Life at High Temperatures

by Thomas D. Brock

Colorful Yellowstone

Grand Prismatic Spring (Russ Finley)

Although Yellowstone is famous for its geysers, the beautiful colors associated with the geysers and hot springs are often a surprise. One sees hot water flowing over patches of brilliant yellow, orange, red, and green; hot pools lined with color; of even steam which appears to be colored.

The early trappers and explorers noted the colors, as did the geologists who mapped the thermal basins in the 1870s. In fact, it was a geologist, Walter H. Weed, who in 1889 first recognized that the colorful deposits of Yellowstone hot springs were microbial.

The presence of living creatures in water too hot to touch is really amazing. But even more impressive is the fact that the organisms of hot springs are not only living, but thriving. In fact, they are so perfectly adapted to these hot environments that they can live no where else.

Such organisms, called thermophiles, are found in hot environments all over the world - not only in hot springs but also in volcanoes, deserts, and artificial thermal environments such as power plants and hot water heaters But nowhere else in the world are they so obvious, and in such brilliant profusion, as in Yellowstone.

What Makes the Colors?

The colors of the Yellowstone thermal features are justly famous. They were first mentioned by Osborne Russell, a trapper who spent the years 1834-1843 in the Yellowstone region. Viewing what is now called Grand Prismatic Spring, Russell noted: "The steam which arose from it was of three distinct colors. From the West side for one third of the diameter it was white, in the middle it was pale red, and the remaining third on the East light sky blue. Whether it was something peculiar in the state of the atmosphere.. or whether it was some chemical properties contained in the water... I am unable to say and shall leave the explanation to some scientific tourist who may...visit this place in the future."

What causes these colors? In Grand Prismatic spring, and others of similar character, the orange color is due to pigmented bacteria of the microbial mats, and the blue color to refracted skylight. The principal pigment for photosynthesis is chlorophyll, which is green. However, chlorophyll is sometimes masked by carotenoids, pigments related to vitamin A, which are orange, yellow, or red. Carotenoids protect the cells from the bright sunlight that occurs in Yellowstone, especially during the summer.

The color of a mat depends principally upon the ratio of chlorophyll to carotenoids. In the summer the chlorophyll content is often low, so that the microbial mats appear orange, red, or yellow. In the winter, the mats are usually dark green, because at this time of year the sunlight is subdued and chlorophyll dominates over carotenoids. In fact, even a few cloudy days in mid-summer can lead to an increase in chlorophyll and a darkening of the mats.

Thus, it is not just the kinds of bacteria but the response they make to sunlight that determines the colors.

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