Yellowstone National Park (Wyoming, USA) has one of the highest concentrations of thermal features in the world. They include geysers, hot pools, fumaroles (steam vents) and mudpots. The reason for this large concentration of thermal features seems to be that the earth's crust is much thinner here - estimated at about 64 km (40 miles) compared to about 144 km (90 miles) over most other parts of the land surface. Hot magma (molten rock) at depths of about 6-16 km heats the overlying rock and the reservoirs of water lying 3-4 km below the surface. This heated water is forced upwards through fissures in the surface rocks, escaping as boiling pools, gas vents or geysers. Thick deposits of limestone are often seen around the thermal vents, where calcium carbonate is precipitated from the underground water.

Several heat-loving (thermophilic) microorganisms grow within the heated pools or in the channels of water that drain from them. These bacteria and archaea are often brightly coloured by photosynthetic pigments (chlorophylls or carotenoids) and show distinct zonations according to their specific temperature tolerances.

A large channel draining from a hot pool, containing carotenoid-rich microorganisms. The temperature of this channel in the foreground is about 60 ^oC. Layers of white-coloured limestone (forming a rock deposit known as travertine) can also be seen. Note the footprints of buffalo in the foreground. These animals often seek the warmth of thermal areas in the winter months.

Left: limestone terraces formed by precipitation from calcium-rich water flowing from a raised hot pool. Pink, green and brown-coloured microorganisms occupy the thermal gradients in the flowing water (60-100^oC). Right: a pool of water that has condensed from a a fumarole (steam/gas vent at top right of the image), with abundant growth of a green photosynthetic microorganism.

Zones of microbial growth in water draining into the Yellowstone River from a raised thermal vent.

Zonation of microorganisms in the seep from a hot pool. The temperature gradient decreases from right to left of the image, where the temperature is low enough to enable plants to grow.

Part of a water seepage from a hot spring, showing carpets of cyanobacteria and other microbial species. The upper temperature limit for growth of cyanobacteria is 70-74^oC. The rim of this pool is colonised by grasses/sedges and by monkey flower (Mimulus guttatus) - a species also found in Britain (see below).

Monkey flower at the edge of a heated pool.

Go back to Thermophilic microorganisms?

GO TO FULL LIST OF PROFILES?