Chronicles of the Yellowstone Caldera is a weekly column written by scientists and staff at the Yellowstone Volcano Observatory. This week’s post is from Jeff Havig, a researcher in Earth, Planetary, Plant and Microbiology at the University of Minnesota.
Hot springs are natural laboratories that allow us to study how life can adapt and thrive in the conditions that humans consider to be extreme. Heat tolerance, in particular, is something that researchers in Yellowstone National Park have learned a lot about over the past 150 years. Members of bacteria and archaea (two of the three spheres of life) have been found alive and thriving in the hottest chimneys in Yellowstone, literally boiling, while members of the eukarya (the third domain of life where all fungi, plants and animals belong) can grow up to 60 Survive ° C (140 ° F).
For hot springs with spring water that is above the temperature limit for photosynthesis, you can see the point where the water temperature exceeds the threshold for photosynthesis 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 the Biscuit Basin, not far from Old Faithful in the Upper Geyser Basin, has an explosion of green against the white silica of the channel floor. These green colors are pigments in photosynthetic bacteria and show exactly 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) of biomass called a mat. Lower down in the outflow channels of most hot springs, where temperatures are cooler, more complex structures are often found, including mats that can be over an inch thick, mushroom-like features, tower-like pinnacles, and long ropes of filaments.
The pigments that bacteria produce mainly include chlorophylls and carotenoids. Chlorophylls are molecules used by organisms such as cyanobacteria, algae, and plants to convert light into energy and create colors of green, yellow, and purple. Carotenoids are molecules that can be used by phototrophic bacteria to generate light energy and that range in color from yellow to red – the colors that give us yellow squash, orange carrots and pumpkins, red shrimp and pink flamingos, and which the orange ring around them form edge of Grand Prismatic Spring in Yellowstone.
Above 72 ° C we find non-photosynthetic microorganisms in neutral to basic sources, which can be yellow and pink due to the production of carotenoids. The colors of the photosynthetic microorganisms in hot springs depend on what types of microorganisms are present and whether they produce chlorophyll and / or carotenoids. So when you see color changes when temperatures drop in a hot spring spout, you are seeing changes in the types of photosynthetic microorganisms – a change in the microbial community.
With acidic hot springs, limiting photosynthesis can be a little trickier. While temperature limits photosynthesis in these springs to 55 ° C (131 ° F) or below, the concentration of hydrogen sulfide (H2S that smells like rotten eggs) can also inhibit most types of photosynthetic microorganisms. In acidic systems, the underground processes that make up the complex hydrothermal areas with fumaroles, mud pots, and acidic springs (like the ones you can see in the Norris Geyser Basin, the Mud Volcanic Area, and Artist Paint Pots) concentrate the H2S, making it a lot is more in acidic hot springs than in neutral to alkaline hot springs. High H2S concentrations, an acidic pH and high temperatures limit where photosynthesis can take place. Therefore, in acidic springs, you will often see a light yellow zone before you see the bright blue-green of the photosynthetic chlorophyll pigments. This bright yellow is elemental sulfur that precipitates due to chemotrophs (microorganisms that get their energy from chemical reactions) that convert H2S into sulfur, which precipitates as a solid. But when the conditions are favorable, the photosynthetic pigments show up.
You may be wondering why is there an upper temperature limit for photosynthesis? Didn’t life have more than 3 billion years to find out? Well, you are not alone in this question. The upper temperature limit for photosynthesis is an open question in science that is actively researched.
The next time you check out Yellowstone National Park hot springs in person or through photos, watch out for the vibrant yellows, greens, oranges, reds, purples, and browns of the pigments of phototrophic microorganisms, and their complex structure can shapes. And when you discover a distinct stripe of photosynthesis in Yellowstone, you can marvel at and appreciate a scientific mystery with your own eyes.