Microbes Eat the Past
In 1969, astronauts Edwin Eugene ("Buzz") Aldrin and Neil Armstrong bounced along the moon's dusty surface wearing the toughest work gear in the solar system. Moon suits were beautiful cocoons of high-tech polymers, convoluted joints, rubber, and fiberglass; 25 layers thick, resilient enough to shrug off hurtling micrometeoroids and insulate against the deep freeze of space. Not long after Aldrin and Armstrong returned, their suits landed in the National Air and Space Museum in Washington, D.C. But as throngs of visitors queued up to peek at these artifacts from the final frontier, curators were puzzled as the sturdy suits slowly fell apart. Recently, preservation specialists shouted for the help of Ralph Mitchell, McKay professor of applied biology. He reached an unexpected conclusion: people may be the one thing space suits can't handle.
|Jim Irwin's Apollo 15 space suit, above, complete with lunar dust, and a detail (above left) showing fungus in the foreground living on three greatly enlarged strands of fabric from the space suit.
Chris McNamara Samantha Snell/National Air and Space Museum, Smithsonian Institution
People? Yes, Mitchell explains, because visitors provide fertilizer for bacteria living in and on the space suits. When humans exhale, they spray moisture into the air. Give microbes moisture, mix in a few other nutrients, and they thrive, gurgling out waste products like sulfuric acid. These acids corrode and discolor stone, metals, plastics, and cloth and threaten many objects that symbolize our collective history and memory, from the Statue of Liberty to cliff dwellings at Mesa Verde to Abraham Lincoln's letters.
"It turns out that all cultural heritage materials are susceptible, whether we're talking about paper going moldy or a museum artifact that's exposed to moisture," says Mitchell. "The question is, can we preserve them for posterity while still allowing them to be visited?"
Microbes, or "bugs" as Mitchell calls them, live on nearly every surface--human teeth, computer keyboards, sewer pipes--in glazes called biofilms. Well-fed biofilms can morph into a destructive carnival of organisms, often containing several types of microbes and fungi. Mitchell and his researchers, postdoctoral fellow Chris McNamara and Cress Foundation conservator-intern Margaret Bruecker, are studying the processes of microbial degradation and looking for ways to control it.
|Left, above: A section of the Maya Pyramid of the Magician at Uxmal in the Yucatan shows microorganisms growing on its stones. Right, above: A magnified detail of a microorganism on the stone surface.
|Pyramid photo by Chris McNamara
Detail photo by Ralph Mitchell
Basic tenets of microbiology provide a simple equation: (biofilm + high humidity + high temperature + food) = (microbial explosion + acids). Since most materials are biodegradable, acids can eat them away. And human beings, says Mitchell, are increasingly bringing more food to the party. "There are two factors that are causing huge problems for cultural heritage materials: air pollution and tourism. We tend to think of tourism as being strictly physical. But think about the tombs in Egypt that were opened, or the caves in France." As soon as you let people in, he says, you provide the bugs, the moisture, and pollutants like hydrocarbons.
Hydrocarbons from tour buses, cars, and industry could be a bug's best friend, Mitchell explains. As hydrocarbons settle out of the air or "sorb" onto surfaces, microbes feast, fatten, and reproduce. With more buses belching out clouds of hydrocarbons each year near sites like Chichén Itzá, the Taj Mahal, or historic sections of Beijing, the forecast for outdoor monuments is murky.
However, Mitchell hopes to add shots of hard, helpful science to the art of preservation. Two years ago, he and Ralph Mueller, then a Harvard Medical School professor, first used CAT-scan technology to peer deep into microbe-ridden stone and stake out biofilms at work. Now scientists can explore microbial processes without destroying stone. Apart from hustling to save space suits, Mitchell, under a grant from the National Park Service, is studying how microbes use air pollutants, and how this relationship affects stone structures. His lab is also experimenting with consolidants--glue-like polymers injected into stones to keep them from crumbling. Consolidants that can resist microbial activity will be important preservation tools, especially in hot, humid environments.
Sometimes, Mitchell says, blocking microbial growth simply means reducing humidity and lowering temperature. Sounds fine for museum-bound space suits, but what about outdoor sites? Plop a couple hundred air conditioners beside the Sphinx, or close it off completely? "With the large outdoor sites, controlling tourism is important," says Mitchell. "You don't have to close them, you just have to be careful."
For example, to stem the damage moisture-spewing tourists were causing to prehistoric paintings at the Lascaux caverns, French authorities sealed it--and built a visitor-friendly replica of the cave's haunting galleries. A similar project is rumbling forward to protect caves in Spain. Some monument managers, like those at Stonehenge, even keep tourists roped away at a safe--for the stones, that is--distance.
Mitchell's research raises a hefty question: How far will Americans go to preserve their cultural heritage? Reducing tourist hordes and detouring bus caravans away from monuments can provide partial remedies for mushrooming hydrocarbon clouds. But saving artifacts in the United States may require a new national perspective on history. "This country hasn't really come to terms with the fact that it does have a cultural heritage that's part of our environment--as much as wetlands, rivers, and lakes," Mitchell says. "The U.S. has been in a sort of amnesia about its history, and it shouldn't be. There is a rich historic memory here, and you don't want to see it chewed up."