Bacteria-Mediated Synthesis of Metal Carbonate Minerals with Unusual Morphologies and Structures

In this paper, well-defined vaterite hollow spheres and amorphous barium carbonate microrods are synthesized in Proteus mirabilis/urea solution. The urease-generated bacterium Proteus mirabilis is able to convert urea to ammonia and CO2, thereby leading to the precipitation of metal carbonate in Proteus mirabilislurea solution containing Ca2+ or Ba2+ ions. It is found that the vaterite hollow spheres are so-called mesocrystals because they have identified primary particles and single-crystalline nature. Crystallization of CaCO3 using Proteus mirabilis and other two bacteria Bacillus subtilis and Aerobacter aerogenes in dilute ammonia aqueous solutions (pH 8.5) is also investigated, suggesting that the products are all CaCO3 mesocrystals. Therefore, we speculate that bacteria promoting formation of CaCO3 mesocrystals. may be a common phenomenon. In addition, marked morphological changes and structural transition in the CaCO3 particles from amorphous calcium carbonate irregular aggregates to vaterite hollow spheres to a mixture of calcite and vaterite hollow discs and polyhedrons in Proteus mirabilislurea solution are observed depending on the reaction time. BaCO3 particles change from oval to rod in morphology within 7 days of reaction, but the structure of them is Still Morphous even after a month. The biomolecules mainly proteins secreted by the bacteria are probably responsible for the morphologies and structures of metal carbonate minerals by first stabilizing their nanosized precursors, which then transform into mesocrystals or amorphous aggregates via oriented or nonoriented aggregation of nanoparticles. This provides a novel and facile way for the study of biomineralization mechanisms and crystal. growth modification.