Geobacter electron acceptor

The microbial degradation of contaminants under anaerobic conditions using humic acids as electron acceptors has been demonstrated. These included the oxidations (a) chloroethene and 1,2-dichloroethene to CO2 that was confirmed using C-labeled substrates (Bradley et al. 1998) and (b) toluene to CO2 with AQDS or humic acid as electron acceptors (Cervantes et al. 2001). The transformation of l,3,5-trinitro-l,3,5-triazine was accomplished using Geobacter metallireducens and humic material with AQDS as electron shuttle (Kwon and Finneran 2006). 

The reductase in Geobacter sulfurreducens is located in the outer membrane and a soluble Fe(III) reductase has been characterized from cells grown anaerobically with acetate as electron donor and Fe(III) citrate or fumarate as electron acceptor (Kaufmann and Lovley 2001). The enzyme contained Fe, acid-labile S, and FAD. An extracellular c-type cytochrome is distributed in the membranes, the periplasm, and the medium, and functions as a reductase for electron transfer to insoluble iron hydroxides, sulfur, or manganese dioxide (Seeliger et al. 1998). 

Lin WC, MV Coppi, DR Lovley (2004) Geobacter sulfurreducens can grow with oxygen as terminal electron acceptor. Appl Environ Microbiol 70 2525-2528. 

Multiheme cytochromes c play an important role in bacterial iron respiration. In this process, soluble Fe chelates or insoluble Fe oxides serve as terminal electron acceptors of the anaerobic respiratory chain. In Shewanella and Geobacter spp., the two most studied Fe -reducing organisms, a great increase in cellular multiheme c-type cytochromes is observed upon growth by iron respiration. In the case of the genus Shewanella, electron transport to Fe involves CymA in the cytoplasmic membrane, and several... 

The following reactions involving Geobacter have been documented for complete oxidation of aromatic organic compounds to carbon dioxide, while using Fe(III) as sole electron acceptor (Lovley, 1991) ... 

The oxidizing action of Geobacter has another beneficial effect. Tests show that uranium salts can replace iron(III) oxide as the electron acceptor. Thus, by adding acetate ions and the bacteria to the groundwater contaminated with uranium, it is possible to reduce the soluble uranium(VI) salts to the insoluble uranium(IV) salts, which can be readily removed before the water ends up in households and farmlands. 

Figure 2.2 Schematic of a two-chambered MFC or an MEC configuration, both having an anode electrode (AE), where Geobacter biofilms catalyze the oxidation of the electron donor (Djg to Dgjj). A proton-exchange membrane separates the two chambers to allow the diffusion of protons (H" ) from the anode to the cathode chamber. In the MFC, the anode electrode is wired directly to the cathode electrode (CE), and the amount of electrons (e") generated by the anode biofilms is dependent on the reduction potential of the electron acceptor (reaction A to Ajg) used as catholyte. In the MEC, the cathode limitation is bypassed using a potentiostat, which sets a constant potential of the anode electrode versus a reference electrode (RE) and allows the H" " and the e" to combine on the cathode electrode to generate Hj.

Mehta T, Childers SE, Glaven R, Lovley DR, Mester T. A putative multicopper protein secreted by an atypical type II secretion system involved in the reduction of insoluble electron acceptors in Geobacter sidfurreducens. Microbiology 2006 152 2257-2264. Mehta T, Coppi MV, Childers SE, Lovley DR. Outer membrane c-type cytochromes required for Fe(III) and Mn(IV) oxide reduction in Geobacter sulfurreducens. Appl Environ Microbiol 2005 71 8634-8641. 

Coppi MV, O Neil RA, Lovley DR. Identification of an uptake hydrogenase required for hydrogen-dependent reduction of Ee(III) and other electron acceptors by Geobacter sulfurreducens. J Bacteriol 2004 186 3022-3028. 

Ding YHR, Hixson KK, Aklujkar MA, Lipton MS, Smith RD, Lovley DR, Mester T. Proteome of Geobacter sulfurreducens grown with Fe(III) oxide or Fe(III) citrate as the electron acceptor. Biochim Biophys Acta 2008 1784 1935-1941. 

Some microbial metabolites/endogenous mediators are used as mediators by microbes. Some microbes such as Rhodoferax ferrireducens, Geobacter metal-lireducens, Shewanella putrefaciens and Geo-bacteriaceae sulferreducens are bioelectrochemically active. These microbes directly transfer electrons via the membrane. They have the capability to form a biofilm on the anode surface. Hence, in the presence of these microbes the anodes act as an electron acceptor. 

Some microorganisms can directly transfer electrons to the electrode via a physical contact of the cell membrane or a membrane organelle with the anode. No diffusional redox species are involved in this electron transfer process. As illustrated in Figure 2.6a, the direct electron transfer requires the microorganisms to possess (1) membrane-bound protein relays which transfer electrons from the inside of the bacterial cell to its outside, and (2) an outer membrane (OM) redox protein which accepts the electrons and delivers them to an external, solid electron acceptor (a metal oxide or an MFC anode). The most studied OM redox proteins are c-type cytochromes, which are involved in metal-reducing microorganisms such as Geobacter, Rhodqferax and Shewanella. These bacteria often have to rely on solid terminal electron acceptors like iron(lll) oxides in their natural environments. 

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