MORE than a century ago, archaeologists began excavating the ancient ruins of
Delos, an arid speck of an island in the Aegean Sea renowned in mythology as the
birthplace of Apollo. Delos thrived for hundreds of years as a religious and
trading centre until, in 88 BC, it was sacked by Menophaneses, a general from
the ancient kingdom of Pontus in northeast Asia Minor. The island鈥檚 rich
archaeological trove includes inscriptions carved in marble. When these epitaphs
were recovered from the Sanctuary of Apollo, they were sharp and clear. A
century later, they are almost indecipherable鈥攃overed in tiny pits caused
by a fungal infection.
鈥淲e have analysed this fungal attack practically all around the
Mediterranean. It鈥檚 everywhere,鈥� says Wolfgang Krumbein, professor of
geomicrobiology from the University of Oldenburg in Germany. He blames
hydrocarbons spewed into the air by human activities for the growth of about 80
strains of black yeast at Delos. 鈥淲e grow the fungi from the Delos monuments in
the laboratory on kerosene鈥攋ust aircraft fuel鈥攁nd it grows very
nicely into the marble and destroys it,鈥� he says.
Back in 1966, Krumbein was the first person to make the link between
hydrocarbons and the growth of microorganisms that destroy rock. Now, there is
increasing evidence that airborne chemicals, both organic and inorganic, serve
as major food sources for a huge variety of fungi, algae, bacteria and lichens
that live on and damage stonework. Researchers who study this 鈥渂iodeterioration鈥�
are starting to understand how these microorganisms inflict their damage.
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Hard rain
Several leading experts believe that biodeterioration is on the increase,
fuelled by pollution. But although there is a strong correlation between rising
pollution and erosion, it is difficult to pin the blame on microorganisms
because other factors play a role. Acid rain, for example, causes the sort of
damage that mars the marbles of Delos. Stone also crumbles to dust under attack
from moisture, freezing and thawing, and salt.
Despite the difficulties, however, a consensus is growing among scientists.
鈥淭he message that is coming through from everybody in the field is that, while
acid rain may be important, hydrocarbons are even more important,鈥� says
microbiologist Ralph Mitchell from Harvard University. Conservators are also
coming to realise that they must limit biodeterioration if they are to preserve
cultural treasures such as the Taj Mahal, Westminster Abbey and the Brandenburg
Gate.
Most new microbial invasions of stone begin when organisms in the air float
down as passengers on soot or other small particles, and sometimes they arrive
by groundwater. Where they establish their colonies depends on a structure鈥檚
rock type, shape and microclimate. As a result, completely different
microcommunities may live within centimetres of each other. 鈥淛ust moving a few
inches on stone might give you a different amount of sunlight and therefore a
different temperature inside the stone or a different quantity of light
penetrating the stone,鈥� explains Robert Koestler, a biologist working at the
Metropolitan Museum of Art in New York.
Complex microcommunities damage stone in a variety of ways. Some organisms
create surface deposits, others cause discoloration, pitting or accelerated
weathering. Krumbein believes that ever-changing mixtures of
microorganisms鈥攔ather than creative imagination鈥攅xplain why
generations of artists have depicted the Acropolis in hues ranging from red to
grey to black (New Scientist, 19 September 1992, p6).
Much of the damage is caused by the waste products of microbial metabolism.
鈥淭he nitrogen and sulphur that rain down on a building provide the microbes on
the surface with a nutrient supply that they can convert to nitric and sulphuric
acid,鈥� says Mitchell. These acids then react with binding materials such as
calcium and magnesium in the stone to form soluble complexes. When they are
washed away the stone may become pitted and its crystal structure is
weakened.
Crack attack
Microorganisms also produce water-absorbing substances that change a rock鈥檚
porosity and permeability. They can get under the surface, explains Mitchell,
where they excrete hydrophilic polysaccharides. 鈥淚t鈥檚 like putting a gel in
there which soaks up water,鈥� he says. As a result, every time the rock goes
through a freeze-thaw cycle, microcracking occurs which in turn leads to
erosion.
Rocks differ in their response to microbial assault, depending on their
physical and chemical properties. Dense stones such as marble restrict microbial
damage to the surface. Porous rocks may become colonised several centimetres
below the surface. Lichens and cyanobacteria are particularly adept at
penetrating sandstones and other porous rocks. 鈥淚t鈥檚 like having a root grow
down into concrete and crack it,鈥� says Mitchell. Chemical characteristics of
rocks鈥攖heir mineral composition and binding materials鈥攁lso determine
the effects of microbial infection.
