Little Guardians on the Great Wall: A Spring Festival Column

As the Lunar New Year approaches, many people begin preparing for the annual year-end cleaning. When scrubbing exterior walls or rooftops, you may have encountered a familiar frustration: no matter how thoroughly you clean, stubborn fungi, algae, mosses, or even small plants keep reappearing on the surface. Before feeling discouraged, it is worth remembering that even the longest defensive structure ever built—the Great Wall of China—faces the same problem.

Like all ancient constructions, the Great Wall has endured centuries of environmental pressure since its major construction during the Ming dynasty. From the cold winds and sand of the northern frontier to mosses and plants growing within the seams between bricks, countless forces slowly erode the structure over time. Among these factors, mosses and microorganisms attached to the wall surface have long been regarded as culprits responsible for chemical weathering. Yet is that assumption entirely correct?

To clarify the relationship between microorganisms and the Great Wall, an international team of researchers from China, the United States, and several other countries conducted a multidisciplinary investigation of biological soil crusts on the Wall. Their findings were published in 2026 in the journal Current Biology. The team carried out field surveys along approximately 600 kilometers of the Great Wall. Because the study area spans different climatic conditions, the researchers divided the region into six sampling sites and compared three distinct microhabitats: exposed rammed-earth walls, cyanobacteria-dominated biological crusts, and moss-dominated biological crusts.

Figure 1. Ruins of the Ming Great Wall covered with moss。圖片來源：Arian Zwegers，採用  CC BY 2.0 授權。

During the survey, the researchers recorded biological coverage within each microhabitat and measured a series of physicochemical properties of the wall material. These included pH, salinity, organic matter content, nitrogen concentration, moisture levels, and porosity. To determine the microbial composition inhabiting the wall surface, the team applied two DNA sequencing approaches. The first was 16S rRNA sequencing, a standard technique used to identify bacteria, including cyanobacteria. The second was ITS amplicon sequencing, a DNA barcode method specifically designed for identifying fungi. Together, these tools allowed rapid identification of microbial groups present in the environment.

Because environmental microbial communities are extraordinarily complex, the researchers did not attempt to identify every organism down to the species level. Instead, DNA sequences sharing more than 97% similarity were grouped into the same operational taxonomic unit, or OTU, allowing statistical analysis of community composition. Using alpha-diversity analysis, the researchers assessed how many microbial types were present within each microhabitat. Beta-diversity analysis was then used to compare differences in microbial composition between habitats.

Figure 2. Distribution of the Great Wall across historical dynasties。圖片來源：Maximilian Dörrbecker  ，採用  CC BY 2.0  授權。

However, identifying which microbes were present was only part of the story. The researchers also wanted to understand what these microorganisms were doing. To address this question, they performed metagenomic functional analysis. This method analyzes the entire collection of genetic material within the microbial community, revealing metabolic genes, environmental adaptation mechanisms, and interactions among organisms.

The results were surprising. Even on the harsh surfaces of the Great Wall, microbial communities were both abundant and diverse. More importantly, the moss and algal crusts that had often been suspected of accelerating wall deterioration appeared to have a potentially protective role. Genetic analysis revealed that metabolic pathways associated with nitrogen cycling and organic acid production were relatively inactive within these crust communities. This suggests that they do not significantly accelerate chemical weathering of the wall.

Instead, the researchers detected numerous genes associated with antioxidation, drought tolerance, heat resistance, and sulfur metabolism. These findings indicate that microbial communities within the biological crust may form a natural protective layer. By buffering environmental stresses and resisting chemical damage from sulfide compounds or extreme temperature fluctuations, these microorganisms may help stabilize the rammed-earth structure.

This discovery challenges the long-standing assumption that biological crusts inevitably damage historical structures. It also opens new possibilities for conservation strategies. Rather than simply removing all biological growth from heritage sites, conservation teams may eventually identify which biological crusts have protective potential. With proper scientific guidance, some of these microbial communities could even be cultivated or encouraged, becoming active partners in protecting cultural monuments.

Returning to the familiar scene of year-end cleaning, the next time you notice mosses or microorganisms growing on a wall or roof, it may be worth pausing before removing them immediately. These tiny organisms may not merely be unwanted stains. In some cases, they might quietly help the structure withstand the passage of time and the forces of nature. Perhaps in the near future, these inconspicuous forms of life will be recognized as small but valuable allies in preserving the buildings that humans have created.

Figure 2. Distribution map of the Great Wall across historical dynasties. Image source: Maximilian Doerrbecker, licensed under CC BY 2.0.

Author: Rodrigo

Reference:

Cao, Y., Bowker, M. A., Feng, Y., Delgado-Baquerizo, M., & Xiao, B. (2026). The Great Wall of China harbors a diverse and protective biocrust microbiome. Current Biology, 36(1), 16–27.e4. https://doi.org/10.1016/j.cub.2025.10.087