Maghemite (γ-FeO) nanoparticles enhance dissimilatory ferrihydrite reduction by Geobacter sulfurreducens: Impacts on iron mineralogical change and bacterial interactions.

Microbially mediated bioreduction of iron oxyhydroxide plays an important role in the biogeochemical cycle of iron. Geobacter sulfurreducens is a representative dissimilatory iron-reducing bacterium that assembles electrically conductive pili and cytochromes. The impact of supplementation with γ-FeO nanoparticles (NPs) (0.2 and 0.6 g) on the G. sulfurreducens-mediated reduction of ferrihydrite was investigated. In the overall performance of microbial ferrihydrite reduction mediated by γ-FeO NPs, stronger reduction was observed in the presence of direct contact with γ-FeO NPs than with indirect contact. Compared to the production of Fe(II) derived from biotic modification with ferrihydrite alone, increases greater than 1.6- and 1.4-fold in the production of Fe(II) were detected in the biotic modifications in which direct contact with 0.2 g and 0.6 g γ-FeO NPs, respectively, occurred. X-ray diffraction analysis indicated that magnetite was a unique representative iron mineral in ferrihydrite when active G. sulfurreducens cells were in direct contact with γ-FeO NPs. Because of the sorption of biogenic Fe(II) onto γ-FeO NPs instead of ferrihydrite, the addition of γ-FeO NPs could also contribute to increased duration of ferrihydrite reduction by preventing ferrihydrite surface passivation. Additionally, electron microscopy analysis confirmed that the direct addition of γ-FeO NPs stimulated the electrically conductive pili and cytochromes to stretch, facilitating long-range electron transfer between the cells and ferrihydrite. The obtained findings provide a more comprehensive understanding of the effects of iron oxide NPs on soil biogeochemistry.