MICROBIAL IRON(II) OXIDATION: EVIDENCE OF IRON BIOMINERALIZATION IN THE ROCK RECORD

Iron is a nutritional requirement for life. However for many microorganisms iron is not only a nutritional necessity, but also serves as a source of energy or as a terminal electron acceptor in respiratory microbial metabolisms. Microbially-catalyzed iron redox reactions between the Fe(III) and Fe(II) valence states not only plays a fundamental role influencing modern environmental biogeochemistry but also played a role in Earth history. Our recent research demonstrates that microorganisms played a role in the formation of Fe(III)-rich oxide geological formations. Concretions are cemented masses within sediments and sedimentary rocks that record sediment diagenesis and pore water chemistry. Abundant Fe(II)-carbonates were precipitated within the Navajo Sandstone downflow from massive CO₂ reservoirs. The Fe(II)-carbonate concretions were altered as oxidizing water was transported through the paleoaquifer. However, mechanism(s) governing Fe(III) oxide precipitation within these concretions has been poorly understood. Chemical and morphological evidence of microbial biosignatures in association with Fe(III) oxides in the Fe(III) oxide-rich exterior of the spheroidal concretions implicated a microbial role in Fe biomineralization. The amount of total organic carbon in the exterior Fe(III) oxides exceeded measured values in the interior. Mean δ¹³C value of organic carbon within the Fe(III) oxide-cemented exterior, δ¹³C of -20.55‰, is consistent with a biogenic signature from autotrophic Fe(II) oxidizing bacteria. Scanning electron micrographs reveal microstructures consistent with bacterial size and morphology including a twisted-stalk morphotype that resembled an Fe(II)-oxidizing microorganism, Gallionella sp. Nanoscale associations of Fe, O, C and N with bacterial morphotypes further implicate a microbial role in Fe biomineralization. Together these results indicate that autotrophic microorganisms were present during Fe(III) oxide precipitation and microbial catalysis as a mechanism of Fe(III) oxide precipitation. The detection of microbial biosignatures in rinded Fe(III) oxide-rich concretions within an exhumed, Quaternary paleoaquifer has broad implications for detection of life within the geological record on Earth as well as other Fe-rich rocky planets such as Mars.