Exopolymeric substances of sulfate-reducing bacteria: Interactions with calcium at alkaline pH and implication for formation of carbonate minerals

Sulfate-reducing bacteria (SRB) have been recognized as key players in the precipitation of calcium carbonate in lithifying microbial communities. These bacteria increase the alkalinity by reducing sulfate ions, and consuming organic acids. SRB also produce copious amounts of exopolymeric substances (EPS). All of these processes influence the morphology and mineralogy of the carbonate minerals. Interactions of EPS with metals, calcium in particular, are believed to be the main processes through which the extracellular matrix controls the precipitation of the carbonate minerals. SRB exopolymers were purified from lithifying mat and type cultures, and their potential role in CaCO3 precipitation was determined from acid-base titrations and calcium-binding experiments. Major EPS characteristics were established using infrared spectroscopy and gas chromatography to characterize the chemical functional groups and the sugar monomers composition. Our results demonstrate that all of the three SRB strains tested were able to produce large amounts of EPS. This EPS exhibited three main buffering capacities, which correspond to carboxylic acids (pK(a) = 3.0), sulfur-containing groups (thiols, sulfonic and sulfinic acids - pK(a) = 7.0-7.1) and amino groups (pK(a) = 8.4-9.2). The calcium-binding capacity of these exopolymers in solution at pH 9.0 ranged from 0.12g(Ca) g(EPS)(-1) - 0.15 g(Ca) g(EPS)(-1). These results suggest that SRB could play a critical role in the formation of CaCO3 in lithifying microbial mats. The unusually high sulfur content, which has not been reported for EPS before, indicates a possible strong interaction with iron. In addition to changing the saturation index through metabolic activity, our results imply that SRB affect the rock record through EPS production and its effect on the CaCO3 precipitation. Furthermore, EPS produced by SRB may account for the incorporation of metals (e.g. Sr, Fe, Mg) associated with carbonate minerals in the rock record.