Enhancing sandstone surface strength in caves through microbial filaments with calcareous coatings

In a cave, cyanobacteria covered the crown and side walls, causing calcium carbonate to form a protective coating on their surfaces. This coating increased the wall's uniaxial compressive strength by as much as 5.8 times and its surface hardness as much as 1.4 times compared to the uncoated areas. The wall also showed signs of self-repair or strengthening. Observations and humidity monitoring revealed that calcium ions in the sandstone evaporated, leading to carbonate precipitation on the cyanobacteria. DNA analysis identified Leptolyngbya sp., which likely triggered the calcite formation. This process suggests that cyanobacteria can naturally repair and reinforce weathered rock surfaces, preventing collapse even when the outcrop is not submerged. To further support these findings, a simplified experiment simulating the natural calcification process was conducted, showing that calcium carbonate precipitates around filamentous microbial cells as calcium-rich water evaporates from the sandstone surface. These results strongly support the proposed mechanism of surface strengthening.