The Great Wall of China has a “living skin” that has kept it standing for 2,000 years; Understanding | Sciences

The Great Wall of China, one of the UNESCO-listed Seven Wonders of the World, is more than 2,000 years old and extends more than 21,000 kilometers, but it was much longer in the past. What remains today is just a small portion of the barrier that has eroded over time.

What has kept the structure standing until now is the “living skin”, made up of bacteria, algae, lichens and other organisms. This layer of organisms is called the biocrust and has been studied by scientists who published the results on December 8 in the journal Advancement of science.

Research shows that the “living skin” protects parts of the building from wind, rain and other corrosive forces, according to the magazine. Sciences. “Biocrusts improve the mechanical stability and reduce the corrosion susceptibility of the Great Wall,” the researchers wrote in the study.

Scientist Bo Xiao of China Agricultural University and his colleagues collected samples from eight different sections of the biocrust-covered wall and compared their physical properties with those in bare areas of rammed earth. Thus they noticed that the “living skin” parts were less porous and more resistant.

“Compared to bare rammed earth, biocrust-covered sections showed reductions in porosity, water-holding capacity, corrosion and salinity by 2 to 48%, while increasing compressive strength, penetration resistance, shear strength and overall stability by 37 to 321%.” The researchers said.

Scientists estimated that the biocrust covers 67% of the sections studied. The “living skin” is composed of cyanobacteria (microorganisms capable of photosynthesis), mosses and lichens that help strengthen the building, especially in the arid and semi-arid regions of China. The ancient builders knew which materials could make the structure more stable. Xiao said in an email to the site Live Science.

To improve mechanical resistance, the researcher says that the wall’s builders used rammed soil made of clay, sand and other materials such as lime, ingredients that provide fertile soil for the organisms that make up the crustacean biome.

The “live skin” samples analyzed date back to between 1368 BC and 1644 BC, during the Ming Dynasty. The groups were three times stronger than simple rammed earth samples. The study found that those containing algae were particularly potent.

This is because cyanobacteria and other life forms within the biocrust secrete materials, such as polymers, that “bond strongly” to Earth’s packed molecules. This union helps “enhance structural stability,” creating a cementitious material, Xiao explained.

“These cement materials, biological filaments and soil aggregates within the biocrust layer ultimately form a cohesive network with strong mechanical strength and stability against external erosion,” the scientist said.