Sapphire Pool, in the Biscuit Basin area of the Upper Geyser Basin, Yellowstone National Park

Sapphire Pool, an alkaline (pH ~7.5) spring in the Biscuit Basin area of the Upper Geyser Basin, Yellowstone National Park. The vent (top image) is hot (~80°C), with white silica precipitating, but as the water cools down the outflow channel the temperature drops below the upper temperature threshold for photosynthesis (~72°C), as indicated by the sudden appearance of bright yellow-green from photosynthetic pigments produced by phototrophic bacteria (bottom image).