One of the most eye-catching hot springs in Yellowstone National Park is the bright and colorful Grand Prismatic Spring. It’s blue in the middle with bands of colors ranging from green and yellow to orange and reddish-brown.

Peter Larson.

My friend Peter Larson is a geologist at Washington State University who is very curious about hot springs. He spent much of his research career in Yellowstone National Park.

Larson said that when we look into the hot spring, we are seeing the colors of tiny living things called cyanobacteria.

Some cyanobacteria also make pigments such as carotenoids, which help them use chlorophyll. Carotenoids can also provide them with some protection from the sun. When we see orangish parts of the hot springs, there are likely some carotenoid-making cyanobacteria in the water.

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