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

Abstract

Dissimilatory metal-reducing bacteria, such as Geobacter sulfurreducens, 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 imcH, encoding an inner membrane c-type cytochrome, eliminated the ability of G. sulfurreducens 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 imcH mutant retained the ability to reduce Fe(III) oxides with potentials of ≤−0.1 V versus SHE. The imcH 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 imcH mutant doubled 60% slower than at higher potentials. Electrodes poised even 100 mV higher (0.0 V versus SHE) could not trigger imcH mutant growth. These results demonstrate that G. sulfurreducens 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 Geobacter is found.

Orenia metallireducens sp. nov. Strain Z6, a Novel Metal-Reducing Member of the Phylum Firmicutes from the Deep Subsurface

“…Moreover, the availability of specific Fe(III) phases may have important implications for the energetics of microbial iron reduction. A recent study shows that G. sulfurreducens operates multiple ironrelated respiratory pathways depending on the redox potential of the substrate being reduced (67). In this case, as the crystallinity of the ferric substrates increases, the amount of energy generated by their reduction decreases.…”

Potential regulates metabolism and extracellular respiration of electroactive Geobacter biofilm

“…Detected changes of electron conduit using voltammetry and measured NAD(P)H level did not give clear information on these processes (Li et al, ; Marsili, Sun, & Bond, ; Rose & Regan, ; Song et al, ; Yoho, Popat, & Torres, ; Zhu et al, ). Recently, the cytoplasmic CbcL and ImcH are recognized as an electron relay when polarizing the cells at less than or equal to −0.1 V and higher potentials, respectively (Levar et al, , ; Zacharoff et al, ). However, this finding is not sufficient to elucidate the metabolism and redox conduction behaviors of G. sulfurreducens biofilms at different potentials.…”

Cultivating Electrochemically Active Biofilms at Continuously Changing Electrode Potentials

“…Another major variable in the EAB cultivation and operation is the electrode potential. The applied potential has been reported to have an impact on microbial EET pathways, e. g., via the activation of low or high potential electron transfer pathways . In some cases more positive anode potentials proved to be advantageous to achieve higher current densities, whereas other studies reported detrimental effects of too high anode potentials, e. g., due to increasing redox stress, and thus recommended the use of less positive potentials .…”