Curriculum Development > High School 9-12 > Other Supplemental Programs > Frontiers in Microbiology > Microbes and Extreme Environments   Most of the biodiversity on earth is found among the microbes. They have exploited virtually every niche where life is possible. Plants and animals by contrast are very finicky, requiring a relatively narrow range of environmental conditions to survive. Scientists exploring the extreme environments in which microbes live find that these conditions are not too dissimilar from those thought to exist elsewhere in the solar system and presumably in many other places throughout the universe. Indeed, the ability of microbes to thrive under such severe conditions makes the possibility of microbial life elsewhere in the universe seem rather likely to many scientists. Some species of microbes can thrive at high temperatures once thought to be incompatible with life. For example, hot springs such as those found in Yellowstone National Park are teeming with microbes that live at near-boiling temperatures. An even more extreme environment is that found around hydrothermal vents on the ocean floor. First discovered in 1977, hydrothermal vents are underwater geysers. Seawater is sucked into cracks on the seafloor where it encounters molten rock, or magma. The hot magma superheats the water, which is forcibly discharged back into the ocean through a hydrothermal vent. As the mineral-rich superheated water meets the near-freezing water of the seafloor, minerals precipitate out of solution and form tall structures called chimneys. A wide variety of organisms such as tube worms and crabs live near the vents. These animals have no digestive systems but rely on symbiotic relationships with bacteria to obtain nutrients. Bacteria, rather than photosynthetic plants, are the producers and initiate the food chain in this ecosystem. Instead of relying on light energy to power photosynthesis, these thermophilic bacteria oxidize hydrogen sulfide to power chemosynthesis. Until the discovery of these deep-sea vent ecosystems, it was believed that all life on earth depended on sunlight. Microbes that live at high temperatures are not of merely academic interest. Biotechnology companies often use microbes, or enzymes isolated from microbes, to carry out chemical reactions needed to produce a product. Since the rate of enzymatic reactions increases with temperature, microbes that have evolved heat resistance can perform these reactions more efficiently than microbes adapted to cooler temperatures. The most popular heat-resistant bacteria used today is Thermus aquaticus. This bacterium thrives at 70ºC (158ºF). It was first isolated from hot springs in Yellowstone National Park in 1969. Its two discoverers, Thomas Brock and Hudson Freeze from Indiana University, grew cultures of Thermus aquaticus in the laboratory and sent one to the American Type Culture Collection. This allowed other scientists from around the world to obtain cultures for their own research. By 1976, other scientists had isolated a heat- stable DNA polymerase called Taq from Thermus aquaticus. In the early 1980s, a researcher named Kary Mullis at the Cetus Corporation was working on a clever way to amplify short DNA sequences. His technique was called the polymerase chain reaction (PCR). It used a bacterial DNA polymerase to synthesize copies of the DNA to be amplified. He originally used an enzyme isolated from E. coli. Unfortunately, since his technique relied on repeated exposure to near-boiling temperatures, the E. coli enzyme was destroyed and had to be replaced 20 to 30 times during each amplification. This was not only inconvenient but also very expensive. Mullis realized that using a heat-stable DNA polymerase would eliminate this problem. After substituting the heat-stable Taq enzyme for the E. coli enzyme, Mullis was able to add enzyme only at the beginning of the amplification; it would remain suitably active until the end of the process. Use of the Taq enzyme made PCR simple and affordable. Cetus rewarded Mullis with a $10,000 bonus and later sold the patent rights for PCR to Roche Molecular Systems for $300 million. In 1993, Mullis was awarded the Nobel Prize in Chemistry for his invention of PCR. Today, biotechnology companies spend hundreds of millions of dollars on Taq polymerase each year. Just as some microbes are heat-adapted, other species have adapted to the cold. Scientists recognize two categories of microbes that live in cold environments: psychrotolerant and psychrophilic. Psychrotolerant microbes prefer living at warmer temperatures but can survive temporarily under cold conditions. Psychrophylic microbes actually prefer to live at cold temperatures, between 0°C and 20°C. These cold-living microbes consist of various species of unicellular bacteria, algae, and fungi. Some of these organisms have been found in ice samples taken from 3.2 kilometers (two miles) below the earth's surface (Carey, 2005). Of course if the organism freezes, then life processes such as metabolism, growth, and reproduction cannot take place. Microbes employ a variety of strategies for surviving these harsh conditions. Some contain specific sugars and proteins that function as an antifreeze, lowering the temperature needed for ice crystals to form. Other microbes become freeze-dried, forming what appear to be lifeless spores that retain the ability to spring back to life if they are provided with water and higher temperature. Selection pressures to cope with dehydration have led to the appearance of some exotic microbe species. In 1956, Arthur W. Anderson at the Oregon Agricultural Experiment Station in Corvallis noticed reddish-colored bacteria growing on some spoiled meat that had been sterilized by exposure to a high dose of radiation. These bacteria, called Deinococcus radiodurans, are the most radiation-resistant organism known to exist. They can withstand radiation doses 1500 times the amount that would kill a human. For this reason D. radiodurans has been nicknamed Conan the Bacterium (Huyghe, 1998). Although scientists are still trying to figure out how these bacteria survive exposure to extreme radiation, the answer has to do with DNA repair. All living organisms have DNA repair systems that cope with damage to their DNA caused by radiation from the sun, chemicals in the environment, and mistakes made by the cell’s replication machinery. There are limits to how much DNA damage can be repaired. D. radiodurans starts off with an advantage by having multiple copies of its genes, as compared to most microbes that have but one copy of each gene. These extra copies serve as backups; when one gene is damaged, its duplicates can continue to function until the damage is repaired. D. radiodurans also has more effective DNA repair systems in comparison to other radiation-sensitive microbes. Why did D. radiodurans evolve this resistance to radiation? In nature, this species has adapted to surviving long periods without water. Since DNA damage caused by dehydration is similar to that caused by radiation, D. radiodurans has the ability to cope with both types of environment. Scientists are finding microbes living in almost everyplace they look, even in rocks. Microbes classified as endoliths live in rocks or in the spaces between mineral grains. These autotrophs use iron, potassium, or sulfur as the basis for their metabolism. In their inhospitable environment, endoliths can't afford to spend much energy on reproduction. They are thought to undergo cell division about once every 100 years (Carey, 2005). Endoliths have been found living about three kilometers (two miles) below the earth's surface. It is the high temperature deep in the crust, rather than the high pressure, that limits their ability to survive deep in the earth. How deep they go remains to be seen. Photosynthetic microbes called hypoliths are found in polar desert environments. They typically live beneath rocks, where they are protected from ultraviolet radiation and harsh winds. So far, most hypoliths that have been characterized are associated with quartz, which is translucent and allows light to reach the microbes. Microbes have successfully adapted to nearly every environmental niche on earth. This diversity has generated species that use many different types of metabolism. Increasingly, scientists at biotechnology companies are harnessing microbes’ metabolisms and putting them to work. Microbes, sometimes with the introduction of foreign genes, are being used to produce proteins of industrial or medical usefulness. Other applications include bioremediation and fuel production. Presently, about 25 percent of the world’s copper production uses bacteria to leach the ore and help purify the metal. This process takes about two years, but scientists are trying to speed up the process by using different species of bacteria that can work at higher temperatures. After the extractable ore has been removed from a mine, attempts are made to reclaim the land and return it to a more natural state. These efforts are often unsuccessful because the tailings left behind are too acidic to promote plant growth. Bioremediation scientists are using bacteria to neutralize the soil and help plants reclaim former mining sites. Our understanding of the range of extreme environments suitable for microbial colonization has expanded substantially over the past few decades. Today, scientists realize that the conditions of some of earth’s extreme environments may exist elsewhere in the solar system. Recent insights into microbial diversity are informing the search for life elsewhere in the universe. Perhaps one day soon we will have to make room in our taxonomy for extraterrestrial microbes. Previous Table of Contents Next

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