FEATURE
Pollution-Fueled"Biodeterioration" Threatens Historic Stone Researchers are probing the role microorganisms play in the decay of buildings and artwork. PATRICK
YOUNG tensified. Today they seek to learn the rate of biodeterioration, air pollution's role in the growth of microbes, and the best ways to protect and preserve stone structures, including such renowned sites as the Acropolis. A wide variety of microbes dwell on stone and metal structures, including bacteria, fungi, algae, and lichens (2). Most commonly, the organisms arrive by air, riding on aerosols, particularly soot; sometimes they arrive via groundwater. They establish colonies in selected locations, depending on such things as rock type, structure shape, and climate conditions (3). For example, Krumbein and Clara Urzi of the University of Messina in Italy found in a study of marbles exposed to different European climate conditions that coarse-grained and fine-grained marbles differed considerably in the communities of microflora they harbored (4). "There are different niches," said biologist Robert J. Koestler of the Metropolitan Museum of Art in New York City. "There are places where water can accumulate, and microorganisms are protected from sun. You get different things growing in the different niches. We need to know if they are causing a significant amount of deterioration that we need to control."
M icroorganisms appear to pose as great a threat to historic buildings, monu1 merits, and statues as does acid pre1 cipitation, according to recent research findings. Air pollution from * urban and industrial growth may be fueling these microbes and speeding the deterioration of venerated artworks and cultural treasures in many parts of the world—the Taj Mahal in India; the Acropolis and the Delos Sanctuary in Greece; stone Buddhas in Japan; cathedrals in Europe; and ancient temples in Cambodia, Vietnam, and Central America. Geomicrobiologist Wolfgang E. Krumbein of the University of Oldenburg in Germany, a leading figure in the emerging field of biodeterioration, reports that microbes attack more rapidly than does ordinary air pollution. "Laboratory experiments show that biodeterioration of rock is 100 to 10,000 times faster than chemical deterioration," he said. Since the 1880s, scientists have correlated the increased air pollution resulting from industrialization with an accelerated damage to historic buildings and outdoor monuments (J). However, only recently have scientists come to recognize microorganisms as truly significant players in this problem. This has led them to question whether air pollution is fueling a sharp increase in biological damage to historic structures and artworks. Scientists documenting this global biodeterioration stress, however, that much remains in question. "We really don't know how important microbes are to this deterioration," said microbiologist Ralph Mitchell of Harvard University. "We are beginning to get an idea that they are very important because we know that the organisms break down hydrocarbons. We know that they convert nitrogen and sulfur oxides to their acid forms. They are present in large numbers in particles landing on surfaces." Although relatively few researchers worldwide focus on biodeterioration, interest in the issue has in-
Koestler's current research seeks to assess biomass, the number of microorganisms that reside on an object, and their damaging waste products. "The problems in quantification are difficult," he said. Such studies hold the key to determining how serious a threat biodeterioration poses. Different rock types respond differently to microbes, depending on such characteristics as mineral composition, binding materials, porosity, and permeability. Damage to dense carbonaceous marble, for example, is limited to the surface; highly porous carbonaceous sandstone suffers much deeper penetration and deterioration by microorganisms.
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Quantification a challenge
Diopitting of an epitaph near the Delos Sanctuary in Greece (left) has occurred in the last 120 years since the stone was excavated. The pits, which are up to 10 mm across and 4 mm deep, are caused by microbial metabolic activities.
For the most part, microbial damage stems from the metabolic products microbes release (5). "These are chemical reactions that are mediated by microbes," Mitchell said. Depending on the organism, the damage includes surface deposits, discoloration, pitting, and accelerated weathering. Pitting, which creates some of the most prominent damage to m a r b l e , was long a t t r i b u t e d to p h y s i c a l chemical attack. However, studies now indicate this disfiguring etching results from the metabolic activities of such microorganisms as algae cyanobacteria and fungi (4). Similarly, microbial growth can discolor stone, changing its appearance dramatically. Krumbein, for example, argues that variation in the mix of microbes growing on and staining the Acropolis, rather than artistic license, explains why paintings of the structure made over the centuries show it in colors ranging from red to gray to black (6). In February, Krumbein began testing biocides against the microflora growing on the Acropolis. Microbe-caused physical damage to stone mostly stems from two weathering processes. First, organic and inorganic acids excreted by the organisms that feast on airborne sulfur, nitrogen, and organic compounds, including hydrocarbons, leach out the binding materials of rock and weaken its mineral crystal structure. Second, water-absorbing substances excreted by the organisms lead to changes in the porosity and permeability of the rocks. This results in an eroded surface and one that is more open to water and frost attacks. Biodeterioration of stone is a natural process and well recognized in one form. "The development of
soils is nothing else than the deterioration of minerals, and this would not occur on our Earth if there were no microbiology," said geomicrobiologist Thomas Warscheid of the German government's Material Testing Institute in Bremen. Particularly in hot, humid climates, microorganisms can seriously damage stone structures in the absence of air pollution. "What's new is the enormous increase in air pollution over the past 25 years in places we never saw it before," Mitchell said. Yet for all the concern and potential for damage, the biodeterioration issue remains rife with questions. Scientists remain uncertain just how prominent a role microbes play in attacking stone structures. "We don't have a definitive answer," Koestler said. "We have been trying to assess that in studies around the world."
