Moss, fungi and lichen may seem inconspicuous but they can damage cultural relics, or, conversely, protect them from erosion. This dual functionality provides a broad selection of research topics and attracts multidisciplinary scholars to continuously explore and innovate effective measures for conservation.

Domestic and foreign academic efforts in this field over the past decade, based on cases conducted under different climates and of different types of cultural relics, were reviewed at the second International Symposium on Biodeterioration and Protection of Cultural Heritage, held at the Dunhuang Academy in Northwest China's Gansu province in June.

Unlike museum collections, kept in a dry and sterile environment, large outdoor artifacts are subject to weathering and undesirable effects from organisms, or biodeterioration, says Gu Ji-Dong, professor at the Guangdong Technion-Israel Institute of Technology in Shantou, Guangdong province.

He cites the Giant Buddha of Leshan in Southwest China's Sichuan province, a UNESCO World Heritage Site, as an example. The 71-meter-tall Buddha statue has long been plagued by mosses, lichens, mold and weeds growing on it. Parts of the Buddha's face, at times, have taken on a black hue as seen in media coverage, despite continuous monitoring and protection.

An internationally acclaimed microbiologist, Gu used to work for Harvard University in the United States and the University of Hong Kong, and now also serves as editor-in-chief of the academic journal, International Biodeterioration and Biodegradation.

Gu explains that in China and many other countries, most cultural relics conservators have an educational background in chemistry or materials science. Upon seeing dark spots on murals, for example, they naturally tend to analyze the chemical substances and reactions, before they realize it's actually the microbes that are causing the color change and damage.

"Preservation measures for large outdoor artifacts ultimately aim to prolong their survival in the natural environment.

"As microbiologists, we want to identify the microbes that are responsible for specific types of damage and how they cause such damage chemically, and by employing intervention measures we can minimize the activities of these microbes as much as possible or prevent their outbreak and attack, so that protection can be achieved by slowing down the damage to allow future generations to appreciate our cultural heritage," he adds.

He also expresses concern that researchers should not focus simply on the microbial communities colonized on the relics, but also pay more attention to their interactions with the surrounding environment and the materials of their making — more specifically, their reactions to changes in sunshine, water and salts — in order to form more comprehensive conservation plans based on scientific results.

For the past two decades, Gu has been continuously joining conservation projects for the Angkor Archaeological Park in Cambodia, also a UNESCO World Heritage Site, in collaboration with his Japanese counterparts to study and protect the Bayon Temple.

Gu's report during the symposium on June 21, working with Yoko Katayama, researcher at the Tokyo National Research Institute for Cultural Properties, suggests that under the tropical climate, colonization of plants and microbes on stone allows more fluctuation in thermal and water regimes, resulting in dissolution of selective minerals in sandstone, which enlarges its pores and porosity.

The soluble salts, when concentrated in the drying process, can be transported back into sandstone by capillary action, triggering salting-out effect. The outer surface of the sandstone structures can peel off by the internal pressure generated by the crystallization of salts.

Their research expounds on how microbes can contribute in a number of ways to affect the physical and chemical processes involved in sandstone deterioration and has demonstrated how biochemical reactions driven by microbes facilitate the accumulation of highly concentrated nitrate on the sandstone of Bayon Temple, while dissolution of calcite in sandstone further increases water retention and therefore improves microbial colonization and activities.

All these findings indicate the urgency to formulate a holistic approach to investigate the biodeterioration mechanisms of Angkor's sandstone temples and monuments, centered particularly on the materials-water-microbiome continuum, according to Gu.

As cycles of sulfur, nitrogen and carbon catalyzed by microbes can accelerate the corrosion of sandstone, intervention measures against microorganisms to inhibit their proliferation by controlling available water nutrients are fundamental. And in some parts of the Bayon Temple, they endeavor to improve drainage to keep the sandstone surface dry, Gu adds.

