Organisms which live in extraordinarily heated climates are called thermophiles. Thermophiles (means “heat lovers”) live in such places as deep-sea hydrothermal vents, inside volcanoes, and in the hot springs of Yellowstone Park. Those which can withstand temperatures above 80 C are usually referred to as hyperthermophiles. These recently discovered organisms force us to look outside the box and expand our current perception of the habitable zone.

In 1977, organisms were discovered living in deep-sea hydrothermal vents. These microbes live adjacent to magma heated plumes of fluid which are heated in excess of 400 C. The waters in which the microbes thrive range between 120 C and 150 C. Water does not boil at these temperatures because the pressure is well above 150 atm. Even more significant is the fact that these microbes survive in extremely high concentrations of heavy metals and sulfides. These thermophiles are likely to be similar to life, if present, in the seas of Europa.

In 1999 scientists in Hawaii conducted an extremophile collecting mission to Loihi, a submarine volcano rising from slope of Mauna Loa. Located 34 km southeast of the big island of Hawaii, the summit of Loihi is 1000m below the surface of the ocean. The mission proved to be a success as many microbial mats, including a never-seen before jelly-like organism were found in waters at 160 C. Currently, scientists are preparing a new mission to Loihi with a submarine capable of collecting and bringing these organisms to the surface while keeping them in their natural conditions before being transferred to a onshore bioreactor.

Thermophiles have been found in many locations around the world. Some of the more exciting discoveries came in the late 1960s when scientists discovered bacterial life in some of the hottest springs in Yellowstone National Park. The organism Sulfolobus acidocaldarius was found in an acidic hot spring at temperatures about 85 C. These organisms are capable of surviving in extremely hot and acidic conditions. Such life is likely to be similar to that on other planets. If these organisms are capable of these Earthly condition, it is likely that other forms of life could tolerate most any liquid water, no matter the pH.

The most heat resistant of these microbes is Pyrolobus fumarii which grows in the walls of smokers. It reproduces best at a temperature of about 105 C, and can multiply in temperatures up to 113 C. At temperatures below 90 C the hyperthermophile stops growing. It gets too cold!

Today, scientists believe that the upper limit for life comes around 160 C. This is because at temperatures above 160 C, ATP, which is used by all living organisms for energy, begins to deteriorate rapidly. Before the discovery of microbial life above 100 C, it was believed that 80 C was the upper limit for life. This idea was based upon the fact that at sea level, DNA unraveled and is not able to maintain the double-helix structure at such temperatures. However, once organisms were discovered at much higher temperatures, scientists were left dumbfounded that even during reproduction, when the DNA must unwind in order to duplicate itself, the DNA was able to reform to the double-helix formation.

Thermophiles have not only expanded our borders for the habitable zone, but have helped scientists understand processes completed by DNA. It was a bacterium, now called Thermus aquaticus, that made possible the widespread use of a revolutionary technology: the polymerase chain reaction (PCR).

Even though we have yet to explore the potentially habitable planets and moons in our solar system, extremophiles leave the possibility for life in the universe open. Before thermophiles were discovered it was believed that no organism could withstand temperatures above the boiling point of water (100 C). The fact that there do exist such heat resistant organisms capable of withstanding temperatures of 160 C opens up the possibility for life in similar conditions elsewhere in the universe. Such locations could include hydrothermal vents in the deep seas of Europa, or inside the volcanoes on Titan.