It's not hard to find natural wonders within Yellowstone National Park, but the park's largest hot spring might be the most remarkable, and not just for its size. The spring radiates extremely hot water—and stunning prismatic color—from its center.
Don't adjust your color settings—the Grand Prismatic Spring really is rainbow-colored. The spring was first officially described, and named, by the Hayden Expedition in 1871, which was the first federally-funded exploration of what became Yellowstone.
But what is the cause of this vibrant and flamboyant body of water? It's all thanks to the heat-loving bacteria that call the spring "home".
Cracks in the Earth's surface allow heated water to emerge, forming hot springs. In the spring's center, which bubbles up from 121 feet underground chambers, the water can reach temperatures of around 189 degrees Fahrenheit, making it too hot for most organisms to survive. Because of this, the water looks incredibly clear and has a hypnotizing deep-blue color to it.
But as the water spreads out and cools, it creates concentric circles of varying temperatures. As a result of these distinct temperature rings, different kinds of bacteria inhabit very different environments. And it's the different types of bacteria that give the spring its prismatic colors.
Within these rings, cyanobacteria (a type of bacteria that gets energy through photosynthesis) inhabit the waters. In the yellow band of the hot spring (photo above), a certain kind of cyanobacteria live there, called Synechococcus. The temperature of that water is barely cool enough to be habitable for these bacteria, around 167 degrees F. However, there is another problem: the area is devoid of trees, meaning constant light.
That's not just a problem for tourists, it's also a challenge for Synechococcus. There's no escaping the sun, and at the high elevation of Yellowstone, the ultraviolet light from the sun's rays becomes extremely, extremely harsh.
SOMEHOW, despite these terrible living conditions they manage to survive! They survive through a balance of photosynthetic pigments. The primary pigment for photosynthesis is chlorophyll, which we see as green. But chlorophyll levels can, at times, be surpassed by an accessory pigment known as carotenoids. Carotenoids are red, orange or yellow; the yellow of Synechococcus is exactly the same pigment (beta-Carotene) that in high concentrations makes the orange we see in carrots.
This is what makes the vibrant colors that we see in the springs.
The carotenoids protect Synechococcus cells from ultraviolet rays by capturing and passing the energy from the light to chlorophyll pigments, which in turn convert it to chemical energy. Consequently, the Synechococcus living in the yellow temperature band produce more carotenoids than they would if they lived under optimal conditions (like in the outer rings), giving it its yellow color.
Since Synechococcus' color is so dependent on sunlight, it also means that in the winter, when the sun is less harsh, the bacteria produce fewer carotenoids, and therefore look less yellow, and more blue-green.
To view the Grand Prismatic Spring while in Yellowstone, head to the Midway Geyser Basin. The path will lead you alongside the hot spring for a truly spectacular view of the spring's colors.
Citations:
Maoka, Takashi. “Carotenoids as Natural Functional Pigments.” Journal of Natural Medicines, Springer Singapore, Jan. 2020, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6949322/#:~:text=Carotenoids%20are%20tetraterpene%20pigments%2C%20which,algae%2C%20plants%2C%20and%20animals.
“Hot Springs.” National Parks Service, U.S. Department of the Interior, https://www.nps.gov/yell/learn/nature/hot-springs.htm#:~:text=Hot%20Springs%20Colors,appear%20as%20masses%20of%20color.
Szalay, Jessie. “What Are Carotenoids?” LiveScience, Purch, 8 Dec. 2021, https://www.livescience.com/52487-carotenoids.html.
Taylor, Ashley P. “What Makes Yellowstone's Hot Springs so Colorful?” LiveScience, Purch, 1 Aug. 2016, https://www.livescience.com/55605-what-makes-yellowstone-hot-springs-colorful.html.
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