ABSTRACT
Microbes are able to respire extracellular substrates, which are essential to the cycling of metals in sediment and subsurface environments, and they also provide a diverse pool of biotechnological tools for bioremediation, biological production of power and electro-fermentation technologies. The process of transferring electrons from the intracellular oxidation of organic molecules, across insulating cellular membranes, into the extracellular space requires dedicated biochemical pathways that have been difficult to characterize. Two main model organisms have emerged for the study of extracellular electron transport: Shewanella oneidensis for its versatile metabolic strategies and its ability to be easily manipulated under laboratory conditions; and Geobacter sulfurreducens for its wide distribution in metal-rich environments as well as biofilm communities attached to anodes across diverse environments. In this chapter, the current state of the characterization of intracellular components of extracellular electron transport pathways of both of these organisms is presented, highlighting their similarities and unique features. The different models of long-range extracellular electron transport proposed for reduction of substrates not in direct contact with G. sulfurreducens and S. oneidensis cells are discussed. Additionally, we have underlined the current challenges in the detailed characterization of extracellular electron transport pathways which, if overcome, will advance the current efforts to apply microbial electrochemical systems to solve real-world problems.
