Reduction of iron oxide nanoparticles by Geobacter sulfurreducens PCA involves outer membrane proteins and secreted redox-active substances.

Fe reducing bacteria (FRB), through extracellular electron transfer (EET) pathway, can reduce Fe(III) nanoparticles, thereby affecting the migration, transformation, and degradation of pollutants. However, the interaction of Fe(III) nanoparticles with the most commonly identified FRB, Geobacter sulfurreducens PCA, remains poorly understood. Herein, we demonstrated that the synergistic role of outer membrane proteins and periplasmic proteins in the EET process for α-Fe2O3, Fe3O4, and β-FeOOH nanoparticles by construction of multiple gene knockout strain. oxpG (involved in the type II secretion system) and omcST (outer membrane c-type cytochrome) mediated pathways accounted for approximately 67 % of the total reduction of α-Fe2O3 nanoparticles. The residual reduction of α-Fe2O3 nanoparticles in ∆oxpG-omcST strain was likely caused by redox-active substances in cell supernatant. Conversely, the reduction of dissolved Fe(III) was almost unaffected in ∆oxpG-omcST strain at the same concentration. However, at high dissolved Fe(III) concentration, the reduction significantly decreased due to the formation of Fe(III) nanoparticles, suggesting that this EET process is specific to Fe(III) nanoparticles. Overall, our study provided a more comprehensive understanding for the EET pathways between G. sulfurreducens PCA and different Fe(III) species, enriching our knowledge on the role of microorganisms in iron biogeochemical cycles and remediation strategies of pollutants.