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 and Professor Trinity Hamilton, researchers in Plant and Microbial Biology and Earth and Environmental Sciences at the University of Minnesota.

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As one walks along the boardwalks in one of the many hydrothermal areas in Yellowstone National Park, there is a feast for the senses—the rotten egg smell of hydrogen sulfide, the blasting spray of geysers, the bubbling and boiling water in hot springs, the hiss of gas escaping fumaroles, the low thumping of mud pots, and the dazzlingly bright light reflected from white siliceous sinters. Many find sunglasses a necessity to even attempt to open one’s eyes while walking through these regions of the park. The white silica-rich sinter is formed from precipitation of silica (silicon dioxide, or SiO₂, in the form of opal and amorphous silica) from the mineral-saturated hydrothermal waters rising from below the surface.

After precipitation of the silica minerals, the sinter can become broken up by mechanical means—freeze-thaw cycles, bison walking on the surfaces, etc. Thus, you will see flat expanses of broken little pieces of siliceous sinter surrounding hot springs, such as those at Upper Geyser Basin, Fountain Paint Pots, and Norris Geyser Basin. And while orange, green, yellow, brown, and purple bacterial and algal mats in the hot spring outflow channels are very obvious signs of thriving microbial photosynthetic communities, there is actually much more life than you might think just beneath that surface.

Researchers in Yellowstone’s hydrothermal areas have noted that when you disturb the bleach-white broken up (or what geologists would call breccia, pronounced BRET-chya) siliceous sinter surfaces, underneath there can be a thin layer of deep dark green due to the presence of photosynthetic pigments. Were these photosynthetic communities living under the broken sinter throughout all of Yellowstone’s hydrothermal areas? Are they active, and if so, how much primary productivity (in this case photosynthesis) is happening? Curiosity about this form of life goes beyond wanting to learn about these communities as they relate to Yellowstone’s hot springs, as there are also hot spring deposits in 3.5 billion year old rocks in Western Australia which preserve some of the earliest forms of life to emerge on Earth.  These rocks record a time before oxygen was in the atmosphere, and thus no ozone layer to protect life from ultraviolet radiation from the sun. Could the environments present in Yellowstone be an analog for how life could live on the surface of early Earth?

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Studies of these communities, called hypoliths (hypo meaning under, and lith meaning rock, so literally life “under rocks”), revealed robust photosynthetic communities that were every bit as productive as the photosynthetic mats growing in hot spring outflow channels. The surface sinter in fact acted like sunscreen, blocking UV radiation, but allowed longer wavelengths to pass to power photosynthesis. Furthermore, the sinter fragments acted like a mulch, holding moisture to keep the environment wet and preventing the microbial communities from being washed away by passing thunderstorms. These hypolithic photosynthetic communities were present in both higher pH/alkaline areas (such as what is found in the Lower, Midway, and Upper Geyser Basins), as well as in lower pH/more acidic places (such as Norris Geyser Basin, the Mud Volcano area, and the Artists Paintpots area). This seems like a perfect environment for life to make its first foray from an aquatic environment and out onto terrestrial surfaces—a modern example of what the first life on land may have looked like, and an incredibly exciting discovery that allows us to go billions of years back in time!

More information on this interesting form of life can be found in the research paper at https://www.frontiersin.org/articles/10.3389/feart.2019.00015/full, entitled “Hypolithic Photosynthesis in Hydrothermal Areas and Implications for Cryptic Oxygen Oases on Archean Continental Surfaces.”

So when you visit Yellowstone National Park and are enjoying the sights, sounds, and smells of the hydrothermal areas, take a moment to reflect on the innocuous expanses of bleach-white broken up siliceous sinters. Think about how just under the surface is a thriving photosynthetic microbial community hidden away, and how that community might represent how life first moved onto land 3 billion years before the first land plants evolved.