RESEARCH ARTICLES CURRENT SCIENCE, VOL. 90, NO. 1, 10 JANUARY 2006 58 *For correspondence. (e-mail: rbaskargjuhisar@yahoo.com) Microbially induced calcite precipitation in culture experiments: Possible origin for stalactites in Sahastradhara caves, Dehradun, India Sushmitha Baskar 1 , R. Baskar 1, *, L. Mauclaire 2 and J. A. McKenzie 2 1 Department of Environmental Science and Engineering, Guru Jambheshwar University, Hisar 125 001, India 2 Laboratory of Geomicrobiology, Geological Institute, ETH-Zentrum, 8092 Zurich, Switzerland Laboratory experiments involving calcium carbonate precipitation by bacteria isolated from stalactites sampled from three caves in Sahastradhara, Dehradun, India were conducted to determine whether geomicrobiolo- gical processes might be involved in stalactite forma- tion. Dominant bacteria inhabiting the Sahastradhara caves, confirmed by PCR amplification of 16S rRNA genes (16S rDNA), were Bacillus thuringiensis and Ba- cillus pumilis. Culture experiments confirmed their role in mineral precipitation. The ability of these bacteria to form CaCO 3 crystals at different incubation tempe- ratures (5, 15, 25, 32°C) indicated that 25°C was optimum for calcite precipitation. The microbial community de- tected by DAPI staining showed a significant number of cells (9 × 10 5 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 sul- phate-reducing bacteria (representing 10% of the to- tal microbial community), but Archaea were also present. Thin section petrography reveals that the sta- lactites consist of microcrystalline calcite, which occurs in chains probably attributed to bacterial precipita- tion. Thus, microbial activity and optimum temperature appear to be key factors promoting calcite precipita- tion and ultimately stalactite formation. Keywords: Bacteria, biomineralization, calcite, caves, geomicrobiology. MICROORGANISMS have been agents for geochemical change for over 85% of the earth’s history, and linkages between the geochemical and biological evolution of the earth are profound. It is widely accepted that microorganisms were largely responsible for production of oxygen in the earth’s atmosphere and that through their metabolism they can dramatically alter elemental distributions. Interactions be- tween the biosphere and geosphere are complex because organisms are able to transform the chemistry of their en- vironment. Calcium carbonate precipitation is a general phenomenon in the bacterial world under appropriate conditions 1 . Indeed, some bacteria and fungi can induce precipitation of calcium carbonate extracellularly through a number of processes that include photosynthesis, ammonifi- cation, denitrification, sulphate reduction and anaerobic sulphide oxidation 2–4 . Additionally, the activity of sulphate- reducing bacteria has been shown to mediate precipitation of dolomite 5,6 . The primary role of bacteria in the precipi- tation process has been ascribed to their ability to create an alkaline environment through various physiological activities 3,7 . In addition to field observations, calcium carbonate has been formed in the laboratory in association with different bacterial cultures, such as marine bacteria 8 , soil bacteria 9 , Pseudomonas flurescens, Myxococcus xan- thus 10 , and various other autotrophic and heterotrophic bacteria 3 . Caves host diverse microbial populations and are sites of active mineral precipitation. Chemical processes mediated by microbial activity are fundamentally related to the dis- tribution of microbes throughout the cave system. Min- eral precipitation is, however, commonly considered to be abiogenic despite the fact that microbes are present in caves. Analysis of cave substrates from a geological per- spective shows that microbes can mediate constructive and destructive processes. Microorganisms have been shown to be important active and passive promoters of redox re- actions influencing geological processes. Potentially these processes can significantly influence the formation and preservation of any cave deposit. Although there is extensive documentation of microbial precipitation of calcium carbonate in the non-cave carbonate literature 11 , studies of microorganisms in caves have been predominantly de- scriptive, with only a few experimental studies reported. The past decade has produced extensive research into micro- bial interactions with minerals within cave environments. Fungi, algae and bacteria are implicated in the precipitation of carbonate dripstone in caves 12,13 . A large variety of het- erotrophic microbial communities in stalactites are well documented in cave ecosystems 14 . Monitoring modern sites of active precipitation can provide valuable insights