A laboratory investigation of cyanobacterial extracellular polymeric secretions (EPS) in influencing CaCO3 polymorphism

Bahamian stromatolites are well-laminated structures. consisting of lithified layers alternating between unlithified layers containing fine-grained carbonate ooids. The lithified layers consist of abundant aragonite needles embedded within a matrix of extracellular polymeric secretions (EPS) by cyanobacteria. Schizothrix sp. Laboratory investigations were conducted using EPS extracted from natural stromatolites and laboratory isolates of Schizothrix sp., to chemically characterize EPS. and determine in vitro how EPS may influence CaCO3 polymorphism. EPS mainly consisted of acidic polysaccharides and proteins. Biochemical analyses indicated that contents of uronic acids and carbohydrates in EPS from lithified layers decreased when compared with unlithified layer EPS, while the protein content remained relatively constant. CaCO3 nucleation experiments demonstrated that EPS from the lithified layer. induced aragonite crystal formation in vitro, as confirmed by scanning electron microscopy and Fourier transform infrared (FT-IR) spectroscopy. In contrast, EPS from the unlithified layer or laboratory-cultured Schizothrix sp, induced calcite crystal formation. These laboratory results suggest the possibility that the biochemical composition. specifically small proteins, of EPS influences the resulting mineralogy of CaCO3. (C) 2002 Elsevier Science B.V. All rights reserved.