Abstract

Dissimilatory Fe(III)-reducing bacteria occupy a central position in a variety of environmentally important processes, including the biogeochemical cycling of carbon and iron, the bioremediation of radionuclides and organohalides, and the generation of electricity in microbial fuel cells. Fe(III)-reducing bacteria are scattered and deeply rooted throughout both prokaryotic domains, an indication that microbial Fe(III) reduction may also have been one of the first respiratory processes to have evolved on early Earth. The metal-reducing γ-proteobacterium Shewanella oneidensis is one of the most extensively studied Fe(III)-reducing bacteria. This chapter examines the molecular mechanism by which S. oneidensis transfers electrons to Fe(III) ranging from highly soluble organic-Fe(III) complexes to highly insoluble Fe(III) oxides. S. oneidensis employs four novel respiratory pathways for dissimilatory Fe(III) reduction, including: i) localization of c-type cytochromes to the cell surface where they deliver electrons to external Fe(III) (Mechanism No. 1, Direct contact by outer membrane-localized c-type cytochromes); ii) localization of c-type cytochromes along extracellular nanowires where they deliver electrons to external Fe(III) (Mechanism No. 2, Direct contact by nanowire-localized c-type cytochromes); iii) delivery of electrons to external Fe(III) via endogenous or exogenous electron shuttles (Mechanism No. 3, Extracellular electron shuttling); and iv) non-reductive dissolution of Fe(III) oxides to form more readily reducible soluble organic-Fe(III) complexes (Mechanism No. 4, Fe(III) oxide solubilization followed by reduction of the produced soluble organic-Fe(III) complexes); The chapter highlights the mechanistic details associated with each of the four Fe(III) reduction pathways of S. oneidensis, including a concluding discussion of the future research directions for each pathway.