Microbes Can Damage butAlso Help Restore ArtifactsDespite damaging wood and stone artifacts in diverse settings,microbes also may be used to restore damaged items

Nick R. Konkol, Christopher J. McNamara, Robert A. Blanchette,

Eric May, and Ralph Mitchell

From the prefabricated wooden huts ofAntarctic explorers in the early 20thcentury to the enduring limestone ru-ins of the Mayan city Ek’ Balam(Black Jaguar) built more than 1,200

years ago (Fig. 1), conservators, archaeologists,and other specialists are striving to understandand combat the physical, chemical, and biolog-ical processes responsible for the deteriorationof these historic structures and artifacts. Each ofthese sites represents a cultural trove, one as atestament to a lost civilization and the other as atribute to a bygone era of exploration. Separatedby more than 10,000 km and residing in twovery different climates, these sites were con-structed of drastically different materials. Yetthe artifacts at each location are slowly wastingaway, presenting two very different cases of

biodeterioration that reflect distinct microbialcommunities and modes of decomposition.

With their ability to use diverse energysources and survive under varied environmentalconditions, microorganisms are superbly capa-ble of degrading culturally important materials.Given the proper conditions of humidity, tem-perature, and pH, many components of ourprecious cultural heritage such as pigments, tex-tiles, metals, polymers, wood, and stone can fallvictim to microbial degradation. Biodeteriora-tion of culturally significant artifacts and struc-tures takes on new levels of importance as con-servators and archaeologists weigh the benefitsof unearthing and displaying artifacts, or evencleaning or repairing such artifacts, against therisks of exposing them to further damage bymicrobes.

Fungi Damage Wood Structures

in Antarctica

The early 20th century marked a heroicage of polar exploration. In 1901, RobertScott and Ernest Shackleton began theirquest to reach the South Pole, launchingthe first of several expeditions to the RossSea region of Antarctica. The menbrought wooden huts with them to use astheir base of operations. Scott himselfwould perish on an expedition launchedfrom one of these huts in 1911. Shackle-ton went on to lead the Endurance expe-dition but succumbed to a heart attack 11years later while attempting to circumnav-igate the frozen continent by ship. Despite

Summary

• The fungal species Cadophora, which is causingsoft rot in and outside historically importantbuildings at Antarctic sites, is distinct from mi-croorganisms that typically damage woodenbuildings in temperate and tropical zones.

• Microorganisms living along and also below thesurfaces of Mayan stone artifacts may threatenarchaeological sites throughout the Yucatan re-gion of Mexico.

• Bacteria that can remove sulphate crusts or formlayers of calcite to consolidate mineral surfacescould prove helpful to conservators who areworking to restore damaged building stones.

Nick R. Konkol is aPostdoctoral Fellowin the HarvardSchool of Engineer-ing and Applied Sci-ences, ChristopherJ. McNamara is aResearch Associatein the HarvardSchool of PublicHealth and the Har-vard School of Engi-neering and AppliedSciences, Robert A.Blanchette is a Pro-fessor in the De-partment of PlantPathology at theUniversity of Min-nesota, Eric May isa Reader in Micro-biology at Ports-mouth University,and Ralph Mitchellis a Gordon MckayProfessor of Ap-plied Biology in theHarvard School ofEngineering andApplied Sciences.

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the harsh environment that killed their formeroccupants, the huts still stand. Yet even insubzero temperatures, fungi are gradually de-grading these wood structures.

Investigations of these historic sites reveal de-cay in the exterior woods, and blooms of fungion the surfaces of the wood, textiles, paper, andother artifacts inside the huts (Fig. 2). The decayin these huts is not the type usually encounteredin temperate and tropical zones, where wood-destroying basidiomycetes are prevalent. In-stead, the ascomycete Cadophora is the predom-inant species causing soft rot by producingcavities inside wood cell walls (Fig. 2). Thispoorly known group of fungi appears to becommon in Antarctica, where they live in soilsand utilize varied carbon sources. Although ithas taken decades of relatively slow growthduring the short Austral summers, their cumu-lative damage to historic wood and other arti-facts is substantial, posing serious conservationproblems not only in the Antarctic huts but alsothroughout the world.

