The role of bacterial community in the formation of a stalactite in coral limestone areas of Taiwan by 16S rRNA gene amplicon surveys

This study investigated the 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 ureolytic microbial CaCO₃ precipitation. The narrow-sky located in the upper part of Tangshan is a small gulch of Pleistocene coralline limestone formation in southern Taiwan. A 16S-metagenomic approach was used to determine the relationship of microbial community structures on the landscapes in various habitats. Our results showed that the biomass of habitats in the opening of the gulch was two times higher than the 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 sequence reads of operational taxonomic units relative to urease-producing bacteria and weathering-associated bacteria from each habitat were determined by NCBI database. Our data revealed that the 16S-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. The semi-quantitative PCR for determining bacterial urease gene (ureC) levels confirmed that the inner limestone habitat had higher ureC gene levels than the outer limestone habitat. This study revealed the pivotal role of microorganisms in governing the geological evolution in the lightless limestone landscape.