Grand Prismatic Spring in Yellowstone is located in the Midway Geyser Basin and teems with the microbial life of thermophilic bacteria. These photosynthetically active microorganisms produce the vivid colors of this hot spring, whose temperature reaches up to 70°C (158°F). The color zonation marks the different inhabitants adapted to the different temperature ranges around the spring. In addition, algae and protozoa thrive in the outer areas of the spring, where temperatures are lower.
These microorganisms form mats by secreting a sticky organic substance that holds the cells together. The main pigment involved in photosynthesis is green chlorophyll. Masked by carotenoids, it produces yellow, orange, and red colorations. This coloration becomes significantly more pronounced in summer, as carotenoids protect the microorganisms from excessive exposure to ultraviolet radiation. In winter, therefore, greener colors predominate.
The color sequence also indicates the temperature zoning of the hot spring. The microorganisms thrive best at 45–60°C (113–140°F) and therefore prefer to settle around the outer periphery. The blue water in the center of the spring is too hot for living organisms and therefore reveals the shortwave blue light that is not absorbed by the microorganisms.
In the transition zone toward deeper water, which provides better protection from the sun’s ultraviolet radiation, the green tones of chlorophyll increasingly predominate. The sharply defined yellow transition zone marks the boundary between the chlorophyll- and carotenoid-dominated bacterial mats.
In contrast to geysers, which erupt at regular or irregular intervals, hot springs exhibit a relatively calm and steady flow of hot water. This is due to the lack of constrictions in their underground conduit systems. As a result, the hot water can slowly cool as it rises toward the surface. Becoming slightly denser, it sinks back into the depths and is continuously replaced by rising hot water. This process produces a continuous vertical circulation of water.
