JUSTIN: When bacteriologist Thomas Brock started probing the hot springs in Yellowstone National Park in the 1960's, he wasn't looking to overthrow a ground rule of biology but to study bacteria in a simplified, real-world environment. As the University of Wisconsin-Madison professor sampled his way up a hot stream, 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 degrees Celsius, 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 degrees C).

ERIC: The prize of his collection was an organism he named Thermus aquaticus and was deposited in a public repository for study by other scientists. Over the years, Thermus aquaticus proved to be very interesting. It was one of at least 50 species of thermophilic bacteria which could tolerate or require temperatures near water's boiling point. It was also the first of the Archaea, ancient microorganisms that some scientists now regard as a separate kingdom of life.

JUSTIN: Archaeans include inhabitants of some of the most extreme environments on the planet. Some live near rift vents in the deep sea temperatures well over 100 degrees Celsius. Others live in hot springs, in extremely alkaline or acidic waters, or in extremely saline water. Archaeans may be the only organisms that can live in extreme habitats such as thermal vents or hypersaline water. They may be extremely abundant in environments that are hostile to all other life forms. However, archaeans are not restricted to extreme environments. New research shows that archaeans are also quite abundant in the plankton of the open sea.

ERIC: There are 3 main types of Archaeans: Methanogens, Halophiles, and Thermoacidophiles. Methanogens are found in anaerobic environments such as the muck of swamps and marshes, the rumen of cattle, sewage sludge, and the gut of termites. They are autotrophic and use hydrogen as a source of electrons for reducing carbon dioxide to food and give off methane as a byproduct. Halophiles are found in extremely saline environments such as the Great Salt Lake and the Dead Sea. They maintain osmotic balance with their surroundings by building up the solute concentration within their cells. Thermoacidophiles are found in such places as acidic sulfur springs and undersea vents and like hot temperatures and acidic surroundings.

JAMIE:The expression �pH' is used to denote the relative acidity or alkalinity of water. Water which is exactly neutral has a pH of 7; waters which are acid have a pH of less than 7, and waters which are alkaline have a pH greater than 7. Most acid springs in Yellowstone have a pH between 2 and 4. The neutral-alkaline springs have a value from 7 to 9. The ranges for particular pools in Yellowstone is shown on this chart.

In several of the acidic hotsprings in Yellowstone, thermoacidophilic prokaryotes in the domain Archaea are the only known inhabitants. The prokaryotes are the genera Sulfolobus or Acidianus. Both these genera contain organisms which oxidize elemental sulfur to sulfuric acid and use this energy to grow autotrophically, using carbon dioxide as their carbon source. This chart shows the various pH limits for organisms - the lowest they are known to be able to live in.

TOM: I'm going to try to answer the question: how hot can the surroundings be for living organisms?

In general, plants can grow at temperatures no higher than 50 degrees Celsius (122 F). Plants can survive higher temperatures, but they will not be able to grow or reproduce, only survive. Animals - that is, multicellular organisms - are similar. Their limit is around 50 degrees Celsius - they can survive higher, but not live or grow in environments that are constantly that high.

Microorganisms are different - they can grow in temperatures much higher than that. Many microorganisms can live in temperatures well above 50 Celsius, in fact up to 115 Celsius, which is past the boiling point of water at sea level. Only the organisms in the domain Archaea are able to do this.

Here is a graph showing maximum temperature ranges. You can see plants and animals live in temperatures up to 122 F. But one of the organisms that lives in the Yellowstone hot pots, Sulfolobus acidocaldarius, lives in temperatures up to 194 F, and doesn't live in temperatures below 158 F. And another organism in the hot pots, Pyrobacterium brockii, lives in temperatures up to 239 F, and doesn't live in temperatures below 176 F.

JUSTIN: Yesterday we took a trip to Yellowstone, to observe some biologists and a geologist as they took samples of the microorganism Sulfolobus acidocaldarius to take back to the lab. Here are pictures of the biologists measuring the temperature and pH of the hot pots... scooping up samples of the water in the hot pots... and labeling their samples, and treating them so the microorganisms will survive the journey back to the lab.