Microbial chemolithotrophy mediates oxidative weathering of granitic bedrock

The flux of solutes from the chemical weathering of the continental crust supplies a steady supply of essential nutrients necessary for the maintenance of Earth's biosphere. Promotion of weathering by microorganisms is a well-documented phenomenon and is most often attributed to heterotrophic microbial metabolism for the purposes of nutrient acquisition. Here we demonstrate the role of chemolithotrophic ferrous iron [Fe(II)]-oxidizing bacteria in biogeochemical weathering of subsurface Fe(II)-silicate minerals at the Luquillo Critical Zone Observatory in Puerto Rico. Under chemolithotrophic growth conditions, mineral-derived Fe(II) in the Rio Blanco Quartz Diorite served as the primary energy source for microbial growth. An enrichment in homologs to gene clusters involved in extracellular electron transfer was associated with dramatically accelerated rates of mineral oxidation and ATP generation relative to sterile diorite suspensions. Transmission electron microscopy and energy dispersive spectroscopy revealed the accumulation of nanoparticulate Fe-oxyhydroxides on mineral surfaces only under biotic conditions. Microbially-oxidized quartz diorite showed greater susceptibility to proton promoted dissolution, which has important implications for weathering reactions in situ. Collectively our results suggest that chemolithotrophic Fe(II)-oxidizing bacteria are likely contributors in the transformation of rock to regolith.