Bacterial extracellular polymeric substances (EPS) mediate CaCO3 morphology and polymorphism

The effect of bacterial extracellular polymers (EPS) on calcium carbonate morphology and polymorphism was investigated by means of free-drift mineralisation experiments, conducted using cells of the EPS-producing thermotolerant Bacillus licheniformis S-86. The cells either had the EPS layer intact (native cells) or had the EPS layer removed (EPS-free cells). Experiments were also undertaken in the absence of cells, but with the addition of the extracted EPS solution. It was found that the abiotic control and EPS-free cell experiments contained vaterite and calcite when sampled after 12 h and 48 h, whereas the native cell and EPS-solution experiments contained only calcite. These results suggest that the presence of EPS inhibits vaterite precipitation. It is proposed that dissolved organic carbon (DOC) released from the EPS complexes Ca2+ ions in solution, reducing the calcium carbonate saturation and favouring calcite precipitation over vaterite. After 7 days, all experiments contained only calcite. However, distinct morphological differences between the calcite crystals precipitated in the four experiments were preserved after a period of 7 days, suggesting that crystal morphology may indicate the biological conditions under which calcite precipitated.