The Khmer Empire was thriving at its peak in the 9-12th centuries CE with a large territory in South Asia including what is now Cambodia, Vietnam, Thailand, Laos and other countries. A large number of temples were built under different kings and some of the best known today include Angkor Wat and the Bayon temple of Angkor Thom in the Siem Reap area of Cambodia.
These temples are valuable cultural heritage examples for the study of the Khmer civilization of the past and also serve as major tourism attractions for the local economy today. They were composed structurally of sandstone, mainly from the Kulen Mountain, which still serves as a major source of building material today.
The site of Angkor and its monuments became UNESCO World Cultural Heritage site in 1992, and subsequently different countries became involved with their protection and conservation.
The Japanese Government Team for Safeguarding Angkor chose the Bayon temple in Angkor Thom as one of the main site to base their extensive protection work covering architecture, archaeology, geotechnical engineering, materials sciences and microbiology of the sandstone.
The Bayon temple is today covered with a wide range of micro-organisms with different physiological functions showing different colours as a result of adaptation to the local environmental conditions.
The directly exposed surfaces are general dark brown while those inside the monuments can range from rich green to dark green.
At the initial stage of colonisation, the pioneering micro-organism lichens, a symbiotic association between algae and fungi, can settle on the fresh surface of sandstone and these are still widely found today. This form of colonisation actually has a very important protective property to the underlying sandstone; this protection however turns into deterioration in the late stage of colonisation development because the lichen layer dissolves the sandstone constituents significantly to weaken the sandstone surface and its integrity, resulting in damage to the sandstone. Because of the damage by initial colonisation, further microbiological communities on sandstone surface becomes possible and produce more complex communities of additional micro-organisms with a wide range of physiological characteristics.
Other microbial micro-organisms are also widely found displaying different colours on the surface of sandstone, especially on bas-relief, making viewing the carving very difficult or impossible. The colours pink, green, violet, black or dark green is associated with different bacterial groups predominantly of Rubrobacter, Cyanobacteria, Cyanobacteria and Chloroflexi, Chloroflexi, and Deinococcus-Thermus, Cyanobacteria and Rubrobacter, respectively.
There are also different minerals associated with different colours. A comparison of the microbial communities between fresh and old biofilms showed that the bacterial communities are very similar, but the eukaryotic communities are distinctly different, indicating a shift in community composition.
This information illustrates the dynamic formation and succession of microbial communities on sandstone under tropical climate condition.
Using an innovative and non-destructive sampling method, biofilms on sandstone can be collected non-destructively to provide detailed information of spatial distribution of micro-organisms in the biofilms on the surface of sandstone.
The biofilm layers on sandstone collected by this sampling technique showed a stratified structure: the blue–green biofilm, associated with serious deterioration of sandstone, is rich in Cyanobacteria and Actinobacteria on the top surface, and Chloroflexi in the deeper layers at the bottom. The characteristic distribution of bacteria at different depths provides valuable information on not only the specific micro-organisms in biofilm and the formation process, but also the sandstone weathering process by these micro-organisms under the humid tropic climate.
The sandstone on these temples shows characteristic deterioration; the destruction is associated with activity of sulphur-oxidising bacteria, producing acidity to initiate acid attack to the sandstone. This has been confirmed at Angkor Wat, the Bayon temple and other sites in Cambodia through our investigations over the last 20 years. In addition, the bas-relief of sandstone at Bayon temple was found to accumulate extremely high concentration of nitrate, but not ammonium or nitrite. The source of nitrate is most likely due to the transformation of ammonia from organics and the micro-organisms responsible for oxidation of ammonia are ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). AOA are easily detected at several temples in high abundance, but AOB are either detected at detection limit or non-detectable. Using nitrogen stable isotope analysis, the nitrate in the sandstone cannot have originated from decomposition of phototrophic micro-organisms colonising on the sandstone and alternative source is sought.
The microbiological investigations conducted at Angkor temples in Cambodia collectively indicate that a wide range of micro-organisms can be detected on surfaces of sandstones through time, and they are responsible for the colour development on the surface and the activity of some selective groups is responsible for bio-deterioration of the sandstone integrity through acid production and dissolution of minerals in the sandstone to contribute to the overall sandstone deterioration observed at these sites.
About the Authors:
Ji-Dong Gu is an associate professor of the School of Biological Sciences, The University of Hong Kong, P.R. China. His research is mainly focused on environmental microbiology and toxicology and he has been working on the protection of Angkor monuments for more than 10 years.
Yoko Katayama is a professor at the Graduate School of Agriculture, Tokyo University of Agriculture and Technology, Japan. Yoko is an environmental microbiologist and has worked on the deterioration of the Angkor monuments from the viewpoint of activities of micro-organisms especially of the sulphur-oxidizing type.