China's Great Wall Has Its Own Wall of Protection

The Great Wall of China continues to stand millennia after its first sections were built around 221 BC, but not without help from bacteria, moss, and lichens. Biocrusts, communities of living organisms that develop on the soil surface and prevent erosion in arid ecosystems, cover about 12% of Earth's land surface and, it turns out, various rammed earth sections of the Great Wall, per Science. Though some of the most famous sections of wall are built from stone or brick, others are built from compacted soil, making them particularly vulnerable to the elements. Researchers noticed that biocrusts formed on sections of the Great Wall and set to work to determine whether they had any benefit.

They found moss-dominated biocrusts covered more than 67% of the sections they studied. In comparing those sections to rammed earth sections without biocrusts, they found the former were less porous with higher shear strength and compressive strength. "Compared with bare rammed earth, the biocrust-covered sections exhibited reduced porosity, water-holding capacity, erodibility, and salinity by 2 to 48%, while increasing compressive strength, penetration resistance, shear strength, and aggregate stability by 37 to 321%," reads the study published Friday in Science Advances. The researchers believe biocrusts give greater stability to the wall through the "secretion of tightly bound polymers," per Phys.org.

They also reduce erosion from wind and water—which contrasts with the prevailing view in heritage conservation that plant growth is damaging. That idea stems from penetrative root systems, but biocrusts have none of those, says study co-author Matthew Bowker, an ecologist at Northern Arizona University, per Science. Researchers note biocrusts are likely to suffer from a warming climate; at least thicker, moss-dominated biocrusts are likely to yield to thinner, less protective cyanobacterial crusts. However, scientists are investigating ways in which bacteria could be grown to aid preservation of the Great Wall. The study notes "only 5.8% of its total length remains well preserved, while 52.4% has either vanished or become severely deteriorated." (More Great Wall of China stories.)