Mineral transformations associated with goethite reduction by Methanosarcina barkeri

To investigate the interaction between methanogens and iron-containing minerals in anoxic environments, we conducted batch culture experiments with Methanosarcina barkeri in a phosphate-buffered basal medium (PBBM) to bioreduce structural Fe(III) in goethite with hydrogen as the sole substrate. Fe(II) and methane concentrations were monitored over the course of the bioreduction experiments with wet chemistry and gas chromatography, respectively. Subsequent mineralogical changes were characterized with X-ray diffraction (XRD) and scanning electron microscopy (SEM). In the presence of an electron shuttle anthraquinone-2,6-disulfonate (AQDS), 30% Fe(III) in goethite (weight basis) was reduced to Fe(II). In contrast, only 2% Fe(III) (weight basis) was bioreduced in the absence of AQDS. Most of the bioproduced Fe(II) was incorporated into secondary minerals including dufrénite and vivianite. Our data implied a dufrénite–vivianite transformation mechanism where a metastable dufrénite transformed to a more stable vivianite over extended time in anaerobic conditions. Methanogenesis was greatly inhibited by bioreduction of goethite Fe(III). These results have important implications for the methane flux associated with Fe(III) bioreduction and ferrous iron mineral precipitation in anaerobic soils and sediments.