Magnetite as a precursor for green rust through the hydrogenotrophic activity of the iron-reducing bacteria Shewanella putrefaciens

Magnetite ((FeFe2O4)-Fe-II-O-III) is often considered as a stable end product of the bioreduction of Fe-III minerals (e.g., ferrihydrite, lepidocrocite, hematite) or of the biological oxidation of Fe-II compounds (e.g., siderite), with green rust (GR) as a mixed Fe-II-Fe-III hydroxide intermediate. Until now, the biotic transformation of magnetite to GR has not been evidenced. In this study, we investigated the capability of an iron-reducing bacterium, Shewanella putrefaciens, to reduce magnetite at circumneutral pH in the presence of dihydrogen as sole inorganic electron donor. During incubation, GR and/or siderite ((FeCO3)-C-II) formation occurred as secondary iron minerals, resulting from the precipitation of Fe-II species produced via the bacterial reduction of Fe-III species present in magnetite. Taking into account the exact nature of the secondary iron minerals and the electron donor source is necessary to understand the exergonic character of the biotic transformation of magnetite to GR, which had been considered to date as thermodynamically unfavorable at circumneutral pH. This finding reinforces the hypothesis that GR would be the cornerstone of the microbial transformations of iron-bearing minerals in the anoxic biogeochemical cycle of iron and opens up new possibilities for the interpretation of the evolution of Earth's history and for the understanding of biocorrosion processes in the field of applied science.