Viability of Micro-organisms
TABLE 1.2 Microorganisms with Particular Physiological and Nutritional Characteristics
| Physiological Characteristic | Description |
| Temperature | |
| Psychrophile/ facultative psychrophile |
Optimal temperature for growth is 15 °C or lower, maximal temperature is approximately 20 °C, and minimal temperature is 0 °C or lower |
| Psychrotroph | Capable of growing at 5 °C or below, with maximal temperature generally above 25 °C to 30 °C; term in this case is a misnomer because it does not indicate nutritional characteristics |
| Mesophile | Generally defined by optimal temperature for growth, which is approximately 37 °C; frequently grows in the range from 8 °C to 10 °C and from 45 °C to 50 °C |
| Thermophile | Grows at 50 °C or above |
| Hyperthermophile | Grows at 90 °C or above, although optimal temperature for growth is generally above 80 °C; maximal growth of pure cultures occurs between 110 °C and 113 °C, although the maximum (113 °C) may well increase as further research is done |
| Oxygen | |
| Aerobe | Capable of using oxygen as a terminal electron acceptor; can tolerate a level of oxygen equivalent to or higher than the 21 percent oxygen present in an air atmosphere and has a strictly respiratory-type metabolism |
| Anaerobe | Grows in the absence of oxygen; some anaerobes have a fermentative-type metabolism; others may carry out anaerobic respiration in which a terminal electron acceptor other than oxygen is used |
| Facultative anaerobe | Can grow aerobically or anaerobically—characteristic of a large number of genera of bacteria including coliforms such as Escherichia coli |
| Microaerophile | Capable of oxygen-dependent growth but only at low oxygen levels; cannot grow in the presence of a level of oxygen equivalent to that present in an air atmosphere (21 percent oxygen) |
| pH | |
| Acidophile | Grows at pH values less than 2 |
| Alkalophile | Grows at pH values greater than 10 |
| Neutrophile | Grows best at pH values near 7 |
| Salinity | |
| Halophile | Requires salt for growth: extreme halophiles (all are archaea), 2.5 M to 5 M salt; moderate halophiles, usually low levels of NaCl as well as 15 to 20 percent NaCl |
| Hydrostatic pressure (100 atmospheres per 1,000-m depth) |
|
| Barophile | Obligate barophiles, no growth at 1 atmosphere of pressure; barotolerant bacteria, growth at 1 atmosphere but also at higher pressures. A number of deep-sea bacteria are called barophilic if they grow optimally under pressure and particularly if they grow optimally at or near their in situ pressure (0.987 atm = 1 bar = 0.1 megapascal [Mpa]) |
| Nutrition | |
| Autotroph | Uses carbon dioxide as its sole source of carbon |
| Heterotroph | Unable to use carbon dioxide as its sole source of carbon and requires one or more organic compounds |
| Chemoorganoheterotroph | Derives energy from chemical compounds and uses organic compounds as a source of electrons |
| Chemolithoautotroph | Relies on chemical compounds for energy and uses inorganic compounds as a source of electrons. Five classes: hydrogen bacteria, iron bacteria, sulfur bacteria, ammonia oxidizers, and nitrite oxidizers. Specific nutritional groups of bacteria that do not clearly fit in this category include obligate methane oxidizers and the carbon monoxide oxidizers. There are also photoorganoheterotrophs and photolithoautotrophs among the anoxigenic photosynthetic bacteria. |
| Mixotroph | Capable of growing both chemoorganoheterotrophically and chemolithoautotrophically; examples include some of the hydrogen bacteria and some species of Thiobacillus (sulfur-oxidizing bacteria) |
| Oligotroph | Can develop at first cultivation on media containing minimal organic material (1 to 15 micrograms carbon per liter) and grow on such media in subsequent cultivation |
| Copiotroph | Requires nutrients at levels 100 times those of oligotrophs |
Microbes are far more likely than multicellular organisms to retain viability on small solar system bodies because they can adapt to a much wider range of environmental conditions. Single-cell organisms have infiltrated virtually every corner of Earth's biosphere and still constitute the bulk of the earth's biomass. They grow in temperate marine and terrestrial settings, within other microbial or multicellular organisms, in deep subsurface niches, and in extreme environments that would be lethal for other life forms. They often influence geochemical reactions within the biosphere and frequently play key roles in food webs and complex ecosystems.
The physiological state of microorganisms influences their ability to survive extreme environmental conditions (see Table 1.2). For example, organisms with active DNA repair mechanisms or a protective layer of material are more likely to endure exposure to ultraviolet (UV) or ionizing radiation than are cells without equivalent capabilities. Viable microorganisms are either metabolically active (vigorous) or dormant (quiescent). Communities of metabolically active microbes may increase in biomass or simply maintain a constant density by replacing dead cells with new ones. Sometimes metabolic activity is restricted to the repair of macromolecular machinery without cell division. Examples of metabolically active microorganisms include exponentially growing cells, cultures that have reached a stationary phase during which cell division occurs either very slowly or at a rate equivalent to that associated with cell death, and organisms that have shifted from exogenous to endogenous metabolism to survive starvation conditions. Dormant cells are metabolically inactive but are capable of returning to an active state. Examples include spores that are metabolically "frozen," freeze-dried cells, and cells that remain viable at suboptimal or freezing temperatures and/or in a desiccated state.
from Evaluating the Biological Potential in Samples
Returned from Planetary Satellites and Small Solar System Bodies
-- Space Studies Board (SSB)
The SSB, which has served the space science community since 1958, is a program office within the Commission of Physical Sciences, Mathematics, and Applications of the National Research Council, which is in turn the operational arm of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The SSB operates a number of standing committees, task groups, and workshops that perform studies in space science and policy for the federal government.