An Inner Membrane Cytochrome Required Only for Reduction of High Redox Potential Extracellular Electron Acceptors

<jats:title>ABSTRACT</jats:title> <jats:p> Dissimilatory metal-reducing bacteria, such as <jats:named-content content-type="genus-species">Geobacter sulfurreducens</jats:named-content> , transfer electrons beyond their outer membranes to Fe(III) and Mn(IV) oxides, heavy metals, and electrodes in electrochemical devices. In the environment, metal acceptors exist in multiple chelated and insoluble forms that span a range of redox potentials and offer different amounts of available energy. Despite this, metal-reducing bacteria have not been shown to alter their electron transfer strategies to take advantage of these energy differences. Disruption of <jats:italic>imcH</jats:italic> , encoding an inner membrane <jats:italic>c-</jats:italic> type cytochrome, eliminated the ability of <jats:named-content content-type="genus-species">G. sulfurreducens</jats:named-content> to reduce Fe(III) citrate, Fe(III)-EDTA, and insoluble Mn(IV) oxides, electron acceptors with potentials greater than 0.1 V versus the standard hydrogen electrode (SHE), but the <jats:italic>imcH</jats:italic> mutant retained the ability to reduce Fe(III) oxides with potentials of ≤−0.1 V versus SHE. The <jats:italic>imcH</jats:italic> mutant failed to grow on electrodes poised at +0.24 V versus SHE, but switching electrodes to −0.1 V versus SHE triggered exponential growth. At potentials of ≤−0.1 V versus SHE, both the wild type and the <jats:italic>imcH</jats:italic> mutant doubled 60% slower than at higher potentials. Electrodes poised even 100 mV higher (0.0 V versus SHE) could not trigger <jats:italic>imcH</jats:italic> mutant growth. These results demonstrate that <jats:named-content content-type="genus-species">G. sulfurreducens</jats:named-content> possesses multiple respiratory pathways, that some of these pathways are in operation only after exposure to low redox potentials, and that electron flow can be coupled to generation of different amounts of energy for growth. The redox potentials that trigger these behaviors mirror those of metal acceptors common in subsurface environments where <jats:italic>Geobacter</jats:italic> is found. </jats:p> <jats:p> <jats:bold>IMPORTANCE</jats:bold> Insoluble metal oxides in the environment represent a common and vast reservoir of energy for respiratory microbes capable of transferring electrons across their insulating membranes to external acceptors, a process termed extracellular electron transfer. Despite the global biogeochemical importance of metal cycling and the ability of such organisms to produce electricity at electrodes, fundamental gaps in the understanding of extracellular electron transfer biochemistry exist. Here, we describe a conserved inner membrane redox protein in <jats:named-content content-type="genus-species">Geobacter sulfurreducens</jats:named-content> which is required only for electron transfer to high-potential compounds, and we show that <jats:named-content content-type="genus-species">G. sulfurreducens</jats:named-content> has the ability to utilize different electron transfer pathways in response to the amount of energy available in a metal or electrode distant from the cell. </jats:p>