Iron Cycle Tuned by Outer-Membrane Cytochromes of Dissimilatory Metal-Reducing Bacteria: Interfacial Dynamics and Mechanisms In Vitro

Abstract

The biogeochemical cycle of iron is of great importance to living organisms on Earth, and dissimilatory metal-reducing bacteria (DMRB) with the capability of reducing hematite (α-Fe₂O₃) by outer-membrane (OM) cytochromes play a great role in the iron cycle. However, the dynamic binding of cytochromes to α-Fe₂O₃ at the molecular level and the resulting impact on the photon-to-electron conversion of α-Fe₂O₃ for the iron cycle are not fully understood. To address these issues, two-dimensional IR correlation analysis coupled with molecular dynamics (MD) simulations was conducted for an OmcA-Fe₂O₃ system as OmcA bonds stronger with hematite in a typical DMRB,<i>Shewanella</i>. The photoelectric response of α-Fe₂O₃ with the OmcA coating was evaluated at three different potentials. Specifically, the binding groups from OmcA to α-Fe₂O₃ were in the sequence of carboxyl groups, amide II, and amide I. Further MD analysis reveals that both electrostatic interactions and hydrogen bonds played essential roles in the binding process, leading to the structural changes of OmcA to facilitate iron reduction. Moreover, the OmcA coating could store the photogenerated electrons from α-Fe₂O₃ like a capacitor and utilize the stored electrons for α-Fe₂O₃ reduction in dark and anoxic environments, further driving the biogeochemical cycle of iron. These investigations give the dynamic information on the OM protein/hematite interaction and provide fundamental insights into the biogeochemical cycle of iron by taking the photon-induced redox chemistry of iron oxide into consideration.