Innovative strategy for the conservation of a millennial mausoleum from biodeterioration through artificial light management

Artificial lights can cause critical microbial biodeterioration of heritage monuments by promoting the outbreak of phototrophic microbiomes when they are used for touristic viewing. Here, with the ultimate aim of providing innovative solutions for the conservation and visiting of such monuments, we conducted a pioneering two-year in situ manipulative experiment to evaluate the impacts of different artificial light wavelengths (i.e., blue, green and red lights compared to white light) on the phototrophic microbiome of a millennial Chinese imperial mausoleum. Our results show that artificial light can shape the ecophysiological features of the phototrophic bacteriome in this monument and reduce its potential for further biodeterioration. In general, Cyanobacteria dominated (42.0% of the total relative abundance) the phototrophic bacteriome of this cultural relic; however, they were also very sensitive to the choice of artificial light. Compared to white light, monochromatic light, especially green light, reduced Cyanobacteria abundances (18.6%) by decreasing photosynthetic pigment abundances (42.9%); decreased the abundances of heterotrophic species belonging to Proteobacteria (4.5%) and the proportion of genes (6.1%) associated with carbon (i.e., carbon fixation), nitrogen (i.e., denitrification), and sulfur (i.e., dissimilatory sulfate reduction) cycling; and further decreased organic acid (10.1–14.1%) production of the phototrophic bacteriome, which is known to be involved in biodeterioration. Taken together, our findings constitute a major advancement in understanding how light wavelengths influence the phototrophic microbiome in cultural relics, and we found that artificial lights with certain wavelengths (e.g., green light) can help long-term conservation while allowing tourism activities.