2011 Annual Science Report
University of Wisconsin Reporting | SEP 2010 – AUG 2011
Project 5F: Roles of Extracellular Polysaccharides From Sulfate-Reducing Bacteria in Governing Dolomite Composition and Crystallization
Ca-Mg-carbonates ranging from Mg-calcite to Mg-dolomite can be synthesized in presence of extracellular polymeric substances (EPS) excreted by sulfate-reducing bacteria (SRB). The role of EPS in catalyzing dolomite formation is similar to the agar-bearing systems, because polysaccharides are the major components in EPS. Low-temperature dolomite and Ca-Mg-carbonates with continuous compositional variations between calcite and dolomite could be potential biosignatures, because they require catalysts like polysaccharides.
To characterize the role of extracellular polymeric substances (EPS) excreted by SRB in dolomite formation, we enriched and isolated SRB from Deep Springs Lake, California, where modern dolomite is still precipitating. EPS were extracted from the growing cultures of these microbes and further purified for synthesis experiments. Our data evidently demonstrated the catalytic effect of EPS on dolomite formation (Fig. 1). For example, the MgCO3 contents in synthetic carbonates increases with EPS concentrations in experimental solutions. With as low as 100 mg/L EPS, Ca-dolomite with 45.8 mol% MgCO3 can be synthesized (Fig. 1a). What is more interesting is that even no Mg-calcite was produced from EPS-removed dead cells-bearing solutions, while control experiments can still produce high Mg-calcite (Fig. 1b, c). Our data further deciphered the roles of SRB in dolomite formation by defining the catalytic effect of EPS.
We propose that the adsorption of dissolved polysaccharides in EPS on Ca-Mg carbonate surfaces through hydrogen bonding is much stronger than that of water so that they can displace the residue hydration shells on surface Mg2+ ions, and thus promote the Mg incorporation and Ca-Mg carbonate precipitation. Published molecular dynamic calculations demonstrated a stronger surface adsorption of polysaccharides on calcite through hydrogen bonding, which support our hypothesis. Our studies can shed light on the understanding of the “dolomite problem” and the roles of microorganisms in dolomite formation. Low-temperature dolomite and Ca-Mg-carbonates with continuous compositional variations between calcite and dolomite could be potential biosignatures, because they require catalysts like polysaccharides.
PROJECT INVESTIGATORS:Huifang Xu
PROJECT MEMBERS:Eric Roden
RELATED OBJECTIVES:Objective 7.1
Biosignatures to be sought in Solar System materials
Biosignatures to be sought in nearby planetary systems