Mineralogy and Mineral Weathering: Fundamental Components of Subsurface Microbial Ecology

By Philip C. Bennett, Jennifer Roberts Rogers, Franz K. Hiebert, and Wan Joo Choi.

ABSTRACT

This is an investigation of the interplay between mineral chemistry, microbial ecology, and mineral weathering in the oil-contaminated Bemidji aquifer using field and laboratory observations and experiments over a period of 8 years. We observed microbe-mineral interactions at two scales of interaction: a macroscale interaction where the metabolism of both attached and planktonic organisms perturb the bulk groundwater chemistry and therefore, the mineral-water equilibria; and a microscale interaction where attached organisms perturb mineral-water equilibria only in the near vicinity of the organism or biofilm. The two scales reveal a tightly linked system whereby the microbial ecology controls mineral weathering, while the mineralogy and mineral chemistry control microbial colonization.

In this aquifer, carbonate chemistry is influenced primarily at the macroscale, with few native microorgansms observed on carbonate surfaces. In the upgradient oxidizing zone aerobic hydrocarbon oxidation produces excess carbon dioxide, accelerating the dissolution of calcite and dolomite, but with little weathering of silicates. Under the anoxic, leading edge, of the oil pool calcite (but not siderite) precipitates on uncolonized surfaces, while under the trailing edge of the oil, iron reduction dominates over methanogenesis and here siderite precipitates in addition to calcite. As molecular oxygen begins to diffuse into the aquifer, aerobes again dominate the residual hydrocarbons and ferrous iron are oxidized, resulting in macroscale carbonate mineral dissolution and iron precipitation.

Feldspars, in contrast, are weathered exclusively near attached microorganisms, and only at the leading edge of the anoxic sub-oil pool zone. Here native organsims preferentially colonize only those feldspars that contain trace phosphorus as apatite inclusions, apparently as a consequence to the P-limiting environment. These feldspars are rapidly weathered while nearby feldspars without trace P are uncolonized and unweathered. The weathering of feldspars results in the release of dissolved silica which increases to near-equilibrium with amorphous silica. Comparison of the distribution of dissolved silica between 1990 and 1998 shows that the area of most intense feldspar weathering has moved downgradient, parallel with the area of most intense iron reduction.

We hypothesize that the weathering of the P-rich feldspar releases the limiting phosphate, offering the colonizing population a competitive advantage over planktonic organisms or organisms that occupy other mineral surfaces. This suggests that minerals, and the nutrient content of minerals can influence microbial processes, microbial colonization, and contaminant degradation efficiency, while the microbial ecology directly influences the mineral weathering rate and sequence.