Our understanding of such microorganismsand how they damage wood in such variedenvironments is not well developed. However,from Antarctic huts, we know there are manypreviously undescribed species of fungi present,leaving us with much to learn about their abilityto tolerate high salts and other toxic substances.With so little known about counteracting thesedamaging fungi, we and other microbiologistsnow recommend to the Antarctic HeritageTrust, the nonprofit organization that oversees

preservation efforts, that microenvironments inand around the huts be changed to make themless conducive for fungal growth. These changesinclude improving snow melt drainage, reducingmoisture levels of wood in contact with theground, and reducing relative humidity insidethe huts. After visitors track snow and ice intothe huts, frost typically forms on interior walls,while moisture from storms may filter into thehuts, furnishing additional moisture to microbesthat augments their growth and leads to furtherdamage. Restricting moisture remains the mostpractical option for limiting these detrimentalmicrobial effects.

If little is known about the microorganismsthat affect the historic huts of Antarctica, lesscan be said about microbial diversity in polarregions. The fungi causing decay in the historicstructures and artifacts appear to be dominantorganisms responsible for carbon utilizationand nutrient recycling in Antarctic soils. Thesefungi are distributed not only in Antarctica, butalso in the Arctic region. Undoubtedly impor-tant in these ecosystems, they were little knownand poorly appreciated before investigators rec-ognized that they are responsible for attackingcultural artifacts left behind by early polar ex-plorers.

Archaeologically interesting wooden artifactstypically are found intact at sites where ex-treme environmental conditions limit micro-bial growth. Whether in arid terrestrial sites orwater-logged at the bottom of the sea, microbescan colonize and degrade wood, raising serious

F I G U R E 1

Areas of study. Left: Shackleton’s hut at Cape Royds, Ross Island, Antarctica. Right: the Mayan city of Ek’ Balam, Yucatan, Mexico.

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challenges over how to preserve valu-able historic structures worldwide. Pro-tecting these important cultural re-sources is an enormous challenge forconservators, one to which microbiolo-gists can contribute by providing a bet-ter understanding of these organisms,which should help toward developingbetter strategies for controlling them.

Bacterial Communities Threaten

Mayan Stone Works

Mayan archaeological sites in SouthernMexico are among the most importantcultural artifacts in the Western Hemi-sphere. Most of these cities were part ofthe great Mayan civilization until the8th and 9th centuries when they wereabandoned by their inhabitants and re-claimed by the jungle. Unearthed in re-cent years, these stone structures aresubject to the high temperatures andhumidities that prevail throughout theYucatan Peninsula, fostering substantialmicrobial growth. In some instances,growth of pigmented microorganismsmerely alters the appearance of the historicstone, presenting a purely aesthetic challenge.However, in other cases, microorganisms are amajor cause of stone deterioration.

The epilithic microflora on the surfaces ofMayan stone structures make up a diversecommunity of microorganisms, including het-erotrophic bacteria, Cyanobacteria, algae,fungi, and lichens. Their growth, particularlythe Cyanobacteria and algae, can stain stonesurfaces green and black. Yet their threat isnot purely aesthetic. The autotrophs that col-onize these surfaces appear to be a source oforganic matter that supports successivegrowth by large communities of heterotrophicbacteria and fungi. Once established, fungalhyphae can then penetrate deep into stone.Subsequent shrinking and swelling of the hy-phae severely degrades the surrounding mate-rial, providing entry sites for water and lead-ing to further deterioration.

