The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Preventing the Forward Contamination of Europa
The cryptoendolithic environments are good examples of absolute extreme environments, i.e., regions where the physical conditions are beyond adaptability. The organisms colonizing the rocks are not adapted to their environment; they survive by tolerating it. While all metabolic activity occurs at ~±10 °C, the optimum temperatures for organisms, as measured in the laboratory, range from 15 to 25 °C, temperatures rarely reached in the Antarctic. Thus microorganisms in Antarctic rocks live near the lower limits of their physiological potential, and they have no reserves to compensate for changes in the environment, should conditions deteriorate. As a consequence, even a minor change in climate can result in local extinctions. In fact, close to 80 percent of the cryptoendolithic communities in Antarctica are dead or fossilized.16
Permafrost microorganisms have been studied most extensively in Siberia 17and have recently been found in Antarctica.18Permafrost microorganisms originate in the soil where they have been immobilized by freezing, while new soil continues to be formed on the surface. In Siberia, the oldest permafrost is 3.5 million to 5 million years old. Recent drilling in Antarctica revealed permafrost some 8 million years old. Permafrost temperatures are extremely stable, around -10 °C in Siberia and down to about -30 °C in Antarctica.
The number of viable bacteria (up to 10 million colony-forming units per gram of dry weight) and the abundance of species found in permafrost decrease with increasing depth (i.e., with increasing age). The viable microbial community in permafrost—mostly psychrotrophs and only very few psychrophiles—is dominated by prokaryotes (organisms whose cells lack a nucleus). Eukaryotic algae (i.e., algae whose cells contain a nucleus) do not survive beyond 5,000 to 7,000 years, but viable yeasts are found in 3 million-year-old permafrost. The composition of bacterial communities found in permafrost mirrors that of the soil from which they originate. Most bacteria isolated from permafrost are aerobes; only a few are anaerobes, mostly methanogens. Permafrost at -10 °C and below is frozen solid. Yet a thin film of unfrozen water envelopes both the inorganic soil particles and microorganisms. The thickness of this unfrozen water film is temperature-dependent and is reduced to about 0.5 nm at -5 °C and below.
In permafrost, microbial growth is in a stationary phase and cell division probably does not occur. This, together with the fact that the number of species decreases with age, suggests that in permafrost a slow selection process takes place, and bacteria that are not able to tolerate the physical conditions of their environment eventually become extinct. In permafrost there is no adaptation, only selection.
1 R.A. Herbert, “A Perspective on the Biotechnological Potential of Extremophiles, ” Trends in Biotechnology 10: 395 1992.
3 E.I. Friedmann, Antarctic Microbiology, Wiley-Liss Inc., New York, N.Y., 1993.
4 C.Lange, L. Wackett, K. Minton, and M.J. Daly, “Construction and Characterization of Recombinant Deinococcus radiodurans for Organopollutant Degradation in Radioactive Mixed Waste Environments, ” Nature Biotechnology 16: 929, 1998.
5 M.J. Daly, personal communication.
6 K.W. Minton and M.J. Daly, “A Model for Repair of Radiation Induced DNA Double-Strand Breaks in the Extreme Radiophile Deinococcus radiodurans,” Bio Essays 17: 457, 1995.
7 F. Krasin and F. Hutchinson, “Repair of DNA Double-Strand Breaks in Escherichia coli, Which Requires recA Function and the Presence of a Duplicate Genome,”Journal of Molecular Biology 116: 81, 1977.
8 M.J. Daly et al., “In Vivo Damage and RecA-Dependent Repair of Plasmid and Chromosomal DNA in the Radioresistant Bacterium Deinococcus radiodurans,” Journal of Bacteriology176: 3508, 1994.