatic pressure, that exceed those found in natural environments.
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From page 41... ...
2005. Oxygen isotopes indicate most intracellular water in log-phase Escherichia coli is derived from metabolism.
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From page 42... ...
nov., two hyperthermophilic archaea from deep-sea hydrothermal vents that resist ionizing radiation. Extremophiles 8:219-227.
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