ARCHITECTS OF RUST: MINERAL PRECIPITATION AND EVASION STRATEGIES OF FE-OXIDIZING MICROBES

Mineral-forming microbes are the smallest architects on our planet. Fe-oxidizing microorganisms (FeOM) produce beautiful “rusty” iron-oxyhydroxide filamentous structures that accumulate into fluffy mats at groundwater seeps and hydrothermal vents. These tiny builders face challenges of living in highly mineralizing environments, and must adapt to avoid being coated in their own, or abiotically-precipitated minerals. I will discuss two FeOM, the marine Zetaproteobacterium Mariprofundus ferrooxydans and the newly-isolated freshwater Betaproteobacterium Gallionellales strain R-1 (93.6% similar by 16S rDNA to Gallionella ferruginea). Remarkably, these two distantly-related organisms appear to have similar blueprints for survival. Both can grow in low oxygen environments, where abiotic Fe oxidation is slow. We measured Fe(II) and O₂ gradients, growth and mineralization by a combination of voltammetric electrodes and light microscopy of microslide cultures. Both deposit their oxidized minerals in a rapidly-formed fibrillar, twisted ribbon-shaped stalk, as viewed in light microscopy and scanning and transmission electron microscopy (SEM and TEM). Previous cryo-TEM and X-ray spectromicroscopy showed that Mariprofundus cells avoid becoming coated in Fe; Our SEM-EDX and TEM-EELS analyses confirm this is also true of R-1. Both FeOM appear to avoid mineral attachment by adapting cell surface charge and hydrophobicity. Zeta potential measurements of both live as well as azide-inhibited cells revealed that the cells have a consistently small, near-neutral surface charge (< -4 mV) over a pH range of 4-10. Microbial adhesion to hydrocarbons (MATH) assays, water contact angle measurements, and thermodynamic and extended-DLVO calculations show that the cell surfaces are hydrophilic, and hydrophilic repulsion prevents microbe-mineral attachment. These results suggest that mineralizing microorganisms can design their cell surfaces, local chemical environments, and precipitation strategies for an efficient mineral-producing lifestyle.