Magnetite biomineralization induced by Shewanella oneidensis

Shewanella oneidensis is a dissimilatory iron reducing bacterium capable of inducing the extracellular precipitation of magnetite. This precipitation requires a combination of passive and active mechanisms. Precipitation occurs as a consequence of active production of Fe2+(aq) when bacteria utilize ferrihydrite as a terminal electron acceptor, and the pH rise probably due to the bacterial metabolism of amino acids. As for passive mechanisms, the localized concentration of Fe2+(aq) and Fe3+(aq) at the net negatively charged cell wall, cell structures and/or cell debris induces a local rise of supersaturation of the system with respect to magnetite, triggering the precipitation of such a phase.These biologically induced magnetites are morphologically identical to those formed inorganically in free-drift experiments (closed system; 25 °C, 1 atm total pressure), both from aqueous solutions containing Fe(ClO4)2, FeCl3, NaHCO3, NaCO3 and NaOH, and also from sterile culture medium added with FeCl2. However, organic material becomes incorporated in substantial amounts into the crystal structure of S. oneidensis-induced magnetites, modifying such a structure compared to that of inorganic magnetites. This structural change and the presence of organic matter are detected by Raman and FT-IR spectroscopic analyses and may be used as a biomarker to recognize the biogenic origin of natural magnetites.