ABSTRACT
Speleothems are secondary mineral deposits normally formed by water supersaturated with calcium carbonate percolating into underground caves. Caves are terrestrial subsurface ecosystems. They are characterized by the absence of phototrophic primary production. Further, these environments have nutrient deficiency. Despite this limitation, cave ecosystems harbor huge microbial diversity. Microorganisms are key players in cave biogeochemical cycles. Speleothems are structures found in cave systems. There are more than 250 different kinds of cave mineral deposits, but the majority of speleothems are composed of calcium carbonate in the form of calcite and aragonite. Calcitic speleothems in caves are produced through diverse abiogenic or biogenic processes or through a combination of both. Caves host a remarkable microbial diversity despite oligotrophy and perpetual darkness. In the cave ecosystem having an allochthonous energy source, microbes break down complex organic matter and serve as a food source for higher-level organisms. Caves having little to zero allochthonous energy harbor chemolithotrophic bacteria. Chemolithoautotrophs may sustain cave ecosystems by driving primary production and the biogeochemical cycle. Several factors such as the presence of mineralized microbes, fabrics, structures such as stromatolite, and geochemical proxies (carbon and oxygen isotopes and lipid biomarkers) indicate the microbial involvement in their formation. This chapter deals with the study of the microbiome of different speleothems such as stalactites, stalagmites, cave wall deposits, and moonmilk.
