ROLE OF EXTRACELLULAR POLYSACCHARIDES IN REGULATING CALCITE NUCLEATION KINETICS AND INSIGHTS TO THE BIOGEOCHEMISTRY OF MICROBIALITES

Microbial carbonate deposits are formed by complex assemblages of proteins and polysaccharides within extracellular polymeric substances (EPS) whose roles in mineralization are not well understood. The acidic or negatively charged functional groups of these organic molecules are often attributed to promoting calcium carbonate precipitation, however, little is known about the mineralization mechanism.

Here we quantify the kinetics of calcite nucleation onto a suite of high purity polysaccharide (PS) substrates under controlled conditions. The energy barriers to nucleation are PS-specific by a systematic relationship to PS charge density and substrate structure that is rooted in minimization of the competing substrate-crystal and substrate-liquid interfacial energies. Chitosan presents a low energy barrier to nucleation because its near-neutral charge favors formation of a substrate-crystal interface, thus reducing substrate interactions with water. Progressively higher barriers are measured for negatively charged alginates and heparin that favor contact with the solution over the formation of new substrate-crystal interfaces.

The results support a directing role for PS in microbialite formation and demonstrate that substrate-crystal interactions are one end-member in a larger continuum of competing forces that regulate calcite precipitation in sedimentary environments. The finding that PS charge and structure significantly control nucleation rates suggests EPS chemistry may contribute to the variety of textures and fabrics seen in microbialites and other lacustrine carbonate deposits.