Extremophiles
"Extremophiles" is a term that refers to bacteria that are able to exist and thrive in environments that are extremely harsh (harsh, that is, in comparison with those environments classically envisioned as being hospitable to bacterial growth).
The discovery of extremophiles, beginning in the 1970s, has had three major influences on microbiology and the biotechnology industry. Firstly, the discovery of bacteria growing in environments such as the hot springs of Yellowstone National Park and around the hydrothermal vents located on the ocean floor (where the bacteria are in fact the fundamental basis of the specialized ecosystem that is fueled by the vents) has greatly increased the awareness of the possibilities for bacterial life on Earth and elsewhere. Indeed, the growth of some extremophiles occurs in environments that by all indications could exist on planets such as Mars and other stellar bodies. Thus, extremophilic bacteria might conceivably not be confined to Earth.
The second major influence of extremophiles has been the broadening of the classification of the evolutionary development of life on Earth. With the advent of molecular means of comparing the genetic sequences of highly conserved regions from various life forms, it became clear that extremophiles were not simply offshoots of bacteria, but rather had diverged from both bacteria and eukaryotic cells early in evolutionary history. Extremophilic bacteria are grouped together in a domain called archaea. Archae share similarities with bacteria and with eukaryotes.
Thirdly, extremophiles are continuing to prove to be a rich trove of enzymes that are useful in biotechnological processes. The hardiness of the enzymes, such as their ability to maintain function at high temperatures, has been crucial to the development of biotechnology. A particularly well-known example is the so-called tag polymerase enzyme isolated from the extremophile Thermus aquaticus. This enzyme is fundamental to the procedures of the polymerase chain reaction (PCR) procedure that has revolutionized biotechnology.
There are several environments that are inhospitable to all but those extremophilic bacteria that have adapted to live in them. The best studied is elevated temperature. Heat-loving bacteria are referred to as thermophiles. More than 50 species of thermophiles have been discovered to date. Such bacteria tolerate temperatures far above the tolerable limits known for any animal, plant, or other bacteria. Some thermophiles, such as Sulfolobus acidocaldarius, are capable of growth and reproduction in water temperatures that exceed 212° F (100° C) (the boiling point of water at sea level). The most heat-tolerant thermophile known so far is Pyrolobus fumarii, that grows in the walls of the hydrothermal vents where temperatures exceed 200° F (93.33° C). In fact, the bacterium requires a temperature above 194° F (90° C) to sustain growth. The basis of the thermophile's ability to prevent dissolution of cell wall constituents and genetic material at such high temperatures is unknown.
Other examples of extreme environments include elevated salt, pressure, and extreme acid or base concentrations.
Salt-loving, or halophilic, bacteria grow in environments where the sodium concentration is extremely high, such as in the Dead Sea or Great Salt Lake. In such an environment, a bacterium such as Escherichia coli would compensate for the discrepancy in sodium concentration between the bacterium's interior and exterior by shunting all the internal fluid to the exterior. The result would be the collapse and death of the bacterium. However, salt-loving bacteria such as Halobacterium salinarum content with the sodium discrepancy by increasing the internal concentration of potassium chloride. The enzymes of the bacterium operate only in a potassium chloride-rich environment. Yet the proteins produced by the action of these enzymes need to be tolerant of high sodium chloride levels. How the enzymes are able to accommodate both demands is not clear.
Acid-loving extremophiles prefer environments where the pH is below pH=5, while alkaline-loving bacteria require pHs above pH=9. Thriving populations of acid-loving bacteria have been isolated in the runoff from acidic mine drainage, where the pH is below one, which is more acidic than the contents of the stomach. Interestingly, these bacteria are similar to other bacteria in the near neutral pH of their interior. Very acidic pHs would irreversibly damage the genetic material. Acid-loving bacteria thus survive by actively excluding acid. The enzymes necessary to achieve this function at very acidic pH levels.
Similarly, alkaline-loving bacteria maintain a near neutral interior pH. The enzymes that function at such alkaline conditions are of interest to manufacturers of laundry detergents, which operate better at alkaline pHs.
Some extremophiles grow and thrive at very low temperatures. For example, Polaromonas vacuolata has an ideal growth temperature of just slightly above the freezing point of water. These bacteria are finding commercial applications in enzymatic processes that operate at refrigeration temperatures or in the cold cycle of a washing machine.
The discovery of bacteria in environments that were previously disregarded as being completely inhospitable for bacterial life argues that more extremophiles are yet to be found, as other environments are explored. For example, in 2001, living bacteria were recovered from drill samples kilometers beneath the Earth's crust, in an environment where virtually no nutrients were present other than the solid rock surrounding the bacteria. By as yet unknown enzymatic mechanisms, these bacteria are able to extract elemental components including sulfur from the rocks and utilize them as nutrients.
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