Abnormal Crystallization Sequence of Calcium Carbonate in the Presence of Synechococcus sp. PCC 7942

Abstract

Studying the crystallization sequence of calcium carbonate (CaCO3) polymorphs provides great insights into bacterial mineralization mechanisms, and carbonate rock genesis in recent geological history. CaCO3 biomineralization studies were conducted using Synechococcus sp. PCC 7942. Changes in bacterial optical density, polysaccharide content, carbonic anhydrase activity, pH, Ca2+, Mg2+, phosphate, and bicarbonate concentrations in medium, and precipitation levels were recorded, while precipitate morphology and polymorphisms were characterized. We identified an abnormal crystallization sequence order of carbonate polymorphs: calcite, aragonite, and vaterite, which failed to obey Ostwald’s rule. We investigated saturation indices (SI) and calculated calcite, aragonite, and vaterite quantities. Also, two mineralization studies were performed using PCC 7942 in the medium-plus different phosphates levels and extracellular polymeric substances (EPS) secreted by PCC 7942. Calcite was precipitated due to phosphate levels >7 µmol·L−1 and SIvaterite values <2.1. SIaragonite values >2.3 and phosphate levels <7 µmol·L−1 exerted crucial roles in aragonite formation. Finally, decreased phosphate and Mg2+ levels promoted vaterite crystallization. We suggest this unique crystallization order may be attributed to changes in bacterial density and PO4 3− levels. Our study provides new insights into bacterial mineralization mechanisms. HIGHLIGHTS Show that calcite, aragonite, and vaterite crystallized in order in MICP. The abnormal crystallization sequence of CaCO3 polymorphs does not follow Ostward’s theory. The metabolic activity of Synechococcus sp. PCC 7942 and phosphate levels should be responsible for this unique crystallization order.