Extracellular Polymeric Substances as Catalysts for Dolomite Crystallization

The "Dolomite problem" has been a subject of scientific debate for decades. It has been proposed that the involvement of microorganisms, especially anaerobic microorganisms, is necessary to overcome the kinetic barriers to dolomite crystallization. However, an unequivocal explanation of this catalytic effect of microorganisms has yet to be defined. Here we show that extracellular polymeric substances (EPS) can catalyze dolomite precipitation. In contrast to previous dolomite syntheses in live microbial cultures, we extracted EPS from cultures of a natural anaerobic microbial consortium isolated from sediments of a dolomite-precipitating lake, and conducted dolomite synthesis in vitro in EPS-bearing solutions. Our data showed that with as low as 0.1 g/L EPS, disordered dolomite precipitated from solutions containing similar concentrations of Ca2+ and Mg2+ as that of modern sea water, whereas aragonite and hydrous Mg-carbonates precipitated from solutions containing the dead cell residues without EPS. High throughput sequencing analyses indicated that the anaerobic consortium was dominated by fermenters. To our best knowledge, this is the first report of the catalytic effect of fermenters on dolomite crystallization. Based on previous studies on dolomite synthesis in polysaccharides-bearing solutions (Zhang et al., 2012), we propose that polysaccharides in EPS may contribute significantly to dolomite precipitation. We suggest that polysaccharides may be strongly adsorbed on the growing Ca-Mg carbonate surfaces to lower the energy barrier to the dehydration of surface Mg2+-water complexes, and therefore to enhance dolomite crystallization. In natural environments, polysaccharides can also be produced by organic matter decay in addition to microbial excretion. All these polysaccharides may be key factors in sedimentary dolomite formation. This study sheds new light on understanding the role of anaerobic microorganisms in dolomite formation and the formation mechanism of sedimentary dolomite. Reference Zhang, F., Xu, H., Konishi, H., Shelobolina, E.S., and Roden, E.E. (2012) Polysaccharide-catalyzed nucleation and growth of disordered dolomite: A potential precursor of sedimentary dolomite. Am. Mineral., 97(4), 556-567.