"Microorganisms have been shown to be important active and passive. promoters of redox reactions that influence the precipitation of various minerals,. including calcite. Many types of secondary minerals thought to be of purely inorganic. origin are currently being reevaluated, and microbial involvement has been demonstrated. in the formation of pool fingers, stalactites and stalagmites, cave pisoliths, and. moonmilk. We studied the possible involvement of bacteria in the formation of a new. type of speleothem from Grave Grubbo Cave, the third-largest gypsum cave in Italy.. The speleothem we studied consisted of a large aggregate of calcite tubes having a. complex morphology, reflecting its possible organic origin. We isolated an abundant. heterotrophic microflora associated with this concretion and identified Bacillus, Burkholderia,. and Pasteurella spp. among the isolates. All of the isolates precipitated CaCO3. in vitro in the form of calcite. Only one of the isolates solubilized carbonate. The relative. abundance of each isolate was found to be directly related to its ability to precipitate. CaCO3 at cave temperature. We suggest that hypogean environments select for microbes. exhibiting calcifying activity. Isotopic analysis produced speleothem d13C values of about. – 5.00%, confirming its organic origin. The lightest carbonates purified from B4M agar. plates were produced by the most abundant isolates. SEM analysis of the speleothem. showed traces of calcified filamentous bacteria interacting with the substrate. Spherical. bioliths predominated among the ones produced in vitro. Within the crystals produced in. vitro, we observed bacterial imprints, sometimes in a preferred orientation, suggesting. the involvement of a quorum-sensing system in the calcium-carbonate precipitation process."
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