Role of Microbial Communities in the Weathering and Stalactite Formation in Karst Topography

This study investigated the long-term effect of environmental physical factors on the relative abundance of bacteria and the consequential landscape evolution in karst topography, focusing mainly on the effects of limestone weathering and calcite precipitation. The Narrow-Sky located in the upper part of Takangshan is a small gulch of Pleistocene coralline limestone formation in southern Taiwan. The landscapes were different in the karst walls between the opening and the inner of gulch due to the variation of physical parameters such as sunlight penetration, humidity, and temperature. A metagenomic approach was used out to determine the relationship of microbial community structures on the landscapes in various habitats around the gulch, namely on the inner and outer limestone wall, the water collected from speleothems surface, and the ground soil at the outer wall. The total organic carbon content was measured in solid samples to evaluate the biomass of the habitats. Our results showed that the biomass of habitats in the opening of the gulch was two times higher than the that inside where light penetration was lower. We also found that speleothems only occurred at the inner wall inside the gulch, where the environment exhibited water drips running through the surface of speleothems and less light penetration. The metagenomics in each habitat was surveyed to measure the sequence similarity of operational taxonomic units relative to urease-producing bacteria and weathering-associated bacteria available in the National Center for Biotechnology Information database. Our data revealed that the metagenomics of the inner wall and water samples exhibited more sequences that were similar to those of urease-producing bacteria, whereas the outer wall showed more sequences that were similar to those of weathering-associated bacteria, suggesting that bacteria facilitated the formation of limestone weathering and calcite precipitation for various habitats. This study revealed the pivotal roles of microorganisms in governing the geological evolution of the limestone landscape.