Signs of Life A Report Based on the April 2000 Workshop on Life Detection Techniques (2002) / Chapter Skim
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3 Detecting Extant Life
3 Detecting Extant Life
Pages 12-24
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The detection of extremely low levels of microorganisms after spacecraft sterilization involves increasing refinement of laboratory techniques designed to detect known types of terrestrial organisms. It also requires research into the possibilities and consequences of failure to detect unknown or poorly known microorganisms that exist within Earth's environment.
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The workshop discussions and the session papers represent a snapshot of the kinds of techniques and their capabilities available at a time when interest in finding life beyond Earth has reached a new crescendo. BIOMOLECULES AND MOLECULAR TECHNIQUES New technologies originating in the biomedical community will be available for use in detecting extant life on other planets.
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For example, the crystal structure and other structural or chemical aspects of biologically produced magnetite crystals can be diagnostic, as described in the paper by Kirschvink. He argues that a detailed and systematic comparative analysis of biogenic magnetite from terrestrial bacteria leads to the conclusion that the magnetite crystals in the SNC meteorite ALH84001, generally accepted to have come from Mars, are the signatures of martian biology.
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Fluorescent dyesspecific for different macromolecules such as proteins, lipids, and nucleic acids are routinely used to identify organisms in natural samples using epifluorescence microscopy. These techniques include the following: Light microscopy.
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Recent experiments couple flow cytometry with fluorescence and fluorescent antibodies or fluorescently labeled DNA or RNA probes. Large volumes of water are required.2~-23 (Technique not discussed during the workshop.)
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The direct methods rely on microscopic observations of dividing cells and measurements of increasing numbers of cells in growth chambers or on glass slides in situ. Radio-labeled substrates are used to make indirect measurements of growth rates based on the rate of DNA, RNA, and protein synthesis.
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Other culture methods, such as the ability to grow in liquid nutrient and solid nutrient media, tissue culture, or animal hosts, are best suited for microorganisms that have complex nutrient requirements or are animal pathogens. Culturing methods including inoculation of samples into suitable hosts should be applied to martian samples for detection of viable microorganisms including animal pathogens.42~4 (See the paper by Soffen in Session 1.)
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are measured. These methods are particularly useful for measuring rates of methane production and consumption, CO production and consumption, hydrogen oxidation, and incorporation of phosphorus into nucleic acids and sulfur into S-amino acids.58-62 (See the paper by Soffen in Session 1.)
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Molecular Methods At the present time, molecular methods are being developed to determine some specific genes that are being transcribed in situ, and new methods are being developed for determining the genes being transcribed by microbial communities in situ. These techniques include the following: · mRNA analyses.
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White, and J.A. Baross, "Phospholipid Fatty Acid-Derived Microbial Biomass and Community Dynamics in Hot, Hydrothermally Influenced Sediments from Middle Valley, Juan de Fuca Ridge," in R.A.
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White, and J.A. Baross, "Phospholipid Fatty Acid-Derived Microbial Biomass and Community Dynamics in Hot, Hydrothermally Influenced Sediments from Middle Valley, Juan de Fuca Ridge," in R.A.
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Fuhrman, "Combined Microautoradiography-16S rRNA Probe Technique for Determination of Radioisotopic Uptake by Specific Cell Types In Situ," Appl. Environ.
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Ludwig, "Ribosomal RNA-Targeted Nucleic Acid Probes for Studies in Microbial Ecology," FEMS Microbiol.
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