Endolithic microorganisms also colonize the in-terior of the stone through pores and cracks thatform through physical weathering. At the ruins ofthe city Ek’ Balam, 160 km west of Cancun, we

discovered that the 16S rDNA diversity of endo-lithic bacteria residing 1–2 cm below the surface oflimestone differs dramatically from the epilithicmicroflora residing above it. Actinobacteria dom-inate this endolithic community, which also con-tains large numbers of Acidobacteria and Firmi-cutes (Fig. 3).

The bacteria in these endolithic habitats maybe damaging culturally important limestone ob-jects. For example, the biofilm matrix in whichthey reside absorbs water, which shrinks andswells their extracellular polysaccharides. Inturn, this mechanical stress opens cracks andfissures in the limestone. Like that of fungalhyphae, this process also exfoliates surface lay-ers and crusts.

Endolithic microorganisms may also acceler-ate stone-damaging chemical processes. For in-stance, bacteria can increase the mineral disso-lution rate of the limestone in which they resideby producing simple acids. Indeed, in the pres-ence of the endolithic bacteria, limestone re-leases Ca2� twice as fast as when those bacteriaare not added to the mix (Fig. 3).

These microbial communities may be endan-gering Mayan archeological sites. For example,

F I G U R E 2

Biodeterioration of cultural properties from Scott’s Cape Evans hut. Left: mold growingon a historic crate (top) and explorer’s boot (bottom). Right: Wood decay cross sectionfrom Shackleton’s Cape Royds hut showing extensive cavities in wood cell walls. (Figureby Benjamin Held, University of Minnesota).

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endolithic growth protects bacterial populationsagainst low temperatures, UV radiation, anddesiccation. To what extent does it also insulatethese organisms from attempts by conservatorsto protect and restore these surfaces? Findingbacteria within such artifacts calls into questiontraditional techniques, such as the application ofconsolidants, long used for preserving carbon-ate stone such as limestone and marble. Consoli-dants, which penetrate the stone surface andbind its components, typically contain biode-gradable organic polymers that endolithic bac-teria might degrade. We need to better under-stand the metabolism of endolithic bacteria if weare to preserve Mayan sites, as well as otherstatues, tombstones, monuments, and historicbuildings.

Using Bacteria to Clean Stone

Surfaces

Atmospheric pollution and weatheringcan disfigure or damage stone surfacesby forming salt crusts—typically, gyp-sum. Such sites present opportunitiesfor bioremediation. While bioreme-diation is now a routine approach fortreating sites contaminated with haz-ardous chemicals, its application toameliorate the effects of stone deteri-oration is rarely practiced.

However, conventional techniques,such as mechanical removal of saltcrusts and the use of slaked lime or limewash, have several disadvantages. Theymay change the color of the stone, inter-fere with the ordinary movement of saltswithin the stone, or remove excessamounts of the original surface. For ex-ample, some conservators continue tocriticize the mechanical removal of blackcrusts and patinas from the Parthenon inAthens, while also questioning whetherthose crusts are linked to past treatmentregimes.

Meanwhile, there is growing evi-dence that bacteria can be used to treatdamaged stone surfaces. In particular,some bacteria have the potential to re-move sulphate crusts or to form sacrifi-cial layers of calcite that consolidatemineral surfaces. Thus, bioremediationmight offer a new means for restoringstone surfaces of heritage buildings.

Conservators involved in the BIOBRUSHproject (www.biobrush.org), which is funded bythe European community, are evaluating low-hazard bacteria to remove salt crusts from stonesurfaces and other bacteria to consolidate stonesurfaces. Applying the bacteria directly to stonesurfaces minimized the risk to the stone itself, tothe immediate environment, and to those whohandle these materials.

In one case, multiple short-term applicationsof aerotolerant sulphate-reducing bacteriawithin an appropriate delivery system success-fully removed black gypsum crusts from marblein the laboratory and in situ on buildings. Thisapproach to removing such crusts occurs in anatural way, as these same microorganisms playan active role in the environment where they

F I G U R E 3

Top: comparison of epilithic and endolithic bacteria. Percentage of clones in eachphylogenetic group from epilithic bacteria (left) and endolithic bacteria (right). Bottom:Ca2� concentration in uninoculated flasks and flasks inoculated with endolithic bacteria.

