Calcium carbonate precipitation in natural habitats: a possible microbial origin

Calcium carbonate precipitation is commonly considered to be abiogenic, in spite of the fact that microbes are present in CaCO3 substrates (Castanier et al., 1999; Barton et al., 2007). Laboratory experiments involving CaCO3 deposition by bacteria isolated from stalactites, moonmilk, travertines and calcareous soils, confirmed their role in mineral precipitation (Cacchio et al., 2003; Cacchio et al., 2004). Under laboratory conditions, we demonstrated that: 1) cultivable bacterial cells isolated from carbonate substrates are abundant, ranging from 1.0 x 103 – 1.6 x 105 CFU g-1 sample dw, suggesting that their presence is not occasional; 2) a large proportion of these bacterial strains (75-100%), were able to form CaCO3 crystals in vitro, at different temperatures, confirming the data of Boquet et al. (1973). 3) the relative abundance (R.A.) of each calcifying strain with respect to the total cultivable microflora showed that: - in calcareous environments, calcifying isolates are dominant, suggesting that calcification is of evolutionary concern in these habitats; - the most frequent strains produced in vitro the largest amounts of crystals, confirming that the ability to precipitate carbonate may therefore be advantageous and subject to evolutionary selection in karst environments; 4) control experiments without bacteria or inoculated with autoclaved bacterial cells precipitate no carbonate minerals: metabolic activity is necessary for precipitation. 5) the type of CaCO3 bioliths obtained in vitro depends on both growth temperature and bacterial strains; 6) X-ray analyses indicated that calcite was always the predominant carbonate polymorph produced; 7) none or a small percentage of the calcifying bacteria derived from caves or calcareous soils dissolves detectable amounts of carbonate, but 50% of the calcifying bacteria associated with moonmilk solubilise CaCO3, confirming the role of this process in the origin of this particular calcareous formation. 8) SEM observations of purified crystals obtained in vitro showed calcified bacterial cells, bacterial imprints and microbial biofilms. The culture experiments reported here support that Bacillus spp., Kocuria spp., Burkholderia spp., Staphylococcus spp., Acinetobacter spp., Arthrobacter spp. play a major role in carbonate deposition in natural habitats.