Forecast: Hot and humid
When bacteriologist Thomas Brock started probing the hot springs in Yellowstone National Park in the 1960s, he was not looking to overthrow a ground rule of biology. Instead, the University of Wisconsin-Madison professor was looking to study bacteria in a simplified, real-world environment.

These hot springs at Yellowstone owe their vibrant colors to heat-loving microorganisms. Courtesy of Thomas D. Brock.

Then -- as now -- precious little was known about how bacteria live their lives -- except for those that cause disease.

As Brock sampled his way up a hot stream in Yellowstone, he approached the hot spring supplying it. And the water got warmer and warmer.

At the time, biologists thought life would not tolerate temperatures anywhere near 80° C. But Brock kept finding bacteria, so he kept on looking. Eventually, he found organisms that could live and reproduce near the temperature of boiling water -- 100° C.

Until then, life and heat seemed incompatable. Because of our anthropocentric point of view, living a normal life in boiling water seems ludicrous! Why would any organism choose to live at such high temperatures? Not to mention the "How?" of such an existance.

Just who do these guys think they are?
Organisms that thrive at high temperatures are called thermophilic (heat-loving). Thermophiles grow their best at temperatures between 45° C (113° F) and 80° C (176° F). That's already pretty toasty, but a few organisms, called hyperthermophiles, grow best at temperatures higher than 80° C. The hottest microbe isolated so far, Pyrolobus fumarii, can function normally at 113° C (235° F)!

Only procaryotes from the domains Archaea and Bacteria have been found that grow at > 80° C. Of these organisms, the most thermophilic are Archaea. (Not all Archaea are thermophiles, but the most extreme hyperthermopiles are classified as Archaea.) The Eukaryotes appear to be limited to more moderate temperatures.

Thermus aquaticus in an outflow channel of a hot spring at Yellowstone. Courtesy of Thomas D. Brock.

Oddly enough, hyperthermophiles not only have the ability to grow and live a normal life at high temperatures, they require the heat. A hyperthermophile cannot live at less than 80° C-- it's too cold!

Hyperthermophilic microbes found in nature grow exclusively in volcanic environments. On land, hot springs, steam vents (fumaroles), and boiling mud-holes, such as those found at Yellowstone, are favorite hangouts for these heat-loving critters. In the oceans, hyperthermophiles have been found at deep-sea hydrothermal vents, where superheated fluids erupt out of the ocean floor.

Now that scientists are finding microbes living in water previously thought to be way too hot for anything to survive, an obvious question presents itself: What is the upper temperature limit for life?