Historical monuments are fundamental to preserving human history and culture for the future generations. However, when they are open to visitors, these monuments can suffer rapid deterioration. For example, illumination with artificial lights is essential for exhibiting cultural heritage and facilitating tourism in indoor environments. Yet, this light can result in important microbial-driven biodeterioration of these relics. Finding a compromise between conserving these relics for future generations while promoting their value and history to current citizen is tricky, and thus is a major challenge for managers, scientists and curators. 

The deterioration of phototrophic microbiome under illumination.

 In theory, cell pigments can control the capacity of phototrophic microbes to absorb photons within a specific light spectrum. For example, white light has the full visible spectrum of 400~700 nm. Two Synechococcus-type picocyanobacteria both have strong absorbance at ~430 nm (i.e., blue light). In addition, each of them has the orange‒red part (620~630 nm) and the green‒yellow part (560~570 nm) of the spectrum, respectively, by which red or green light vs. blue or white light had different influences. Thus, light wavelengths can influence the ecophysiological features of phototrophic microbes as well as their driving ecological functions. As primary producers, phototrophic microbes facilitate the colonization and growth of heterotrophic organisms by providing small molecular weight organic acids derived from atmospheric carbon fixation and available nitrogen derived from atmospheric nitrogen fixation. In this respect, we hypothesize that changing light wavelength can impact the status of conservation of historical monuments associated with the phototrophic microbiome, namely phototrophic microbes and their subsided heterotrophic microbes.

The cultural heritages in the Two Mausoleums of the Southern Tang Dynasty (937 A.D. to 975 A.D.).

 Here, we conducted a manipulative experiment to evaluate the impact of different light wavelengths on the microbiome of two mausoleums of the Southern Tang Dynasty from China. The Qinling Mausoleum of Emperor Bian Li (built in 943 A.D.) and the Shunling Mausoleum of Emperor Jing Li (961 A.D.) constitute the Two Mausoleums of the Southern Tang Dynasty (937 A.D. to 975 A.D.), which are the two largest imperial mausoleums in the Five Dynasties and Ten Kingdoms period (902 A.D. to 979 A.D.) of China with more than 1,060 years of history. Their relics authentically record various aspects of daily life (e.g., sacrifice, agriculture, entertainment, and costumes) during that period, and they provide extremely precious historical materials for research on Chinese history and culture. 

The in situ monochromatic light manipulative experiment in the Shunling Mausoleum of Emperor Jing Li (961 A.D.). 

After two years of continuous exposure to monochromatic light wavelengths, the structural and functional profiles of the phototrophic bacteriome on the walls of this monument shift, compared to white light. The abundance of Cyanobacteria photosynthetic pigments is observed to decrease in response to green light compared to white light. Consequently, Cyanobacteria populations and their functional genes associated with carbon and nitrogen fixation all decrease. Due to identity of the primary producer of Cyanobacteria, via bottom-up food web, the abundances of heterotrophic species and their held functional gene abundances related to biodeterioration are all found to have decreased. Further, the data of metabonomics and multiple extracellular enzymatic activities indicate that such changes in the ecophysiological features of the phototrophic bacteriome can help deactivate biodeterioration potential.

Mechanistic map of artificial lights impacting the phototrophic microbiome on monuments in indoor environments. 

Together, our work provides new evidence from a manipulative experiment in a millennial mausoleum that changing light wavelengths (i.e., blue, green and red lights) could be cheap and effective method to limit the impact of humans on cultural relics and monuments while allowing them to be visit. This information helps build a blueprint essential for developing approaches and strategies aimed at supporting tourism benefits while preserving indoor cultural monuments exposed to artificial lights worldwide for future generations. The above information has been recently published in npj biofilms and microbiomes (https://www.nature.com/articles/s41522-023-00438-9). 

Youzhi Feng, Nanjing Forestry University

Yonghui Li, Southeast University

Manuel Delgado-Baquerizo, Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS)