Microbially induced calcite precipitation in culture experiments: Possible origin for stalactites in Sahastradhara caves, Dehradun, India

Laboratory experiments involving calcium carbonate precipitation by bacteria isolated from stalactites sampled from three caves in Sahastradhara, Dehradun, India were conducted to determine whether geomicrobiological processes might be involved in stalactite formation. Dominant bacteria inhabiting the Sahastradhara caves, confirmed by PCR amplification of 16S rRNA genes (16S rDNA), were Bacillus thuringiensis and Bacillus pumilis. Culture experiments confirmed their role in mineral precipitation. The ability of these bacteria to form CaCO₃ crystals at different incubation temperatures (5, 15, 25, 32°C) indicated that 25°C was optimum for calcite precipitation. The microbial community detected by DAPI staining showed a significant number of cells (9 × 10⁵ cells, g sed⁻¹). Application of fluorescence in situ hybridization techniques, based on the presence of rRNA, shows a large number of active microbial cells (around 55% of the total cell number). The microbial community is dominated by Eubacteria, mainly sulphate-reducing bacteria (representing 10% of the total microbial community), but Archaea were also present. Thin section petrography reveals that the stalactites consist of microcrystalline calcite, which occurs in chains probably attributed to bacterial precipitation. Thus, microbial activity and optimum temperature appear to be key factors promoting calcite precipitation and ultimately stalactite formation.