Microbial control of mineral-groundwater equilibria: Macroscale to microscale

Using field and laboratory experiments, the geomicrobiology of a petroleum-contaminated aquifer and the relationship between mineral alteration, groundwater chemistry, and microbial colonization were examined. Results indicate that indigenous microorganisms influence mineral weathering at two scales of interaction: macroscale processes that perturb general groundwater chemistry and therefore mineral-water equilibria; and microscale interactions, where attached organisms locally perturb mineral-water equilibria, potentially releasing limiting trace nutrients from the dissolving mineral. In the contaminated unconfined glacio-fluvial aquifer near Bemidji, Minnesota, USA, carbonate chemistry is influenced primarily at the macroscale. Under oxic conditions, respiration by native aerobic heterotrophs produces excess carbon dioxide that promotes calcite and dolomite dissolution. Aerobic microorganisms do not colonize dolomite surfaces and few occur on calcite. Within the anoxic groundwater, calcite overgrowths form on uncolonized calcite cleavage surfaces, possibly due to the consumption of acidity by dissimilatory iron-reducing bacteria. As molecular oxygen concentration increases downgradient of the oil pool, aerobes again dominate and residual hydrocarbons and ferrous iron are oxidized, resulting in macroscale carbonate-mineral dissolution and iron precipitation. Feldspars, in contrast, weather exclusively at the microscale near attached microorganisms, principally in the anoxic region of the plume. Native organisms preferentially colonize feldspars that contain trace phosphorus as apatite inclusions, apparently as a consequence of the low P concentration in the groundwater. These feldspars weather rapidly, whereas nearby feldspars without trace P are uncolonized and unweathered. Feldspar dissolution is accompanied by the precipitation of secondary minerals, sometimes on the bacterial cell wall itself. These observations suggest a tightly linked biogeochemical system whereby microbial processes control mineral diagenesis at many scales of interaction, and the mineralogy and mineral chemistry influence microbial ecology. Only the macroscale interaction, however, is easily observable by standard geochemical methods, and documentation of the microscale interactions requires microscopic examination of microorganisms on mineral surfaces and the locally intense diagenetic reactions that result.