Being able to withstand extremely hot temperatures. In terms of microorganisms or enzymes, a thermophile is generally considered one that maintains life or activity at temperatures above 40 degrees Celsius. Thermophilic microorganisms are able to survive because of their ability to produce thermophilic enzymes. Most normal enzymes, being proteins, begin to denature around 40 degrees, and are generally completely inactive beyond 50-60 degrees. Some thermophilic enzymes have been discovered that can maintain at least half of their specific activity at temperatures as high as 80 degrees, or in rare cases even higher. In other cases, enzymes might partially denature at high temperatures but have adaptive systems that allow them to renature once removed from such extreme conditions.
Microorganisms that can tolerate high temperatures, also called extremophiles, are found in hot springs, or deep-sea thermal vents, and are typically part of the group of Archaea. Although the Archaea might not grow fast enough or produce enough enzyme under "normal" conditions to make their harvest practical, their enzymes do have many potential applications in a wide variety of industrial processes (eg. pulp and paper processing) where extreme conditions are required. Therefore, once the gene for a potentially useful thermophilic enzyme has been identified, cloning techniques can be applied to express the enzyme under the control of a strong promoter, in a fast-growing organism that has been proven to show robust growth in large-scale fermentation systems.