Yellowstone Caldera

Life at high temperatures

Introduction to heat-loving (thermophilic) micro organisms

These grow in Yellowstone National Park (YNP) within the heated pools or in the channels of water that drain from them. They are mostly prokaryotes (bacteria and archaea), specially adapted to grow in these environments, but at the lower temperatures eukaryotic algae are also present. They show distinct zonations according to their temperature optima and to the acidity of the water (pH). Waters at YNP have pH values ranging from 1 to 10, temperatures from ambient to about 93°C (boiling at YNP’s altitude), and high concentrations of As, H2S, SO4, and HCO3. Numerous chemical reactions occur, which also cause the deposit of limestone terraces of Mammoth Hot Springs.

Some like it hot

When researchers began to study the biology of Yellowstone hot springs in the 1960s, the upper temperature limit for life was thought to be around 73 °C. But in the late 1960s, to their astonishment, Thomas D. Brock and colleagues identified in several springs in the Great Fountain area of the Lower Geyser Basin, a neutral or weakly alkaline area, the first extremophile capable of growth at even higher temperatures: it was called Thermus aquaticus and it was the first of the Archaea, ancient micro organisms that some scientists now regard as a separate kingdom of life.

It lives at 50-80 °C, a temperature range which overlaps that of the photosynthetic bacteria so that in many Yellowstone springs it lives in association with cyanobacteria. It gets its energy from the photosynthesis of these organisms. However, it may also be found at temperatures too high for photosynthesis, and here Thermus aquaticus live on tiny amounts of organic matter present in the source water, making it visible to the naked eye. The unique thermostable enzyme of Thermus aquaticus, called Taq polymerase, is finding wide industrial and medical use (AIDS diagnosis, for instance) and forensics (DNA fingerprinting).

Some like it very hot

Brock also found the first hyperthermophile in an extremely hot and acidic spring, the archaean Sulfolobus acidocaldarius. Lots of it grows at temperatures as high as 85-90 °C. Sulfolobus species gain their energy by oxidising the sulphur granules around hot springs, generating sulphuric acid and thereby lowering the pH of the water (as in Norris Basin, very acid area). After collecting Sulfolobus solfataricus from the Solfatara volcanic area near Naples, an Italian team used ADH enzyme for crystallization aboard the Space Shuttle.

Evolution and colours

Life as we know it might first have arisen about three billion years ago in high-temperature environments. Such thermophiles would then have continued to exist on earth in the intervening period, finding refuges in the hot springs. In addition, these thermophiles might well have been the forerunners of all other life forms.

The thermal features at Yellowstone owe their colours to heat-loving micro organisms. They are often brightly coloured by green photosynthetic pigments or carotenoid pigments, related to vitamin A, which are orange, yellow, or red. Carotenoids protect the cells from the bright sunlight that occurs in Yellowstone. The colour of a mat depends mainly on the ratio of chlorophyll to carotenoids. Thus, it is not just the kinds of bacteria but the response they make to sunlight that determines the colours.

Compiled by Valeria Perin.