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, groun dwater chemistry, and microbial colonization were examined. Results indicat e that indigenous microorganisms influence mineral weathering at two scales of interaction: macroscale processes that perturb general groundwater chem istry and therefore mineral-water equilibria; and microscale interactions, where attached organisms locally perturb mineral-water equilibria, potentia lly releasing limiting trace nutrients from the dissolving mineral. In the contaminated unconfined glacio-fluvial aquifer near Bemidji, Minneso ta, USA, carbonate chemistry is influenced primarily at the macroscale. Und er oxic conditions, respiration by native aerobic heterotrophs produces exc ess 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 calc ite cleavage surfaces, possibly due to the consumption of acidity by dissim ilatory iron-reducing bacteria. As molecular oxygen concentration increases downgradient of the oil pool, aerobes again dominate and residual hydrocar bons and ferrous iron are oxidized, resulting in macroscale carbonate-miner al dissolution and iron precipitation. Feldspars, in contrast, weather exclusively at the microscale near attached microorganisms, principally in the anoxic region of the plume. Native orga nisms preferentially colonize feldspars that contain trace phosphorus as ap atite 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 a ccompanied by the precipitation of secondary minerals, sometimes on the bac terial cell wall itself. These observations suggest a tightly linked biogeochemical system whereby m icrobial processes control mineral diagenesis at many scales of interaction , and the mineralogy and mineral chemistry influence microbial ecology. Onl y the macroscale interaction, however, is easily observable by standard geo chemical methods, and documentation of the microscale interactions requires microscopic examination of microorganisms on mineral surfaces and the loca lly intense diagenetic reactions that result.