Microbial diagnesis in terrestrial aquifer conditions : laboratory and field studies

Abstract

The results of laboratory and field experiments demonstrated that bacteria indigenous to a shallow oil-contaminated aquifer caused minerals to weather rapidly in microreaction zones created in the near vicinity of attached bacteria. Bacteria readily colonized clean mineral surfaces in both laboratory and field microcosms. Nannobodies (0.1 μm diameter spheres) coated all minerals collected from organic rich and bacterially active waters. Nannobodies may be metabolizing bacteria, starved or shrunken bacteria, clots of congealed amorphous organic matter, and/or inorganic calcite crystal nuclei. Feldspars, quartz, and, in some cases, calcite dissolved in the immediate vicinity of attached bacteria, even though the saturation indices for these minerals in the bulk ground water indicated that little to no dissolution should occur. Metabolic production of CO₂ and HCO₃ resulting from acetate methanogenesis and the oxidation of aromatic hydrocarbons coupled with iron reduction provided a supply of reactants to the aquifer-scale bulk ground water for calcite precipitation. Dolomite, with no evidence of direct bacterial involvement, dissolved as expected from results of geochemical modeling and observations of increased dissolved Mg concentrations in the anaerobic zone. Calcite, in contrast, precipitated in a wide variety of spiky morphologies to an absolute uniform elevation above the underlying crystal surface. The uniform ultimate elevation of calcite precipitate suggests a physical control on reactant supply. Bacteria colonizing quartz and microcline were in close proximity to etch pits, exhibiting a spatial correlation between bacteria location and etch pits. The etch pits, however, were not localized at the actual contact surface of the microbe, and the euhedral shape and extent of etching suggest the mineral surface was in contact with an aqueous weathering fluid. Localized mineral etching is proposed to have occurred in a reaction zone at the bacteria/mineral interface where high concentrations of organic acids, formed by bacteria during metabolism of hydrocarbon, selectively mobilized silica from the mineral surface