Climate concern

Ten years have passed since the Dunhuang Academy hosted the first edition of this symposium. This year's event, joined by more than 100 researchers and conservation practitioners from China, Italy, France, Spain and Japan, also serves as part of the academy's celebration of the 80th anniversary of its founding.

Gu is among the microbiologists who attended both editions of the symposium held at the academy, which oversees the Mogao Caves, a UNESCO World Heritage Site, and several other relics sites in Gansu, such as the Maijishan Grottoes in Tianshui and the Beishiku Temple (North Grotto Temple) in Qingyang.

He got in touch with cultural relics preservation practitioners on the Chinese mainland shortly after returning from the US to teach in Hong Kong in 1999, and cooperated several years later with the Dunhuang Academy and Lanzhou University.

Both are among the earliest organizations in China to study microorganisms' influence on cultural relics, according to Wu Fasi, deputy director of the academy's Conservation Institute.

In 2007, when he was a graduate student at the university, Wu joined a project to study whether increasing visitors to the Mogao Caves would bring changes to the concentration, quantity and variety of airborne microorganisms inside and outside the grottoes.

That study provides scientific proof for optimizing cave tourism and defining tourist capacity, and ever since, the academy and the university have been continuously conducting research programs and training professionals together in this regard.

Even though not many research teams in China were studying the microbiological influence on cultural relics 10 years ago, the academy and the university, together with the International Biodeterioration and Biodegradation Society, managed to initiate the first ever symposium in China specifically focused on biodeterioration of cultural relics and the corresponding preservation measures.

That event also saw participants from the Chongqing China Three Gorges Museum, the Shanghai Museum, Guangdong Museum, as well as the Cold and Arid Regions Environmental and Engineering Research Institute attached to the Chinese Academy of Sciences.

This year's symposium covered a large variety of topics and fully demonstrated the progress achieved over the past 10 years.

Climate change has brought challenges for cultural heritage preservation as cave temples, earthen relics and some ancient architecture in the open are more sensitive to the impact of the global phenomenon. Higher temperatures, extreme storms and prolonged high humidity can exacerbate or trigger biodeterioration on the relics, Wu says.

Located amid verdant forests and immersed in a warm and rainy environment, the Maijishan Grottoes, built in the 5th century and one of China's most well-known cave temples, are suffering from periodic microbial outbreaks on the murals and clay sculptures due to increasing rainfall in recent years.

In July 2018, staff members spotted some white mycelia on murals. Researchers from the Dunhuang Academy, Lanzhou University and the Guangdong Technion-Israel Institute of Technology confirmed that these were mainly the mass of interwoven filaments of fungi and actinomyces — filamentous or rod-shaped bacteria.

They then managed to compare, experiment and establish a technical process, combined by applying biocides and tools like soft brushes and modified vacuum pumps, to clean and control microbial mycelia on the murals.

Moreover, various animals from the Sciuridae family, such as Chinese rock squirrels and complex-toothed flying squirrels, like to scramble in and around the Maijishan Grottoes, nesting and excreting there.

Their brownish black excreta — solid and liquid — were widely distributed in the grottoes, on the cliffs and the trestle paths, as well as in the beam holes.

He Dongpeng, research associate at the Dunhuang Academy, says that based on experiments, they used bio-enzyme reagents such as artificial saliva to clean the contaminated areas, while setting up metal spikes and spraying capsaicin, a chili pepper extract, to deter animals.

Feng Youzhi, professor at the Nanjing Forestry University in Jiangsu province, introduced how his team, in collaboration with the Dunhuang Academy, investigated the Dahuting Tombs of the late Eastern Han Dynasty (25-220) in Zhengzhou, Central China's Henan province, where symbiotic relationships between species have led to microbial outbreak in the subterranean relics.

To be more specific, the Actinobacteria attract springtails by releasing a pheromone called geosmin and, with the help of the insects, migrate into the tombs. The Actinobacteria can inhibit other microbial species but proliferate to become the predominant species on the tomb walls and murals.

