Mineral Weathering

Mineralogy, microbial ecology, and mineral weathering in the subsurface are an intimately linked biogeochemical system. While bacteria are implicated in the accelerated weathering of minerals, it is not clear, if this is simply the coincidental result of microbial metabolism, or if it represents a specific strategy offering the colonizing bacteria a competitive advantage. We find evidence that silicate weathering is sometimes driven by the nutrient requirements of the microbial consortia, and controlled by the trace nutrient content of the minerals. This occurrence was observed in reducing groundwaters where carbon is abundant but phosphate is scarce, and here even resistant feldspars are rapidly destroyed. This suggests that the progression of mineral weathering may be influenced by a mineral’s nutritional potential, with microorganisms only destroying beneficial minerals. The rock record therefore may contain a remnant mineralogy that reflects early microbial destruction of biologically valuable minerals, leaving a residuum of “useless” minerals, where “value” depends on the organism and diagenetic environment and has little correlation to laboratory rates of feldspar dissolution kinetics. Conversely, the subsurface distribution of microorganisms may in part be controlled by the mineralogy and by the ability of an organism to take advantage of mineral-bound nutrients.

Rocks, such as basalt, can be heavily colonized and weathered by microorganisms after only 3 months of  in situ exposure to native groundwater.  The images represent a time series of attachment by microorganisms, the establishment of possible diffusion boundaries and initial weathering, and finally heavy etching of the mineral surface. Note the precipitation of a secondary phase onto the organism itself. Bar scale is 2 um.