Directing Effect of Bacterial Extracellular Polymeric Substances (EPS) on Calcite Organization and EPS–Carbonate Composite Aggregate Formation

Mineralized structures generated under biological control are hierarchical composites that consist of biopolymer matrices and minerals. The biopolymer matrix in biological material is developed as membranes or as a network of fibrils, which has structural as well as functional roles for the composite hard tissue. Microorganisms are surrounded by microbial extracellular polymeric substances (EPS), which consist of self-organized macromolecules and have a fibrous fabric. The EPS enables the survival of microorganisms with their modes of organization in different physicochemical states. To understand the influence of biopolymer matrices on the composite material formation, we conducted growth experiments with the EPS of the bacteria Pseudomonas putida. We synthesized EPS–agarose–calcite aggregates, characterized aggregate morphologies, EPS–hydrogel distribution, and mineral organization. We find that P. putida EPS exerts a tremendous influence on morphology, polymer distribution, and crystal assembly. Aggregates containing P. putida EPS are spherical polycrystalline entities without texture, while aggregates without EPS are branched dendrites. Polymer distribution in aggregates where EPS is intercalated is inhomogeneous. It is mainly present as membranes, which induces the formation of irregular-shaped, different-sized, highly misoriented subunits. P. putida EPS changes the microstructure of the mineral in a specific manner. This can be used as a tool for the identification of bacterially mediated calcification.