High temperatures are most commonly identified as an example of extreme conditions on Earth. Maybe this is because we sterilize things like drinking water or medical instruments using high temperatures. Regardless, high temperatures do present a unique extreme environment on our planet. Although they would restrict the existence of humans and many other complex organisms, environments of extremely high temperature aren't considered as inhabitable as they once were. This is because scientists are discovering a wide variety of organisms that not only survive, but thrive in high temperatures.
In general, organisms that live in environments that are extreme with respect to humans are called extremophiles. In particular, the organisms that thrive in high temperatures are known as thermophiles. The temperatures at which thermophiles grow optimally start at 45 °C (113 °F) and extend well beyond that. Scientists have known about thermophiles for over 40 years. It has been only recently that microbiologists have discovered that the temperatures at which some life can thrive far exceeds 45 °C. In fact, several organisms have been identified that optimally grow at temperatures above 80 °C (176 °F)! These organisms are known as hyperthermophiles to distinguish them further from thermophiles.
There are many environments in which thermophiles (and hyperthermophiles) can be found. The most widespread and extreme environments of high temperature are often identified with volcanic events. High temperature environments are located on or near the surface of Earth, such as the hot springs found in Yellowstone National Park, in the subterranean areas of Earth, and in submarine environments, near hydrothermal volcanic vents. Currently, the microbe that holds the record for high-temperature growth is Methanopyrus kandleri, which survives at a temperature of 122 °C, closely followed by Pyrolobus fumarii, which can grown in temperatures up to 113 °C and which was first discovered in the walls of a hydrothermal vent in the bottom of the ocean.
So why is an environment of high temperature considered to be extreme? With respect to humans, it should be obvious as to the extremity of temperatures above the boiling point of water. But what about single celled organisms; why do high temperatures present an extreme environment for them? Living cells are all made up of similar building blocks. Many of these building blocks are sensitive to high temperatures. For instance, enzymes are a type of protein that helps catalyze chemical reactions within living cells. An enzyme, or more generally a protein, is made up of a long string of molecules called amino acids. This string of amino acids then folds up to make a three-dimensional macromolecule. Often times heat causes deformations in or complete destruction of the three-dimensional folding of these macromolecules, rendering things like enzymes useless. DNA can also be denatured by high temperature; the hydrogen bonds holding the two strands of DNA together are disrupted, causing the DNA to break apart into single strands. Furthermore, the membranes that enclose living cells can experience negative effects due to high temperature, often resulting in the lipid bilayer being torn apart.
Thermophiles have evolved to combat the consequences of high temperature. First of all, biochemists have noted that enzymes in thermophiles vary little when compared to enzymes in mesophilic (existing in temperate conditions) organisms; the sequences of amino acids are only different by an amino acid or two. However, the small differences in amino acid sequence allow the enzyme to fold in a manner that is more resilient to high temperature. Specifically, heat-resistant enzymes and proteins will have more salt bridges, ionic bonds between charged amino acids, and will fold up into a tighter three-dimensional configuration. Tighter packing is naturally resistant to unfolding in normal conditions. Thermophilic organisms also employ special mechanisms to maintain DNA integrity. One way that the DNA is made more resilient to high temperature is by introducing positive supercoils, rather than negative supercoils, into the molecular structure, which show greater stability. Also, in some hyperthermophiles a heat-resistant protein is produced that binds to the DNA which serves to stabilize the DNA and thus lower its melting point. Finally, the cell membranes of thermophiles are different from organisms living at moderate and cold temperatures. The primary difference is in the type of lipid used to construct the membrane. In most cells, a lipid bilayer is formed. However, thermophiles use a different lipid that forms a monolayer instead of a bilayer, which is therefore immune to the tendency of high temperature to pull apart bilayers.
Thermophilic organisms are interesting for more than just their ability to resist high temperatures. Many astrobiologists contend that the last common ancestor of all life on Earth was a hyperthermophilic organism. If this is true, then unraveling the biochemistry of these organisms may be of great importance to our understanding of the origin and evolution of life on our planet. Thermophiles have contributed to our understanding of the evolution of life in a much more concrete way. Taq polymerase, an enzyme isolated from a thermophilic microorganism, has been used in the polymerase chain reaction (PCR) technique that allows for the amplification and subsequent sequencing of genetic information. It is beyond question that these resilient little organisms will continue to enlighten our search for life in the Universe.