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OCR for page 39
Preventing the Forward Contamination of Europa B Glossary Aerobe —An organism that requires oxygen to grow. See Anaerobe. Anaerobe —An organism with the capacity to grow in the absence of oxygen. See Aerobe. Archaea —Organisms making up one of the three branches onthe phylogenetic tree of life. Their cells do not contain a defined nucleus and they are genetically and biochemically distinct from the Bacteria. See Eukaryotes and Bacteria. Autotroph —Organisms than can use carbon dioxide as their sole source of carbon. See Heterotroph. Back contamination —The biological contamination of Earth withmaterial returned from another planetary body. See Forward contamination. Bacteria —Organisms making up one of the three branches ofthe phylogenetic tree of life. Their cells do not contain a defined nucleus and they are genetically and biochemically distinct from the Archaea. See Eukaryotes and Archaea. Barophiles —Microorganisms that require high (hundreds of megapascals) hydrostatic pressure for growth. Bremsstrahlung —Electromagnetic radiation generated when high-energy, charged particles rapidly decelerate during impact with a target. Cfu (colony-forming unit) —An individual cell that can be grown on a culture plate to form a colony of microorganisms. Chemoautotroph —Organisms with the ability to synthesize organic nutrients directly from simple inorganic compounds using the energy derived from chemical rather than photochemical reactions. Chemolithoautotroph —Organisms deriving all of their carbon and energy requirements from inorganic compounds. The “litho” component of the name implies that they derive energy from the oxidation of hydrogen. Chemosynthesis —The process by which certain organisms use the energy derived from chemical reactions to sustain their metabolism. See Photosynthesis. Cryptoendoliths —Organisms, typically bacteria and lichens, living just below the translucent surfaces of porous rocks found in Antarctica. DNA (deoxyribonucleic acid) —A polymer of nucleotides connected via a sugar-phosphate backbone. This complex biomolecule encodes genetic information in all terrestrial organisms. Eukaryotes —Organisms making up one of the three branches on the phylogenetic tree of life. Their characteristic feature is that their cells have a defined nucleus containing most of the organism’s DNA. See Archaea and Bacteria. Extremophiles —Microorganisms capable of growing under extreme physicochemical conditions such as high temperatures, pressures, and acidity.
OCR for page 40
Preventing the Forward Contamination of Europa Facultative anaerobe —An organism with the capacity to grow in both the presence and the absence of oxygen. See Aerobe and Anaerobe. Forward contamination —The biological contamination of an extraterrestrial body by terrestrial organisms inadvertently carried aboard a spacecraft. See Back contamination. Gram-positive bacterium —A bacterium that shows a purple color from Gram’s stain procedure. The structure of the bacterium’s cell wall determines its ability to retain the dye used in the Gram-stain procedure. Gray —A measure of radiation exposure defined in terms of the total amount of energy absorbed per unit mass of the absorbing material. One gray is equal to 1 joule of energy deposition per kilogram of the target material. Because the amount of energy absorbed depends on the nature of the target material, the unit is often qualified to indicate the nature of the target. One gray is equal to 100 rad. Heterotroph —An organism that survives by the ingestion and breakdown of complex organic materials. See Autotroph. Homoacetogen —A bacterium that produces acetate as an end product and can live as both a chemoautotroph (like methanogens) and a chemoheterotroph. Hydrothermal vents —Springs of hot seawater on the deep ocean floor. They are formed when cold seawater seeps through cracks in the ocean floor, circulates through volcanically heated rock, and returns to the seafloor rich in dissolved minerals. LET (linear energy transfer) —The energy lost per unit length of path when ionizing radiation passes through a material. Magnetosphere —The volume of space surrounding a planetary body that is under the dynamical influence of that body’s magnetic field. Methanogen —A prokaryote that produces methane via the reduction of either carbon monoxide or carbon dioxide in an anerobic environment. Panspermia —A theory suggesting that life on a given planet may have been seeded by life originating on another planetary body. PCR (polymerase chain reaction) —A relatively quick and sensitive technique used to detect and generate copies of specific DNA fragments. See DNA. Photosynthesis —The process by which certain organisms use the energy derived from sunlight to sustain their metabolism. Phylogenic —Pertaining to the relationships between different organisms. Such relationships are typically based on comparisons between the DNA sequences of different organisms. Prokaryotes —Organisms such as the Bacteria and the Archaea whose cells lack a nuclear membrane and other organelles. See Eukaryote. Psychrophiles —Organisms that have a maximum growth temperature of 20 °C, an optimal growth temperature of 15 °C or lower, and a minimum growth temperature of 0 °C or lower. Psychrotrophs — Organisms that have a maximum growth temperature above some 25 to 30 °C and a minimum growth temperature of 5 °C or lower.
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Preventing the Forward Contamination of Europa Rad —A measure of radiation exposure defined in terms of the total amount of energy absorbed per unit mass of the absorbing material. One rad is equal to 100 erg of energy deposition per gram of the target material. Because the amount of energy absorbed depends on the nature of the target material, the unit is often qualified to indicate the nature of the target, e.g., 5 krad [water] per month. Radiation-resistant organisms —Organisms that can survive and grow following acute exposure to radiation. RBE (relative biological effectiveness) —A numerical factor used to compare the biological effectiveness of different types of ionizing radiation. It is defined as the inverse of the amount of absorbed radiation of a particular type required to produce a given effect relative to the absorbed dose of a reference radiation (e.g., X rays or gamma rays) required to produce the same effect. RNA (ribonucleic acid) —A polymer of nucleotides connected via a sugar-phosphate backbone. It plays an important role in protein synthesis and other chemical activities in cells. Stationary phase —The period in the growth of a microbial culture after available nutrients have been depleted and population growth ceases. Sterilization —A procedure that destroys all living microorganisms, including vegetative forms and spores. In practice, a completely sterile state is rarely achieved. Thermophiles —Organisms that can survive and grow in high-temperature environments. TSA (trypticase soy agar) —A solid growth media used to culture microorganisms.
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