Microbially mediated bioreduction of iron oxyhydroxide plays an important role in the biogeochemical cycle of iron.Geobacter sulfurreducens is a representative dissimilatory ironreducing bacterium that assembles electrically conductive pili and cytochromes.The impact of supplementation with γ-Fe2O3 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-Fe2O3 NPs,stronger reduction was observed in the presence of direct contact with γ-Fe2O3 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-Fe2O3 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-Fe2O3 NPs.Because of the sorption ofbiogenic Fe(Ⅱ) onto γ-Fe2O3 NPs instead of ferrihydrite,the addition of γ-Fe2O3 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-Fe2O3 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.