Calcium carbonate binding by microbial exopolymers

Calcium carbonate (calcite) dissolution and precipitation reactions are important in biological, environmental, and industrial settings, and microorganisms mediate many of these reactions. Bacterially-produced exopolysaccharides (EPS) produced by calcite-inhabiting microorganisms vary in their monosaccharide and linkage compositions and have different binding strengths with the calcite surface depending on the chemical nature of the EPS. The chemical compositions of the EPS, the cation-binding sites therein, and their matching to atomic projections of calcium cations and carbonate anions on different calcite faces also dictate crystallographically-specific EPS-calcite interactions. A model polysaccharide, alginic acid, increases the dissolution rate of calcite six-fold at circumneutral and alkaline pH conditions. The dissolution occurs via preferential attack of the obtuse steps of dissolution pits. EDTA is used as a model ligand for comparison to the alginic acid surface reaction and, unexpectedly, occurs by a completely different mechanism, in which step velocity increases linearly with pit depth. Crystal defects initiate different types of pits on the calcite surface that control pit depth and, hence, a bimodal distribution of pits occurs where slow pits are attributed to clusters of point defects and fast pits are attributed to linear defects. Molecular modeling is used to investigate interactions of EPS with the calcite surface. Alginic acid conformation, configuration, and calcium binding all affect the molecular behavior and torsional stability of constituent disaccharides. All disaccharides have similar global energy minimum conformations under in situ conditions but aqueous cation binding allows less-favorable conformations. Development of a new hybrid molecular modeling force field allows dynamic simulation of a calcite surface with multilayered explicit hydration and inclusion of organic molecules. Hydration of the calcite surface alternately affects calcite calcium cation displacement and carbonate anion inversion for successive mineral monolayers with depth. Water molecules organize on the calcite surface in two coordinations, which affects the water molecule surface stability, and exchange and diffusion rates. Alginic acid disaccharides maintain conformations similar to the global energy minimum conformations previously determined and association with calcite-surface canons again allows alternate conformations. The number and proximity of electron-donating groups on the disaccharides determine variable sorption strengths with the calcite surface.