Microbial Mat at Grand Prismatic Spring
Yellowstone National Park, Wyoming, USA
Grand Prismatic Spring is the largest hot spring in the US and the third largest in the world. The vivid colors around the edges of the mineral-rich water in the spring are the result of microbial mats, while the center of the pool is sterile due to extreme heat.
The amount of color in the microbial mats depends on the ratio of chlorophyll to carotenoids (organic pigments produced by plants and algae), and also on the temperature gradient in the runoff. The mats tend to be mainly orange and red in the summer, and usually dark green in the winter.