NUCLEATION AND PRECIPITATION OF LOW-TEMPERATURE DOLOMITE BY METHANOGENS

Despite being abundant in the geological record the nucleation and precipitation of dolomite at low temperature (< 50 ⁰C) under laboratory conditions has been a notoriously difficult endeavor. Recent studies have shown that several physiologic types of microorganisms are capable of overcoming kinetic barriers and driving the formation of low temperature dolomite. This study investigates the mechanisms employed by methanogenic microbes in mediating the nucleation of dolomite, using batch reactors containing a native methanogenic consortium and Archaeal cell wall sorption experiments with Ca²⁺/Mg²⁺ and two pure cultures. Microbial biomass increases from 1 x 10⁵ to 7x 10⁷ cells ml^-1 and the methanogenic population within experimental vessels yields approximately 46 fmoles of CH₄ per cell per day. Indicators of microbial metabolic activity demonstrate a shift from iron-reducing conditions to methane production within 90 days, indicative of population succession. Coinciding with the observed shift in metabolic activity, we precipitate ferroan dolomite from solution after 90 days of incubation. Once Fe(II) is removed from solution (after 166 days) ordered stoichiometric dolomite is precipitated as methanogenesis continues. Characterizations of mineral surfaces using scanning electron microscopy show small rhombohedra in close association with microbial cells and exopolysaccaride. Sorption experiments have demonstrated preferential adsorption of Mg²⁺ over Ca²⁺ to the cell wall of Methanobacterium formicicum, an autotrophic methanogen; such that Mg²⁺/Ca²⁺ ratios associated with the cell wall differ significantly with those originally in solution. These data support the microbial model for low temperature dolomite nucleation and formation and highlight the role of physiologic type in determining dolomite composition and stoichiometry. More complete understanding of dolomite formation in methanogenic systems should serve as models for predicting dolomite distribution in the subsurface.