The Great Wall of China harbors a diverse and protective biocrust microbiome

The Great Wall of China, one of the most emblematic human heritage sites ever built, is largely covered by a living skin that has a potentially distinct microbiome compared with bare wall surfaces. However, the structure and function of this microbiome remain virtually unknown, which hampers any effort to understand the impacts of this microbiome on the long-term conservation of the Great Wall. Here, we investigated the microbiome of the Great Wall at six sampling sites 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 surfaces. We hypothesized that these biocrusts could establish a unique microhabitat and support a microbiome with a community structure and function potentially 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 the 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. Aridity was an additional determinant of the microbiome. Our work serves as a critical step toward understanding the microbiome of the Great Wall, which contributes to conserving this unparalleled human monument for future generations.