Growth of Geobacter sulfurreducens on poorly crystalline Fe(III) oxyhydroxide coatings

Few studies have examined the molecular to micron-scale interactions between dissimilatory Fe(III)-reducing bacteria and poorly crystalline Fe(III) phases which are frequently the most bioavailable Fe(III) sources in the subsurface. Here we describe methods for analysing these interactions using a range of chemical and spectroscopic techniques. Glass slides were coated with a synthetic poorly crystalline Fe(III) phase and then incubated in the presence of the Fe(III)-reducing bacterium Geobacter sulfurreducens and a suitable growth medium. Growth on the Fe(III)-coating was observed via cell staining and environmental scanning electron microscopy while microbial Fe(III) reduction was quantified using a colorimetric assay. However, following microbial reduction, Fe(II) could not be detected on the slide surface using X-ray photoelectron spectroscopy. Fe(II)-coated control slides showed that the mineral surface was not re-oxidised during handling or analysis. Further experiments intended to demonstrate removal of Tc(VII) and Cr(VI) from solution via abiotic reduction mediated by biogenic Fe(II) on the slide surface resulted in far lower levels of Tc(VII) and Cr(VI) reduction than expected. These data may indicate that the electrons transferred from G. sulfurreducens to poorly crystalline Fe(III) involves the deeper mineral structure, so that Fe(II) phases are not detectable on the surface. The environmental implications of this hypothesis are discussed.