The Quarantine and Certification of Martian Samples (2002) / Chapter Skim
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2 Detection of Potential Biohazards
2 Detection of Potential Biohazards
Pages 10-20
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From page 10... ...
A series of tests can be envisioned that will provide evidence of viable or recently dead organisms, detect chemical fossils or probable biological molecules (biomarkers) , and, at the same time, quantify contamination by terrestrial microbes and organic compounds.
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From page 11... ...
will allow the detection of organic compounds and their identification even if they should be novel compounds unique to Mars, existing life detection methods are not so easily adapted to finding Mars life if the latter employs novel and unique biomolecules. Nevertheless, the principles of genetic and metabolic analysis of geological and environmental samples will be extremely useful in efforts to identify and quantify the magnitude and sources of any terrestrial contamination in martian samples, and they may serve for detecting Mars life if it is sufficiently similar to life on Earth.
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From page 12... ...
Thus, although viruses can be very successful among and destructive of the terrestrial organisms with which they have convolved, it is extremely unlikely that viruses for any truly novel form of life—one that is not based in the same genetic materials and exactly the same relationships between gene sequence and amino acid sequence in proteins (the translation table that we call the genetic code) —would be able to replicate on Earthed This means that we need not speculate about a particularly resistant strain of virus that defies the biological principles of terrestrial life.
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From page 13... ...
Researchers anticipate technological advances, in particular advances associated with the monitoring of complex gene expression patterns, that may provide tools for detecting environmental perturbations. However, the necessary first step toward evaluation of how possible alien life forms might affect complex terrestrial ecosystems or human health will be the ability to detect the presence of microbial life in samples returned to Earth.
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From page 14... ...
Contamination is a substantial concern for all aspects of sample analysis, but contamination with viable organisms presents the added complication that the organisms can replicate and their significance thereby increase.l9 The more significant problem with the use of culturing for life detection is that of false negatives, leading to failure to detect the existence of viable organisms that defy cultivation.20 In spite of great advances in available methods, several lines of evidence indicate that the majority of organisms on Earth (whether measured by cell count, cell mass, or genetic diversity) have not been successfully cultivated in the laboratory.
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From page 15... ...
Ribosomal RNA is a common target of PCR because it is universal among terrestrial organisms, and there is an extensive database of the diverse sequences that have been obtained.26 Although, as noted above, it is not realistic to depend on DNA and rRNA genes being present in extraterrestrial life, their presence does provide a sensitive test of terrestrial contamination (even more sensitive than growth, in that it is not limited to organisms that researchers know how to cultivate)
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From page 16... ...
16 THE QUARANTINE AND CERTIFICATION OF MARTIAN SAMPLES Relative Sensitivity and Throughput of Detection Techniques The sensitivities of the various techniques outlined above are not discussed in detail here. To facilitate comparison, Table 2.1 indicates some of the properties of the techniques, characterizing them qualitatively or in terms of broad ranges, since it is frequently possible to improve one parameter at the expense of another.
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From page 17... ...
In addition, PCR can be used to screen samples whose mass ranges from nanograms to a gram without an increase in the complexity of or time required for the test, so it also has a high throughput. Although the use of PCR with rRNA genes will enable detection of virtually any terrestrial organism, the disadvantage of this technique is that it is unlikely to detect novel life forms that originated independently of terrestrial life.
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From page 18... ...
First, lack of detection is not a reliable indicator that the sample tested contained no viable organisms. Under any conditions chosen, most known organisms will not respond to culturing.27 And although PCR is a more general method of detecting viable or recently dead terrestrial organisms than culturing, it too is capable of producing false negatives.
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From page 19... ...
In essence, it would be possible to say only that viable organisms were not abundant in the sample, not that the rest of the sample was free of life. Although it might be possible to achieve comparable sensitivity with microscopic searches for cell morphology, studies of stable isotopes, or analyses of elemental distributions, and although such studies might have the advantage of being able to detect novel life forms, none of these methods has a very high throughput.
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From page 20... ...
. More complex histories would obviously be possible for an amplified sample that produced multiple sequences, but phylogenetic analyses could be used to check consistency with alternative hypotheses regarding origins and transport.
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