Microbial processes that bind heavy metals and form minerals are widespread, and they represent a basic aspect of biogeochemistry. Some microorganisms can crystallize minerals by secreting a specific enzyme. In particular, calcite ($CaCO_3$) precipitation is an important part of biomineralization, and has been studied extensively because of its wide application in civil engineering technology. This process provides an effective way to stabilize heavy metals within a relatively stable crystal phase. In this study, biomineralization of calcite by three urea-hydrolyzing indigenous bacterial strains was investigated by microbiological analyses. Three bacterial strains were isolated from the sludge of B mine in Mexico and each bacterial strain was identified by the cellular fatty acid composition and 16S rRNA partial sequencing analysis. The results of the identification analysis showed that these strains were closest to Sporosarcina pasteurii, Kurthia gibsonii, and Paenibacillus polymyxa. We found that the optimum conditions for growth of these indigenous bacteria were $30-40^{\circ}C$ and pH range of 7-8. Microbiological analyses showed the possibility that the bioaccumulated heavy metals ions were deposited around the cell as crystalline carbonate minerals under the optimum conditions. The findings of our study suggest that the indigenous bacterial strains play an important role in heavy metal immobilization.

Keywords: Heavy metals;Calcite;Biomineralization;Indigenous bacterial strains;Heavy metal immobilization;