National Park Service

Travertine's little helper.
Microbes catalyze the mineralization seen in Yellowstone's hot springs.

The Secret Ingredient in Yellowstone's Travertine

YELLOWSTONE NATIONAL PARK, WYOMING--Bruce Fouke can't leave a rock or microbe unturned. For 10 years, the University of Illinois, Urbana-Champaign (UIUC), marine geologist has studied the origin of terraces made of travertine--a stone commonly used in floors and countertops--in Yellowstone National Park's Mammoth Hot Springs. At a meeting hosted by the Thermal Biology Institute at Montana State University (MSU) in Bozeman held here last week, Fouke presented the first evidence that heat-loving bacteria in hot pools catalyze the mineralization, a process long assumed to be largely inorganic. The finding could improve models used for gas and oil exploration and for estimating groundwater supplies.

For Fouke, Yellowstone's travertine terraces provide a window into how mineralization occurs in coral, clam shells, and even meteorites that have struck Earth. All are composed of calcium carbonate, and because travertine in hot pools grows at a rate of 5 millimeters a day--compared with just 1 millimeter per year for coral--Yellowstone offers an ideal natural lab for observing mineralization in action, he says. Geologists have chalked up the travertine growth to inorganic processes that involve carbon dioxide forming carbonic acid that reacts with minerals and ground water. Meanwhile, microbiologists have focused on the pools' thermophiles: heat-loving microorganisms that have provided blueprints for industrial enzymes. Fouke wondered whether the geologists and microbiologists might have something in common.

To see if the thermophiles might be contributing to travertine formation, Fouke performed fluorescence microscopy on the stone, which revealed microbes encrusted in travertine at the nanoscale level. Next, Fouke's crew set up a siphonlike contraption inside a hot-spring channel to filter out microorganisms. When unfiltered water flowed into the channel, millimeter-long calcium carbonate crystals formed within hours. But when microbes were filtered out, travertine mineralization dropped by 2.5-fold.

How are the microbes helping? Fouke and proteomics expert Peter Yau of UIUC have identified several microbial proteins in samples of water collected from hot pools where mineralization readily occurs. Fouke speculates that such proteins, including heat shock proteins that microbes use to protect themselves from heat damage, could reduce the amount of energy needed for mineralization. "The words 'microbiology' and 'thermophiles' should no longer be spoken without the words 'geology' and 'water chemistry,' " Fouke told meeting attendees.

Fouke's discovery "expands the linkage between microbiology, geochemistry, and geology," says microbiologist Gill Geesey of MSU. In addition, he notes, evaluating the fine structure of calcium carbonate mineralization deposited in hydrothermal systems during different periods of Earth's evolution may offer insights into which microbes lived at which times.

Fouke says his findings could improve models used by oil and gas geologists that predict underground spaces for drilling, because microbe-induced mineral growth may close off these spaces. Moreover, the work could enhance models that predict the availability of ground water for drinking, he notes, as groundwater access could also be impeded by such growth.

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