Proving that damage has been caused by resident microorganisms is not always
possible, however. Koestler cites an example a few years ago when the
Metropolitan Museum of Art acquired Hiawatha, a statue of the legendary
American Indian sculpted by Augustus Saint-Gaudens (1848-1907). When curators
opened the shipping crate, they found a large piece had broken off during the
journey from Florida. Koestler discovered a bright green zone containing living
cyanobacteria about a centimetre below the surface. But did they cause the
damage, or was their presence mere coincidence? There is no way of knowing for
sure, admits Koestler.
It is hardly surprising, then, that controversy rages over whether pollution
is increasing the rate of biodeterioration. 鈥淲hat鈥檚 new is the enormous increase
in air pollution over the past 25 years in places we never saw it before,鈥� says
Mitchell. 鈥淗ydrocarbons are going to be converted by fungal biofilms to a wide
range of organic acids.鈥� He points to growing evidence that biofilms鈥攖hin
mats 50 to 100 micrometres thick made up of microorganisms living in a gel that
they secrete鈥攁re a major cause of biodeterioration. The gels take up
nutrients from the air 鈥渓ike a sponge鈥� and the resident microorganisms feast
upon them and excrete their damaging acids, explains Mitchell.
Biofilms are very destructive. In 1994, a team of chemists from the
University of Hamburg Institute for Inorganic and Applied Chemistry, working
with Thomas Warscheid, a geomicrobiologist from the German government鈥檚 Material
Testing Institute in Bremen, showed that biofilms double pollution deposition on
stone surfaces. Biofilms can also clog up rock. This changes the stone鈥檚
porosity, which alters its humidity and gas exchange and accelerates erosion
from within.
Mitchell is convinced that biodeterioration is on the increase. But other
researchers including Warscheid and Krumbein say the scientific evidence is not
strong enough. Warscheid, however, believes the anecdotal evidence is
compelling. And Krumbein has published figures showing that while sulphur
dioxide levels in air have decreased significantl in the past few years, organic
pollution is steadily rising.
Krumbein adds that it will take a lot more laboratory and field work to know
for sure. His lab analysis indicates that biodeterioration destroys rock between
100 and 10 000 times faster than chemical deterioration. He found that marble
slabs inoculated with black yeast from Delos contained pits up to 400
micrometres deep after just nine months in conditions optimal for fungal growth.
Krumbein鈥檚 observations in the field, both at Delos and in Israel鈥檚 Negev
desert, suggest that fungi can eat away at rocks at up to 5 millimetres in 100
years. But according to Koestler, even these results are controversial. 鈥淲e鈥檙e
still trying to figure out how to measure the rate,鈥� he says.
For conservators such disputes are academic. Historic and artistic stone is
under threat, and finding improved ways to protect it from the ravages of
microorganisms is a priority. Already, conservators have an arsenal of physical,
chemical, and biological weapons at their disposal. These include high-pressure
water sprays, steam and sand blasting as well as high-tech solutions such as
lasers and ultrasonic techniques and chemicals including emulsifiers, acids,
alkalis, organic solvents and absorbent clays. Biofilms are often treated with
heavy doses of biocides but any microorganisms that escape the onslaught will
quickly recolonise the stone.
Even when cleansing is effective it may have unwanted side effects. Koestler
recalls what happened in New Orleans to a stone figure of philanthropist
Margaret Haughery that had spent a century outdoors. Cleaned with a biocide
containing calcium hypochlorite and treated with a consolidant to bind its
surface, the white Carrara marble turned orange. The cause: a reaction between
iodide in the biocide and free calcium in the stone produced orange calcium
iodide crystals.
And compounds used by conservators to seal stone after cleaning have an even
more chequered history. Most are biodegradable, which means they may actually
fuel further deterioration. Exposed to the elements, says Koestler, consolidants
and waterproofing agents may last as few as five years. 鈥淭he polyurethanes, the
polyacrylics and the acrylic polymers are all susceptible to microbial
deterioration,鈥� says Mitchell. 鈥淭he one group that looks interesting are the
epoxies, but even they have some susceptibility.鈥� Krumbein and his graduate
student Chris Heyn have found, in lab tests, that fungi degrade the synthetic
polymers Paraloid, Klucel, Mowilith, Mowiol, and Primal鈥攗sed to conserve
frescoes鈥攚ithin a few months.
As knowledge about biodeterioration grows, ways to treat the problem will, no
doubt, improve. But preservation is a costly business. Inevitably, societies
will have to make choices about which stone structures and artworks to treat.
But, as Norbert Baer, a physical chemist at New York University鈥檚 Conservation
Center, points out, not everyone wants to see buildings and statues stripped of
their microbial inhabitants. 鈥淥ne person鈥檚 corrosion is another person鈥檚
patina,鈥� he says.