Another study conducted by Sichuan University and Sichuan Provincial Cultural Relics and Archaeology Research Institute aims to inhibit microbial growth on ancient ivory excavated from the sacrificial pits of the Sanxingdui Ruins in Guanghan, Sichuan province.

According to Sun Qun, professor at Sichuan University, most fungi and bacteria on the ivories derived from the in-situ soil in the pits. During excavation, cleansing and storage, human activities contributed to the change of microbiota on the ivories.

The researchers identified the key corrosion microorganisms and therefore developed an antibacterial agent from their patented bacillus, and this has been proved effective in controlling microbial biodeterioration on the ivories with sufficient safety.

However, nature has passed its blessing on these epitomes of human wisdom, exemplified in the Great Wall, located mostly in arid and semiarid regions of northern China, suffering from harsh climates, rain and wind erosion. Some parts of the Great Wall date back more than 2,000 years.

A research team from China Agricultural University and the Institute of Soil and Water Conservation under the Chinese Academy of Sciences and the Ministry of Water Resources carried out an investigation into some 600 kilometers of rammed earth Great Wall built during the Ming Dynasty (1368-1644) in the Ningxia Hui autonomous region, Shaanxi and Shanxi provinces, apart from the Inner Mongolia autonomous region.

They found that biocrusts, biological soil crusts that consist of mosses, lichens and photosynthetic bacteria such as cyanobacteria, cover 67 percent of these sections and serve as stabilizers, sacrificial layers and drainage roofs.

These biocrusts enhance the Great Wall's strength by reducing the effects of wind speed, raindrop kinetic energy and runoff scouring force, buffering temperature fluctuations, preventing infiltration, promoting soil mechanical stability and reducing erodibility and salinity, according to Xiao Bo, professor at China Agricultural University.

Expanding community

For the organizers and participants of the symposium, the biggest progress — and joy — was seeing an increasing number of young scholars from various professional backgrounds join this undertaking, yielding impressive academic fruits.

"Both editions of the symposium invited foreign scholars to come and see what their Chinese counterparts are doing, share knowledge and exchange ideas," microbiologist Gu says, adding that the latest event has witnessed a great expansion of this particular academic community.

Wu says, over the past decade, more and more research institutions realized that the combination of zoology, botany and microbiology with archaeology and cultural relics conservation is appealing and has the potential to produce notable academic achievements, resulting in growing emphasis on cultivating graduates and doctoral students in this interdisciplinary field.

Gu says, "These hardworking young people have already made impressive achievements, but they have an even brighter future ahead of them. I hope this symposium can serve as a reference for them to recognize their position in this academic field and better plan for their future directions."

With talent and technologies, China has a great opportunity to advance cultural heritage conservation — going into detail, doing the work earnestly and thoroughly and developing its own model systems of research work, he adds.

Microbiologist Clara Enza Urzi, associate professor at the University of Messina, Italy, and president-designate of IBBS, stressed the importance of a common language shared among the multiple disciplines involved in cultural relics conservation — achieved from exchanges and applied in turn to promote exchanges — to deal with the attractive complexity of this academic field.

She says it's important for China to have this community, with enthusiastic young people, open to the rest of the world. She calls for more exchanges and cooperation, either personal connections or academic ties such as visiting scholar programs.

The Italian scholar also suggests that the younger generation, both of China and the rest of the world, should be more aware of the works of their predecessors and foreign counterparts.

Guo Qinglin, deputy director of the Dunhuang Academy, says that conducting interdisciplinary research is the key path and inevitable trend to achieve breakthrough development in cultural heritage conservation.

He adds that in the future the academy will proactively build a more open platform to deepen cooperation and exchanges with peer researchers and, through multidisciplinary collaborative innovation, contribute to the study, preservation and sustainable utilization of the cultural heritage of Dunhuang and all over the world alike.

"After all, in heritage conservation, collaboration surpasses competition. It brings people together," he says.

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