Dependence of Secondary Mineral Formation on Fe(II) Production from Ferrihydrite Reduction by Shewanella oneidensis MR-1

Although dissimilatory iron reduction and secondary mineral formation by Shewanella oneidensis MR-1 have been widely recognized, questions remain about the effects of biogenic Fe(II) on the rate and extent of secondary mineral formation and the importance of biogenic Fe(II)-induced crystallization processes. In this study, we investigated the effects of different mutants of MR-1 on the bioreduction and mineralization of ferrihydrite. The results indicate that while the reduction rates of ferrihydrite by Delta mtrD, Delta mtrF, and Delta omcA are similar to that of the wild type (WT), the capacity to reduce ferrihydrite decreased dramatically in the mutants cymA and Delta mtrA. The order for Fe(III) reduction by MR-1 WT and mutants was ranked as follows: WT approximate to Delta mtrD approximate to Delta mtrF > Delta omcA > Delta mtrC > Delta cymA > Delta mtrA. Secondary minerals of ferrihydrite were characterized using X-ray diffraction, Fourier transform infrared spectra, and scanning electron microscopy. The results show that goethite and hematite were the main secondary minerals formed during the first 2 days in all treatments, and then magnetite appeared in the WT, Delta mtrD, Delta mtrF and Delta omcA treatments, whereas magnetite began to appear from the sixth day onward in the Delta mtrC treatment. However, no magnetite was observed during the 6 days in the Delta mtrA and Delta cymA incubation treatments. The plausible electron transfer pathways of bioreduction and phase transformation were also verified using thermodynamic calculations of elementary reactions. This study clarified the importance of Fe(II) production in secondary mineral formation processes and highlighted the significance of biogenic Fe(II)-catalyzed crystallization. This information may, in turn, help us to better understand natural microbe-mineral interaction processes.