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Project Progress

It was proposed that both the initial precipitation of Ca-dolomite and subsequent early diagenesis are strongly mediated by microbial activities. It was suggested that possible microbial activities were involved in the dolomite formation, for example, sulfate-reducing bacteria, methongens, and halophilic aerobic bacteria. However, the roles of microbes in the dolomite crystallization are still unclear. We investigated synthetic dolomites and associated phases precipitated from a media with Halomonas meridian activity at 25 °C using X-ray micro-diffraction and high resolution transmission electron microscopy (HRTEM). The results proved that synthetic dolomites are disordered dolomites instead of ordered dolomite. Synthetic dolomites occur as spherulites (100~500 um) where nano-crystals (~10 nm) aligned with their c-axes along the radius direction. The dolomite spheres have compositions ranging from 45 mole % to 55 mole % of MgCO₃. Other co-existing precipitates are hydromagnesite, dypingite, and struvite. By annealing the disordered dolomite at 300 °C for 4 weeks, the Ca-Mg ordering characterized by weak superlattice reflections resulted from Ca-Mg-ordering appear. The dolomite composition and structure are very similar to abiotically synthesized disordered dolomite in presence of agar gel or dissolved agar at room temperature. Agar is water-soluble, gel-forming polysaccharide extract from agarophyte members of the Rhodophyta. Agar is usually composed of repeating D-galactose and 3,6-anhydro-L-galactopyranose units.

We propose that when dissolved in solution, polysaccharides can be adsorbed on Ca-Mg carbonate surface through hydrogen-bonding, which may lower the dielectric constant of the solution nearby the carbonate surface, enhance the desolvation and dehydration of surface Mg²⁺-water complexes, and catalyze Mg²⁺ incorporation into the precipitating Ca-Mg-carbonate. In natural environments, it is possible that polysaccharides produced by microorganisms, e.g., extracellular polysaccharides, may play a key role in promoting disordered dolomite nucleation and crystallization. This study provides insight into the effect of similar polysaccharides produced by microorganisms, i.e., extracellular polysaccharides, on dolomite formation. Low-temperature Ca-Mg-carbonate solid solutions with chemical composition between Mg-calcite and dolomite could be potential biosignatures, because they require catalysts like polysaccharides.