Identification of different putative outer membrane electron conduits necessary for Fe(III) citrate, Fe(III)-oxide, Mn(IV)-oxide, or electrode reduction by Geobacter sulfurreducens .

Abstract

At least five gene clusters in the Geobacter sulfurreducens genome encode putative ‘electron conduits’ implicated in electron transfer across the outer membrane, each containing a periplasmic multiheme c -type cytochrome, integral outer membrane anchor, and outer membrane redox lipoprotein(s). Markerless single gene cluster deletions and all possible multiple deletion combinations were constructed and grown with soluble Fe(III) citrate, Fe(III)- and Mn(IV)-oxides, and graphite electrodes poised at +0.24 V and -0.1 V vs. SHE. Different gene clusters were necessary for reduction of each electron acceptor. During metal oxide reduction, deletion of the previously described omcBC cluster caused defects, but deletion of additional components in an Δ omcBC background, such as extEFG , were needed to produce defects greater than 50% compared to wild type. Deletion of all five gene clusters abolished all metal reduction. During electrode reduction, only the Δ extABCD mutant had a severe growth defect at both redox potentials, while this mutation did not affect Fe(III)-oxide, Mn(IV)-oxide, or Fe(III) citrate reduction. Some mutants containing only one cluster were able to reduce particular terminal electron acceptors better than wild type, suggesting routes for improvement by targeting specific electron transfer pathways. Transcriptomic comparisons between fumarate and electrode-based growth showed all of these ext clusters to be constitutive, and transcriptional analysis of the triple-deletion strain containing only extABCD detected no significant changes in expression of known redox proteins or pili components. These genetic experiments reveal new outer membrane conduit complexes necessary for growth of G. sulfurreducens , depending on the available extracellular electron acceptor.

Extracellular electron transfer-dependent anaerobic oxidation of ammonium by anammox bacteria

“…Brocadia electricigens may have activated this system in order to uptake Fe(III) as an alternative electron acceptor as well as for iron uptake for assimilation. This finding is in agreement with a previous study using the EET-capable model bacteria Geobacter sulfurreducens ( 8 ) , in which it was shown that t he pathways required for EET and metal oxide reduction are distinct.…”

“…Also, growth or electrochemical activity was not quantified in these experiments. Further, these experiments could not discriminate between Fe(III) oxide reduction for nutritional acquisition (i.e., via siderophores) versus respiration through extracellular electron transfer (EET) ( 8 ). Therefore, with these preliminary experiments it could not be determined if anammox bacteria have EET capability or not.…”

Molecular evidence for the adaptive evolution of Geobacter sulfurreducens to perform dissimilatory iron reduction in natural environments

“…The reduction of the soluble extracellular electron acceptor ferric citrate occurs on the outer cell surface of G. sulfurreducens, while the reduction of Fe(III) oxide was shown to involve EET (Jiménez Otero, Chan, & Bond, ). The reduction of Fe(III) was analyzed as previously reported (Ueki et al, ).…”

Extracellular DNA Promotes Efficient Extracellular Electron Transfer by Pyocyanin in Pseudomonas aeruginosa Biofilms

“…One widespread strategy microbes employ to overcome this challenge is to channel electrons derived from intracellular metabolism to extracellular oxidants at a distance (Shi et al, 2016). Known as ''extracellular electron transfer'' (EET), this process requires electron carriers to bridge the gap, be they outer membrane-associated or extracellular cytochromes (Richter et al, 2009;Xu et al, 2018;Jimé nez Otero et al, 2018;Nevin et al, 2009), various structures called ''nanowires'' (Steidl et al, 2016;Subramanian et al, 2018;Wang et al, 2019;Reguera et al, 2005;Malvankar et al, 2011), cable bacteria conductive filaments (Cornelissen et al, 2018), or redoxactive small molecules (Glasser et al, 2017a). Although the putative molecular components underpinning different EET processes have been described in a variety of organisms, a detailed understanding of how these components achieve EET remains an important research goal across diverse systems.…”