Much is known about the bacterial precipitation of carbonate rocks, but comparatively little is known about the involvement of microbes in the formation of secondary mineral structures in caves. We hypothesized that bacteria isolated from calcareous stalactites, which are able to mediate CaCO3precipitation in vitro, play a role in the formation of carbonate speleothems. We collected numerous cultivable calcifying bacteria from calcareous speleothems from Cervo cave, implying that their presence was not occasional. The relative abundance of calcifying bacteria among total cultivable microflora was found to be related to the calcifying activity in the stalactites. We also determined the13C and18O values of the Cervo cave speleothems from which bacteria were isolated and of the carbonates obtained in vitro to determine whether bacteria were indeed involved in the formation of secondary mineral structures. We identified three groups of biological carbonates produced in vitro at 11°C on the basis of their carbon isotopic composition: carbonates with13C values (a) slightly more positive, (b) more negative, and (c) much more negative than those of the stalactite carbonates. The carbonates belonging to the first group, characterized by the most similar13C values to stalactites, were produced by the most abundant strains. Most of calcifying isolates belonged to the genusKocuria. Scanning electron microscopy showed that dominant morphologies of the bioliths were sherulithic with fibrous radiated interiors. We suggest a mechanism of carbonate crystal formation bybacteria.
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Document Type: Research Article
Department of Basic and Applied Biology, Microbiology Laboratory, University of L'Aquila, Coppito, L'Aquila, Italy
INFN-LNF-Dafne Light Laboratory, CNR-Institute of Nanomaterial Study, Rome, Italy
Department of Earth Science, La Sapienza University and IGAG-CNR, Rome, Italy
December 1, 2004
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