Biopolymers from a Bacterial Extracellular Matrix Affect the Morphology and Structure of Calcium Carbonate Crystals

Biomineralization is a mineral precipitation process occurring in the presence of organic molecules and used by various organisms to serve a structural and/or a functional role. Many biomineralization processes occur in the presence of extracellular matrices that are composed of proteins and polysaccharides. Recently, there is growing evidence that bacterial biofilms induce CaCO₃ mineralization and that this process may be related with their extracellular matrix (ECM). In this study we explore, in vitro, the effect of two bacterial ECM proteins, TasA and TapA and an exopolysaccharide, EPS, on calcium carbonate crystallization. We have found that all the three biopolymers induced the formation of complex CaCO₃ structures. The crystals formed in the presence of the EPS were very diverse in morphology and they were either calcite or vaterite in structure. However, more uniformly sized calcite crystals were formed in the presence of the proteins; these crystals were composed of single crystalline domains that assembled together into spherulites (in the presence of TapA) or dumbbell – like shapes (in the presence of TasA). Our results suggest the EPS affects the nucleation of calcium carbonate when it induces the formation of vaterite crystals and that unlike EPS, the proteins stabilize pre-formed calcite nuclei and induce their aggregation into complex calcite structures.

Biomineralization processes induced by bacterial ECM macromolecules make biofilms more robust and difficult to remove when they form, for example, on pipes and filters in water desalination systems or on ship hulls. Understanding the formation conditions and mechanism of formation of calcium carbonate in the presence of bacterial biopolymers may lead to the design of suitable mineralization inhibitors.