Microbes
eating away at pieces of history
Easir Abedin
At Angkor Wat, the dancers' feet are crumbling. 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. Dr. 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, Ms. 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," Ms. 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,"
Dr. 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, Dr. 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
Ms. 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-candy like fungi on flooded African artifacts
housed in a university building in New Orleans when Hurricane
Katrina hit, Dr. 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 Dr. 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.
Dr.
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."