The palatial 12th-century Hindu temple, shrouded in the jungles of Cambodia, has played host to a thriving community of cyanobacteria ever since unsightly lichens were cleaned off its walls nearly 20 years ago. The microbes have not been good guests.
These bacteria (Gloeocapsa) not only stain the stone black, they also increase the water absorbed by the shale in morning monsoon rains and the heat absorbed when the sun comes out. The result, says Thomas Warscheid, a geomicrobiologist based in Germany, is a daily expansion and contraction cycle that cracks the temple's facade and its internal structure. Warscheid, who has studied Angkor Wat for more than a decade, said in an interview that these pendulum swings had broken away parts of celestial dancer sculptures on the temple walls.
"It is getting worse — up to 60 or 70 percent of the temple is black," he added.
Once chalked up to weathering, the damage at Angkor Wat is now seen as the result of a much more complex dynamic: the interaction of micro-organisms with the chemical and physical properties of the temple.
In various places around the globe, from Easter Island to the Acropolis, microscopic organisms are accelerating the deterioration of monuments and historic landmarks. Scientists and conservators have only recently begun to understand the role that common bacteria and fungi play in destroying cultural sites and how — if at all — they can be stopped. This growing recognition is inspiring new techniques to combat microbial damage.
"Our heritage is disappearing," said Ralph Mitchell, a biology professor at Harvard. "Whether it's Angkor Wat or the Mayan sites in Mexico or the Native American archaeological sites in the West of this country, they are all under threat. And the question is, can we preserve them?"
From bacteria that feed on hydrocarbons to endolithic fungi that eke out an existence within porous rock, monument-damaging microbes thrive because they survive in environments inhospitable to other flora and fauna.
"One of the recent discoveries that is of concern is that increased air pollution can sometimes increase biodeterioration," said Eric Doehne, a scientist at the Getty Conservation Institute. Some bacteria feed on chemicals found in pollutants, excreting an acid that eats away at stone, metal and paint.
Microbes pose a serious risk to the monuments at the Acropolis in Athens, including the golden-proportioned Parthenon and the Temple of Athena Nike, said Sophia Papida, conservator for the Acropolis Restoration Service.
Bacteria penetrate the veins of the marble, attract water and expand, cracking the monuments' faces and pillars, Papida said. Lichens burrow circular holes in the marble, a phenomenon known as honeycomb weathering, and exfoliate sculptural friezes that tell the stories of gods and goddesses.
Microbes also thwart painstaking efforts to restore the monuments. Acropolis stones can crumble into thousands of pieces, leaving a near-inscrutable jigsaw puzzle. "Our work is attacked by micro-organisms and we have to go back, remove the micro-organisms and put it back together," Papida said. "The bacteria which are there, they are having a good time, actually."
For decades, researchers struggled to grow laboratory cultures of bacteria that thrive on monuments. Today, genetic techniques allow scientists to better identify micro-organisms, but that does not always mean they can reverse the damage.
"We can use DNA analysis to identify who's there, but it doesn't mean that they cause the problem," said Robert Koestler, director of the Museum Conservation Institute at the Smithsonian.
Some efforts to preserve monuments become the very cause of the problem. Biodegradable polymers used to consolidate the stones of Mayan ruins in Mexico, for example, created conditions ripe for damaging microbes .
An added complication is that the organisms sometimes protect monuments, such as the volcanic rock formations known as the Cappadocian "fairy chimneys" of southeastern Turkey. Just as lichens once kept Angkor Wat from absorbing too much water and heat, scientists discovered that lichens on the chimneys prevented them from taking in too much water, keeping them intact longer.
"It's not always a bad-news story," Doehne said. He is optimistic about scientists' ability to manage microbial attacks. "We are seeing a burst of knowledge coming to the fore, really in the last 20 years."
At Angkor Wat, Warscheid developed a biocide called "mélange d'Angkor" that will be used to whiten parts of the temple. The chemical solution changes the ability of the cyanobacteria to produce their black-staining byproduct. There is no point, he says, in applying the biocide to the whole temple. After 10 years, the bacteria will adapt to it. "In certain places," he said, "where there are carved stone scriptures, you can provide the manpower to do this cleaning on a regular basis."
At the Acropolis, University of Athens researchers are working with Papida to test a biocide, a quaternary ammonium compound that she hopes will get the restoration back on track.
Fighting off microbes is a matter of "vigilant and routine maintenance," said Mark Weber of the World Monuments Fund. People often deal with "stone-eating organisms," as if they are singular events, he added, rather than as adaptive beings.
Another emerging solution is to starve the microbes. Conservators did this to kill off cotton-candylike fungi on flooded African artifacts housed in a university building in New Orleans when Hurricane Katrina hit, Koestler said. The fungi thrive on oxygen; they created an anoxic environment by flushing the objects with argon.
The method is easier, of course, indoors. Outdoors, combating microbes can mean cutting off their water source. "You want to catch it early — just like you diagnose a disease," said Mitchell of Harvard. Once a biofilm, a community of bacteria like the slimy coating that forms on your teeth, develops, any effort may be futile.
In Warscheid's view, protecting monuments, while important, is delaying the inevitable. "We have to accept that at some moment they will disappear," he said. "But we know a lot about how to conserve them for the next 20, 30 years."