Extremophiles
Extremophiles
Extremophiles is a term that refers to bacteria that are able to exist and thrive in environments that are extremely harsh, in terms of those environments classically envisioned as hospitable to the growth of bacteria.
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.
See also Chemoautotrophic and chemolithotrophic bacteria; Economic uses and benefits of microorganisms; Extraterrestrial microbiology
Extremophile
Extremophile
Most animals live in conditions where the temperature is between 4°C and 40°C (39°F-104°F), the pH (which measures acidity and alkalinity) is between 5 and 9, and oxygen is abundant. Some animals, called extremophiles, live in conditions that are outside these ranges. The term "extremophile" is given to micro-organisms that live in extreme environments. These environments range from hot springs to sea ice to brine lakes to deep-ocean hydrothermal (hot water) vents. Each of these environments has conditions that are considered punishing or even unbearable for most animals. However, extremophiles thrive under these conditions. Extremophiles may also provide a glimpse of what the earliest forms of life looked like.
Extremophiles are part of a new kingdom of animals called Archae. The members of this kingdom look like bacteria and were considered a phylum in the Kingdom Monera. In the late-twentieth century, however, scientists separated the Archae from other bacteria based on their genetic and biochemical makeup.
Extremophiles are loosely grouped into categories on the basis of where they live. Thermophiles are found living in temperature extremes. Some thermophiles are found in hot springs with water temperatures that approach boiling. The hydrothermal vents along the midoceanic ridges support extremophiles that not only tolerate high temperatures and acidic conditions but also metabolize, or process, hydrogen sulfide, which is poisonous to most animals. At the other extreme, some extremophiles thrive in very cold conditions. These are found in sea ice and on glaciers.
Other examples of extreme environments include natural salt lakes such as the Dead Sea and Great Salt Lake. The extremophiles living there are called halophiles. Still other extremophiles live in highly acidic or highly alkaline environments. Acidophiles thrive in environments with a pH less than 5 while alkaliphiles live in environments with a pH greater than 9. Acidophiles are found in places such as hydrothermal vents while alkaliphiles are found in soda lakes such as are found in Egypt and the western United States. A final major group of extremophiles are the methanogens. They are found living in places with little oxygen, such as swamps and the intestinal tracts of animals. Methanogens do not use oxygen to metabolize their food and they produce methane gas as a waste product.
Scientists have known about extremophiles for more than forty years. Most scientists considered them curiosities in the animal kingdom. Scientists became interested in extremophiles because of their enzymes. Because extremophiles live in extreme conditions, their enzymes must also work under these conditions. Enzymes extracted from extremophiles have grown into a multibillion dollar industry. The enzymes are used in industrial and medical applications that range from making stone-washed jeans to creating artificial sweeteners to conducting genetic tests.
The enzyme-based process known as PCR is used to amplify DNA for genetic identification or genetic testing for disease and conditions. The enzyme reactions used in the procedure occur slowly at room temperatures. By using enzymes from a thermophile, the reactions are performed at a much higher temperature and so at a faster rate.
In recent years, scientists have began searching extreme environments on earth in hope of discovering clues for finding extraterrestrial life. As scientists searched extreme environments, they found more and more kinds of extremophiles. Many environments that scientists had in the past considered sterile were discovered to be the home of many different organisms. Some scientists came to believe that the total mass of all extremophiles on Earth exceeded the mass of all humans on Earth.
see also Adaptations,Biological Evolution,Kingdoms of Life.
Allan B. Cobb
Bibliography
Taylor, Michael Ray. Dark Life. New York: Scribners, 1999.
Internet Resources
Madigan, Michael T., and Barry L. Marrs. "Extremophiles. " Scientific American. <http://www.sciam.com/0497issue/0497marrs.html>.