Smith Lecture: Frontiers in Molecular Biogeochemistry: Electron Transfer Dynamics in the Redox Cycling of Iron
Life on earth has evolved utilizing the unique redox chemistry of iron. It is an essential element for virtually all life forms due to its presence in heme-containing proteins, with biological functions including oxygen transportation, chemical catalysis, and electron transfer. Electron exchange between ferrous and ferric iron determines the availability of iron in the biosphere by influencing the form of key iron-bearing minerals and their transformation pathways. Under environmentally relevant conditions this exchange involves interaction between aqueous ferrous iron and solid-phase ferrous iron oxides and oxyhydroxides, with complex involvement of solid-state charge migration. Examples include Fe(II)-catalyzed recrystallization of hematite and goethite, and mixed-valent spinels such as magnetite acting as a mineralogic source and sink for reactive Fe(II) due to its topotactic solid-solution property. Ferrous-ferric electron exchange is also essential for microbial respiration via the evolution-optimized molecular machinery present in metal-respiring bacteria. This machinery transmits current across their cell membranes using redox metalloprotein modules comprised of distinct multiheme c-type cytochromes. This presentation explores the dynamics of ferrous-ferric electron exchange in such systems at the atomic and nanoscale levels from theory, computational molecular simulation, spectroscopy, and microscopy. It identifies impacts on reaction rates and mechanisms in the biogeochemical cycle of iron. More generally the topic also illustrates the importance of molecular science for making fundamental advances in understanding the environmental biogeochemistry of the earth’s near-surface.