Biocrusts: Guardians of the Great Wall of China

China's Great Wall, with its vast rammed-earth structures, faces severe weathering. Biocrusts—complex communities of microorganisms—could offer a low-cost, eco-friendly protective layer. Beyond reviewing existing knowledge, this article proposes a core hypothesis: biocrusts could offer a low-cost, eco-compatible protection paradigm, but their effectiveness mainly depends on a critical “beneficial/harmful” threshold set by climate, wall materials, and microbial succession stage. We detail how biocrusts stabilized the wall by reducing wind/water erosion, buffering temperature shifts, and reinforcing soil by emphasizing their dual protective/deteriorative nature and highlight the key knowledge gaps. To enable this paradigm shift, we propose research to clarify the thresholds and underlying mechanisms of biocrust functions across their successional stages and climatic gradients. Future multidisciplinary collaboration among microbiologists, materials scientists, and conservators through nationwide surveys, experiments, and monitoring should address a core question: how to cultivate beneficial biocrusts while minimizing risks? Such hypothesis-driven research could transform this “natural guardian” into a practical framework for sustainable protection for the Great Wall and other earthen heritage sites.

The Great Wall’s dilemma and biocrusts’ potential

The Great Wall of China, a United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage site, stands as a monumental testament to human ingenuity. Today, centuries of exposure have made weathering an existential threat. This vulnerability stems from the rammed-earth construction technique, which renders the structure highly susceptible to weathering. Common conservation methods—including physical reinforcement (e.g., earthen buttresses), chemical treatments (e.g., high-modulus potassium silicate-based agents), and protective shelters—are costly, labor intensive, and often ineffective at scale, leaving many sections unprotected. This has driven the search for sustainable, nature-based alternatives.

Interestingly, the wall itself may host a natural protective layer: biocrusts. These complex microbial communities—mainly consisting of cyanobacteria, lichens, and mosses—cover approximately 67% of the studied sections across a 600-km surveyed area.1,2 Rather than being detrimental, emerging research suggests these biocrusts act as a “living skin,” shielding the wall from degradation through centuries of ecological succession.