Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Dr. Jeff Havig, a researcher in Earth and Planetary Science, and Plant and Microbial Biology, at the University of Minnesota.

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Hot springs are natural laboratories that allow us to explore how life is able to adapt and thrive in what humans consider to be extreme conditions. Heat tolerance, especially, is something that researchers have learned much about over the past 150 years of study in Yellowstone National Park. Members of Bacteria and Archaea (two of the three domains of life) have been found alive and thriving in the hottest vents in Yellowstone, which are literally boiling, while members of the Eukarya (the third domain of life, and to which all fungi, plants and animals belong) can survive up to around 60°C (140°F).

For hot springs with source water that is above the temperature limit for photosynthesis, you can see the point at which the water temperature crosses the photosynthetic limit threshold with the sudden and dramatic appearance of bright pigments (usually greens and yellows) in the outflow channels. For example, the outflow channel of the alkaline (basic pH) Sapphire Pool in Biscuit Basin, not far from Old Faithful in the Upper Geyser Basin, has what looks like an explosion of green against the white silica of the channel bottom. These green colors are pigments in photosynthetic bacteria, showing precisely where the water temperature drops to around 72°C (162°F)—the upper temperature limit of photosynthesis in neutral to alkaline hot springs. Near the temperature threshold, photosynthetic microbial communities form a thin (usually only millimeters, or less than a tenth of an inch, thick) layer of biomass called a mat. Further down the outflow channels of most hot springs, where the temperatures are cooler, you can often find more complex structures, including mats that can be over 2.5 cm (1 inch) thick, toadstool-like features, spire-like pinnacles, and long ropey filaments.

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Pigments that bacteria produce primarily include chlorophylls and carotenoids. Chlorophylls are molecules used by organisms like cyanobacteria, algae, and plants to convert light into energy, producing colors of green, yellow, and purple. Carotenoids are molecules that can be used to harvest light energy by phototrophic bacteria and have colors of yellow to red—the colors that give us yellow squash, orange carrots and pumpkins, red shrimp, and pink flamingos, and that form the orange ring around the edge of Grand Prismatic Spring in Yellowstone.

Above 72°C in neutral to basic springs, we find non-photosynthetic microorganisms that can be yellow and pink due to carotenoid production. For the photosynthetic microorganisms in hot springs, the colors depend on the types of microorganisms present and whether they produce chlorophyll and/or carotenoids. Thus, when you see color changes as temperatures decrease down a hot spring outflow channel, you are seeing changes in which types of photosynthetic microorganisms are present—a change in the microbial community!

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For acidic hot springs, things can be a little more complicated for limiting photosynthesis. While temperature limits photosynthesis to at or below 55°C (131°F) in these springs, the concentration of hydrogen sulfide (H₂S, which smells like rotten eggs) can also inhibit most types of photosynthetic microorganism. In acidic systems, the subsurface processes that create the complex hydrothermal areas with fumaroles, mud pots, and acidic springs (like those you can see at Norris Geyser Basin, the Mud Volcano Area, and Artist Paint Pots) concentrate the H₂S, so that there is much more in acidic hot springs than in the neutral to alkaline hot springs. High H₂S concentrations, acidic pH, and high temperatures all act to limit where photosynthesis can occur. So in acidic springs, you’ll often see a zone of light yellow before you see the brilliant blue-greens of photosynthetic chlorophyll pigments. That light yellow is elemental sulfur precipitating due to chemotrophs (microorganisms that get their energy from chemical reactions) converting H₂S into sulfur, which precipitates as a solid. But once conditions are favorable, the photosynthetic pigments show up.

You may ask yourself, why is there an upper temperature limit for photosynthesis? Hasn’t life had over 3 billion years to figure it out? Well you aren’t alone in wondering this! The upper temperature limit for photosynthesis is an open question in science that is being actively researched.

The next time you are looking at hot springs in Yellowstone National Park, in person or in photos, be sure to take note of the bright yellows, greens, oranges, reds, purples, and browns of pigments from phototrophic microorganisms, and the complex structures they can form. And when you spot a distinct photosynthetic fringe in Yellowstone, you can marvel and appreciate seeing a scientific mystery with your own eyes!