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Published: 2013

Total Pages: 0

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The objective of this study is to systematically investigate the role of anaerobic microorganisms in sedimentary dolomite formation. My results showed that dissolved sulfide, which can be one of the major products of bacterial sulfate reduction, can significantly enhance the Mg incorporation into the calcitic structure, and promote the crystallization of disordered dolomite. In situ atomic force microscopy observations of Ca-Mg carbonate {104} surface growing from supersaturated solutions showed that dissolved sulfide greatly enhanced both the 1-D step nucleation and step growth which had been retarded by Mg2+ ions. These data offer a mechanism by which sulfate-reducing bacteria (SRB) induce dolomite precipitation. The effect of polysaccharides, which can be a major component of extracellular polymeric substances (EPS), was also tested on dolomite precipitation. Both model polysaccharides, carboxymethyl cellulose and agar, were showed to induce disordered dolomite crystallization. The connection between anaerobes and dolomite precipitation was further explored by characterizing the effect of EPS on dolomite crystallization. EPS secreted by a natural consortium of halophilic anaerobes consisted of fermentative bacteria and SRB was extracted and purified for synthesis experiments. EPS was showed to catalyze disordered dolomite precpitation, while dead cells without EPS only induced hydrous Mg-carbonates. Furthermore, inactive biomass from model strains of both fermentative bacteria and SRB was showed to promote dolomite precipitation, which therefore found a new link between fermentative bacteria and dolomite formation. In addition, EPS secreted by a model methanogen which generally inhabit freshwater environments showed similar catalytic effect. Total polysaccharides analyses showed the existence of significant amounts of polysaccharides in EPS. I suggest that these polysaccharides in EPS served as catalysts for dolomite precipitation. I propose that dissolved sulfide and polysaccharides may adsorb on carbonate surfaces through hydrogen bonding between the H in HS-/H2S or the OH group of polysaccharides and the O in CO32- on carbonate surfaces to weaken the rigid Mg2+ hydration shell, resulting in enhanced Mg incorporation and dolomite crystallization. These findings provide insight into the long-standing "dolomite problem" and suggest a fundamental role for microbial processes in the formation of dolomite across a wide range of environmental conditions from freshwater to hypersaline.