Calcium carbonate (CaCO3) minerals are known to be deposited in a wide array of different organisms, ranging from microbes to vertebrates [(1989) On Biomineralization, Oxford University Press, New York]. Calcite, aragonite and vaterite are the major crystalline structural polymorphs of CaCO3 associated with living systems, and participate in a variety of biological functions [(1989) Biomineralization: Chemical and Biochemical Perspectives. VCH Publishers, Weinham, Germany; (1991) Advances in Inorganic Chemistry 36, 137–200]. Here we report on the ability of a soil bacterium to synthesize calcite in a calcium-stressed environment. The elaboration of this exocellular crystalline residue enables the organism to regulate its calcium content. The attainment of calcium homeostasis via the exocellular deposition of bacterial calcite with unique crystal habits is a novel biological phenomenon.
