Rethinking sediment biogeochemistry after the discovery of electric currents

The discovery of electric currents in marine sediments arose from a simple observation that conventional biogeochemistry could not explain: Sulfide oxidation in one place is closely coupled to oxygen reduction in another place, centimeters away. After experiments demonstrated that this resulted from electric coupling, the conductors were found to be long, multicellular, filamentous bacteria, now known as cable bacteria. The spatial separation of oxidation and reduction processes by these bacteria represents a shortcut in the conventional cascade of redox processes and may drive most of the oxygen consumption. In addition, it implies a separation of strong proton generators and consumers and the formation of measurable electric fields, which have several effects on mineral development and ion migration. This article reviews the work on electric currents and cable bacteria published through April 2014, with an emphasis on general trends, thought-provoking consequences, and new questions to address.