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contribute to the sulfur cycle. Based on trialresults, this form of bioremediation comparesfavorably to traditional chemical cleaning withammonium carbonate.

Another BIOBRUSH project explored factorsthat influence biological precipitation of cal-cium carbonate. According to studies in Spain,France, the United States, and Italy, biocalcify-ing bacteria pose little threat to heritage objects.They can deposit a calcite layer without signifi-cantly reducing the porosity of the stone. Hereagain, conservators used bacteria isolated fromthe environment and controlled the release ofnutrients to minimize growth of contaminantmicroorganisms.

Researchers continue to evaluate the effec-tiveness and risks of these procedures to pro-vide conservators with greater confidence. An-

other set of concerns focuses on whether thereare any long-term effects from applying thebacteria and nutrient media. Scientists andconservators should collaborate to addressthese issues.

Despite such questions, the controlled use ofmicroorganisms to help preserve, protect, and re-store building stone appears promising as a meansfor supplementing, not wholly replacing, tradi-tional conservation technologies, some of whichare not particularly effective, while others exposeusers and the environment to toxic materials.During the past 10 years, researchers began toidentify suitable microorganisms that either re-move salt crusts or consolidate pore structures indamaged stone, and have also addressed some ofthe environmental risk factors in applying this newtechnology.

ACKNOWLEDGMENTS

The authors are supported by grants from the National Science Foundation, the Consejo Nacional de Ciencia y Technologica,the Samuel H. Kress Foundation, the European Commission Fifth Framework, Energy, Environment, and SustainableDevelopment Programme, and institutional funds from their respective Universities.

SUGGESTED READING

Arenz, B. E., B. W. Held, J. A. Jurgens, R. L. Farrell, and R. A. Blanchette. 2006. Fungal diversity in soils and historic woodfrom the Ross Sea Region of Antarctica. Soil Biol. Biochem. 38:3057–3064.Blanchette, R. A., B. W. Held, J. A. Jurgens, D. L. McNew, T. C. Harrington, S. M. Duncan, and R. L. Farrell. 2004. Wooddestroying soft rot fungi in the historic expedition huts of Antarctica. Appl. Environ. Microbiol. 70:1328–1335.Cappitelli, F., L. Toniolo, A. Sansonetti, D. Gulotta, G. Ranalli, E. Zanardini, and C. Sorlini. 2007. Advantages of usingmicrobial technology over traditional chemical technology in removal of black crusts from stone surfaces of historicalmonuments. Appl. Environ. Microbiol. 73:5671–5675.Crispim, C. A., P. M. Gaylarde, and C. C. Gaylarde. 2003. Algal and cyanobacterial biofilms on calcareous historic buildings.Curr. Microbiol. 46:79–82.Held, B. W., J. A. Jurgens, B. E. Arenz, S. M. Duncan, R. L. Farrell, and R. A. Blanchette. 2005. Environmental factors influencingmicrobial growth inside the historic huts of Ross Island, Antarctica. Int. Biodeterioration Biodegradation 55:45–53.May, E., M. Jones, and J. I. Mitchell. Heritage Microbiology and Science: Microbes, Monuments and Maritime Materials,Chapter 8 The BIOBRUSH project for bioremediation of heritage stone, p. 76–93. RSC Publishing, Cambridge, in press.McNamara, C. J., and R. Mitchell. 2005. Microbial deterioration of historic stone. Frontiers Ecol. Environ. 3:445–451.McNamara, C. J., T. D. Perry, K. A. Bearce, G. Hernandez-Duque, and R. Mitchell. 2006. Epilithic and endolithic bacterialcommunities in limestone from a Maya archaeological site. Microbial Ecol. 51:51–64.Webster, A., and E. May. 2006. Bioremediation of weathered-building stone surfaces. Trends Biotechnol. 24:255–260.

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