Crystallization and Morphogenesis of Calcium Carbonate in the Presence of Bound EPS and Soluble EPS Secreted by Bacteria

Extracellular polymeric substances (EPS) secreted by bacteria play a role in influencing the mineralization and growth of CaCO3 minerals. However, the specific roles of different EPS forms in bacterially influenced carbonate mineralization remain inadequately understood. Therefore, this study conducted CaCO3 mineralization simulation experiments using bound EPS (BEPS) and soluble EPS (SEPS) separately extracted from Synechococcus sp. PCC7942 and Bacillus cereus LV-1 cultures. The precipitates obtained from mineralization experiments with BEPSPCC7942, SEPSPCC7942, BEPSLV-1, and SEPSLV-1, as well as the control group (CK), were predominantly calcite with minor aragonite, suggesting that high SI and pH facilitated the crystallization of these phases. The mole percentage of aragonite in the EPS-treated groups was lower than that in the CK groups. Compared to the CK group, BEPSPCC7942, SEPSPCC7942, and BEPSLV-1 promoted the formation of rhombohedral crystals with blurred edges, and SEPSLV-1 produced rhombohedral minerals with smooth surfaces. The ratios of large to small crystals in the EPS experiments were lower than those of the CK experiments. Negatively charged EPS can attach to specific crystal faces, thereby inhibiting the growth of certain CaCO3 crystal facets, leading to the formation of rhombohedral crystals with blurred edges and providing additional nucleation sites, which results in the formation of smaller crystals.