Biological Contamination of Mars Issues and Recommendations (1992) / Chapter Skim
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4 LIMITS OF LIFE ON EARTH: EXPANSION OF THE MICROBIAL WORLD AND DETECTION OF LIFE
4 LIMITS OF LIFE ON EARTH: EXPANSION OF THE MICROBIAL WORLD AND DETECTION OF LIFE
Pages 30-42
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From page 30... ...
There is far too little information to assess the possibility that life may exist in subsurface environments associated with hydrothermal activity or in selected microenvironments free from the harsh conditions already mentioned, or to conclude that organisms resembling terrestrial life forms did not evolve on Mars during the planet's early geological history. The primary residual concern of the task group is with forward contamination by intact cells or components of cells that could be detected by sophisticated molecular methods in future expeditions designed to look for evidence of extant or past life on Mars.
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From page 31... ...
They include hyperthermophiles capable of growing at 110°C, obligate barophiles capable of growing at the pressures found in the deepest ocean trenches, and anaerobes capable of using iron, manganese, or even uranium as electron acceptors. Similarly, a variety of strategies have been identified by which microorganisms can survive environmental conditions that do not allow growth, including low temperature and low nutrient conditions.
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From page 32... ...
For example, an amylase from Pyrococcus furiosus -- a heterotroph capable of growing at temperatures up to 103°C -- has a half-life of 2 hours in an autoclave at 120°C and is active at 140°C.6 A purified -glucosidase, with a half-life of 48 hours at 98°C, reaches optimal activity in the temperature range of 105 to 115°C.7 The other extremely thermal stable enzymes studied from hyperthermophiles include ferredoxin, hydrogenase, serine protease, glyceraldehyde-3-phosphate dehydrogenase, and a never-before-described tungsten-iron-sulfur enzyme from P furiosus that catalyzes a dehydrogenase-like reaction of very low potential at 100°C.8 Besides proteins, other macromolecules from hyperthermophiles, including DNA and membrane lipids, must also have some unusual properties.
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From page 33... ...
Some spores germinate whenever there is free water, ranging in temperature from subfreezing to superboiling. Recognition of the ability of spores to survive such harsh conditions has led previous committees to focus on bacterial endospores as a major concern in planetary protection.
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From page 34... ...
The ability of Earth microbes with a full complement of enzymes to exist in relatively suspended animation for extended periods, yet to be ready for instant growth, has direct implications for planetary protection requirements related to forward contamination as well as sample return. The ability to maintain efficient DNA repair in the absence of cell division (which these organisms are apparently able to do)
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From page 35... ...
Cryptoendolithic lipids, which can stay fluid to -20°C, may be important for the organisms to metabolize in such cold conditions.22 Clearly these microbes have adapted to harsh environmental conditions, and these communities may provide reasonable models for survival strategies that might be adopted by microbes as conditions change from above-freezing temperatures and flowing water to temperatures below 0°C and limited free water. Barophiles If liquid water is present at kilometer depths on Mars, microbial life in that environment may face environmental conditions similar to those experienced by the barophilic bacteria isolated from the deep sea on Earth.
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From page 36... ...
LIFE DETECTION FOR PLANETARY PROTECTION (INCLUDING BIOBURDEN DETERMINATION) Techniques for assessing the existence of microorganisms have advanced dramatically since pre-Viking days, and these advances will strongly affect bioburden assessment procedures as well as future lifedetection experiments.
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From page 37... ...
In surface soils, direct microscopic counts of stained bacteria show that less than 1 percent of the organisms seen by epifluorescence microscopy can subsequently be recovered by direct plating and grown to form colonies. Clearly, the previously used procedures for counting viable organisms are insufficient to help assess potential contamination by organisms that could possibly reproduce on another planet or on spacecraft components.
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From page 38... ...
Consequently, the detection of specific polar lipids provides a quantitative definition of the viable or potentially viable cellular biomass and requires no growth or recovery of intact microbes. Because different groups of microbes contain identifiable specific patterns of lipid components, detailed examination of the structure of the lipid allows definition of the community structure of the microbial 38
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From page 39... ...
Thus the lipid analysis can provide direct evidence of lipid synthetic gene activity as well as the viable biomass, community structure, and nutritional status of the community. Since this technique could provide a means to detect the presence of extant or fossil life on Mars, it is important to prevent potential contamination of spacecraft by specific microbial lipids.
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From page 40... ...
This is one of the most characteristic features of life on Earth. Recent advances in the use of chiral derivatizing agents or stationary phases in column chromatography coupled with the detection of specific analytes on cooled germanium disks (which allow matrix-assisted microscopic Fourier transform infrared spectroscopy)
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From page 41... ...
1989. "Distribution of Inorganic Species in Two Cryptoendolithic Microbial Communities." Geomicrobiol.
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From page 42... ...
1978. "Life Under Conditions of High Irradiation." In Microbial Life in Extreme Environments.
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