Grand Prismatic Spring in Yellowstone National Park, the largest hot spring in North America and the third largest in the world. The vivid colors are the result of minerals and algae interacting with a water temperature between 147° and 188° F. The Hayden Expedition named the spring in 1871 for its vivid range of colors.
What Makes the Colors?
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. (Images by: wikimedia.org)