How big a role? Field observations and laboratory experiments demonstrate the ability of microbes to do damage. Yet proving them a significant factor has been difficult. For one thing, the biological mechanisms remain the least understood of the processes that damage stone. And the simple presence of microbes and damage does not prove cause and effect, Koestler points out. When "Hiawatha," a large statue by the Irishborn American sculptor Augustus Saint-Gaudens (1848-1907), arrived at the Metropolitan Museum of Modern Art a few years ago, curators found that a large piece had broken off during shipment from Florida, where it had stood outdoors for 80 years. On examining the broken piece, Koestler discovered a bright green zone of living cyanobacteria about half VOL. 30, NO. 5, 1996/ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS " 2 0 7 A
A yeast-like fungus (Dematiaceae group) produces pits through a physical mechanism that researchers have not yet been able to identify. Differential expansion of the black fungi and the white marble under the heat of the sun is suspected. Photos courtesy W. E. Krumbein.
an inch below the surface. What he cannot say is whether these microbes weakened the statue enough for it to break. Even researchers who agree that microorganisms are an important factor disagree as to how definitive the evidence is that air pollution is increasing the biodeterioration of stone structures. Mitchell sees a solid link. "[Deterioration] is increasing as air pollution increases," he said. "The nitrogen and sulfur that rain down on a building are just providing the microbes on the surface with a nutrient supply that they can convert to nitric and sulfuric acid. Organics are going to be converted by fungi to all of the organic acids." Warscheid finds the anecdotal evidence of increased damage from air pollution compelling, but the scientific proofless so. "I would suggest that we do not have a clear scientific basis for this," he said. Currently, Warscheid is investigating organic air pollution, especially hydrocarbons, trying to characterize its role in biodeterioration. Krumbein's work indicates that the major threats come from chemoorganotrophic and chemolithotrophic microbes that feed on organic and inorganic pollutants, rather than on phototrophic org a n i s m s s u c h as algae a n d l i c h e n s . And the chemoorganotrophic organisms, which devour hydrocarbons released in the burning of fossil fuels, appear to be the greatest threat. Nonetheless, no general agreement exists on the issue, Krumbein said. "The question needs more laboratory and field work."
Biofilms increase pollution deposition Another area of investigation involves trie role of biotiims. Evidence suggests these thin mats, ou to luu micrometers thick and consisting of microorganisms living in a gel they themselves secrete, are the primary cause of biocieterioration. i ne gels constantly take nutrients from the air, iviitchen explained. They re acting like a sponge (7). Microbes consume these nutrients and produce acids as by-products. The mats presence also can change a material s porosity by closing down rock pores, which changes the humidity and gas ex2 0 8 A • VOL. 30, NO. 5, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
change, Warscheid said. In a recently completed study carried out with a team of chemists in Hamburg, Warscheid has found that "the pollution deposition on the surfaces is doubled by the presence of microbial biofilm." Increasingly, conservators seek improved ways to preserve and protect stone. Physical, chemical, and biological methods can help cleanse surfaces. These include the use of sprayed water, steam, and grit blasting, laser and ultrasonic cleaning; surfactants, emulsifiers, acids, alkalis, organic solvents, and absorbent clays; and biocides. However, these methods often provoke further deterioration. Biocides—fungicides, algicides, and bactericides— require careful selection to avoid further ill effects through crystallization or color change. Koestler tells of a white Carrara marble statue in New Orleans that turned orange after it was cleaned with a calcium hypochloride biocide and treated with a consolidant material to bind the surface. It turned out that iodide in the biocide reacted with free calcium in the stone to create calcium iodide crystals and a garish sheen (8). Biofilms greatly resist removal, Mitchell said, and require "high concentrations of biocides." Even then, a few organisms inevitably escape destruction, Warscheid said. "People have to face the problem that the applications of biocides on an intact biofilm will not have the same results as they have in the laboratory." After cleaning, conservators often use waterproofing compounds, mineral paints, polymers, and resins, and consolidants to seal stone from damaging microbes, with only limited success. The products themselves are biodegradable and may encourage further biodeterioration. Consolidants and waterproofing agents have a lifetime "as short as five years" in an outdoor environment, Koestler said. Currently, Mitchell devotes much of his research to testing the biodegradability of an extensive number of polymers, looking for one that will offer extended protection. No foolproof protectant exists, but the search goes on, as does the quest for more definitive answers about the role of microbes in the deterioration of cultural artifacts. "This is not just scientific curiosity," Krumbein said, noting the cultural and religious significance of these historic stone structures.
References (1) May, E. et al. Biodeterior. Abstr. 1993, 7, 2. (2) Griffin, P S.; Indictor, N.; Koestler, R. J. Int. Biodeterior. .991, 28, 187-207. (3) Warscheid, X; Oelting, M.; Krumbein, W. E. Int. Biodeterior. 1991, 28, 37-48. (4) Krumbein, W. E.; Urzi, C. The Conservation of Monuments in the Mediterranean Basin: Proceedings of the 2nd International Symposium; Museum of Art and History: Geneva, Switzerland, 1991, 219-33. (5) Saiz-Iimenez, C. Atmos. Environ. 1993, 27B{\), 77-85. (6) New Scientist Sept. 19, 1992, 6. (7) Ford, T.; Mitchell, R. Adv. Microb. Ecol. 1991, 11, 231-62. (8) Tudor, P B.; Matero, F. G.; Koestler, R. J. In Biodeterioration Research 3; Llewellyn, G. C.; O'Rear, C. E., Eds.; Plenum Press: New York, 1990.
Patrick Young is a Washington-based freelance science writer and former editor of Science News.
