Unraveling the microbiome of the Great Wall of China

The Great Wall of China, one of the most emblematic human heritage sites ever built, is largely covered by a living skin with a potentially distinct microbiome from bare wall surfaces. However, the structure and function of this microbiome remain virtually unknown, which hampers any effort to understand its role in the long-term conservation of the Great Wall. Here, we investigated, for the first time, the microbiome of the Great Wall along a 600 km section, which stretches across arid and semiarid climates and is covered by a mosaic of biological soil crusts (biocrusts) and exposed wall surface. We hypothesized that these biocrusts could establish a unique microhabitat and support a microbiome with a community structure and functional potential distinct from those on bare walls, thereby modulating the biodeterioration processes affecting the Great Wall. Our findings revealed that biocrust-covered sections exhibited a 12%–62% increase in abundance, diversity, and co-occurrence network complexity for bacterial and fungal communities compared with bare walls. Further metagenomic analyses indicated that biocrust cover enhanced the abundance of overall functional genes and stress resistance pathways within the microbiome by 4%–15%, while decreasing the metabolic pathways linked to heritage biodeterioration like carbon fixation pathways. Essentially, biocrust effects on the microbiome were primarily attributed to biocrust influences on rammed earth properties. Aridity was an additional determinant of the microbiome. Our work serves as a first step towards understanding the microbiome of the Great Wall, which holds groundbreaking significance for conserving this unparalleled human monument for future generations.