[[File:Proposal of catalyzing bio-voltage memristors.webp|thumb|''Geobacter sulfurreducens'' and its bacterial nanowires]] An '''exoelectrogen''' normally refers to a microorganism that has the ability to transfer electrons extracellularly. While exoelectrogen is the predominant name, other terms have been used: electrochemically active bacteria, anode respiring bacteria, and electricigens.<ref name=LOG>{{cite journal |author=Logan B.|title=Exoelectrogenic bacteria that power microbial fuel cells |journal=Nature Reviews Microbiology |volume=7 |pages=375–383 |date=May 2009 |issue=5 |doi=10.1038/nrmicro2113 |pmid=19330018|s2cid=2560062 }}</ref> Electrons exocytosed in this fashion are produced following ATP production using an electron transport chain (ETC) during oxidative phosphorylation. Conventional cellular respiration requires a final electron acceptor to receive these electrons. Cells that use molecular oxygen (O<sub>2</sub>) as their final electron acceptor are described as using aerobic respiration, while cells that use other soluble compounds as their final electron acceptor are described as using anaerobic respiration.<ref name=WIL>{{cite book |author=Willey J. |title=Prescott's Microbiology |publisher= McGraw Hill |pages=228–245 |year=2011|isbn=978-0-07-337526-7|display-authors=etal}}</ref> However, the final electron acceptor of an exoelectrogen is found extracellularly and can be a strong oxidizing agent in aqueous solution or a solid conductor/electron acceptor. Two commonly observed acceptors are iron compounds (specifically Fe(III) oxides) and manganese compounds (specifically Mn(III/IV) oxides).<ref name=HAR>{{cite journal |author=Hartshorne R.|title=Characterization of an electron conduit between bacteria and the extracellular environment |journal=Proceedings of the National Academy of Sciences |volume=106 |pages=22169–22174 |date=Dec 2009 |issue=52 |doi=10.1073/pnas.0900086106|pmid=20018742 |pmc=2799772 |bibcode=2009PNAS..10622169H |display-authors=etal|url=https://ueaeprints.uea.ac.uk/20554/1/Hartshorne_et_al_PNAS_2009.pdf|doi-access=free }}</ref><ref name=BAR>{{cite journal |author=Baron D.|title=Electrochemical Measurement of Electron Transfer Kinetics by ''Shewanella oneidensis MR-1'' |journal=The Journal of Biological Chemistry |volume=284 | issue = 42 |pages=28865–28873 |date=Oct 2009 |doi=10.1074/jbc.M109.043455 |pmid=19661057 |pmc=2781432|doi-access=free }}</ref><ref name=SHI>{{cite journal |author=Shi L. |title=Isolation of a High-Affinity Functional Protein Complex between OmcA and MtrC: Two Outer Membrane Decaheme ''c''-Type Cytochromes of ''Shewanella oneidensis MR-1'' |journal=Journal of Bacteriology |volume=188 | issue = 13 |pages=4705–4714 |year= 2006 |doi=10.1128/JB.01966-05|display-authors=etal |pmid=16788180 |pmc=1483021}}</ref> As oxygen is a strong oxidizer, cells are able to do this strictly in the absence of oxygen.<ref name=BRU>{{cite book |author=Logan B. |title=Microbial Fuel Cells |publisher= John Wiley & Sons Inc |pages=4–6 |year=2008|isbn=978-0470239483}}</ref>
Utilization of exoelectrogens is currently being researched in the development of microbial fuel cells (MFCs), which hold the potential to convert organic material like activated sludge from waste water treatment into ethanol, hydrogen gas, and electric current.<ref name=LOG/><ref name=FLY>{{cite journal |author=Flynn J. |title=Enabling Unbalanced Fermentations by Using Engineered Electrode-Interfaced Bacteria |journal=mBio |volume=1 | issue = 5 |pages=1–8 |year= 2010 |doi=10.1128/mBio.00190-10 |doi-access=free|display-authors=etal|pmc=2975363 |pmid=21060736}}</ref>
thumb|right|alt=Electron Transport Chain to move electrons to outer membrane of ''Geobacter Sulfurreducens''|Figure 1. Alternative Electron Transport Chain to move electrons to outer membrane of ''Geobacter Sulfurreducens''.
While the exact process in which a cell will reduce an extracellular acceptor will vary from species to species, methods have been shown to involve the use of an oxidoreductase pathway that will transport electrons to the cell membrane that is exposed to the external environment.<ref name=HAR/> This pathway splits off from the ETC pathway after the cytochrome bc1 complex (Complex III) is oxidized by c-type cytochromes designed to move electrons towards the extracellular face of its outermost membrane instead of towards cytochrome c oxidase (Complex IV). MtrC and OmcA are examples of such c-type cytochromes that are endogenously found in the outer membrane of ''Shewanella oneidensis MR-1'' a gammaproteobacterium, though many other variations exist (Figure 1).<ref name=HAR/><ref name=BAR/><ref name=SHI/><ref name=FLY/><ref name=LOV>{{cite journal |author=Lovley D.|title=The microbe electric: conversion of organic matter to electricity |journal=Current Opinion in Biotechnology |volume=19 |pages=1–8 |year= 2008 |issue=6 |doi=10.1016/j.copbio.2008.10.005|pmid=19000760 }}</ref>
Aside from releasing electrons to an exogenous final electron acceptor, external electron transfer may serve other purposes. First, cells may transfer electrons directly to each other without the need for an intermediary substance. ''Pelotomaculum thermopropioncum'' has been observed linked to ''Methanothermobacter thermautotrophicus'' by a pilus (external cell structures used in conjugation and adhesion) that was determined to be electrically conductive. Second, extracellular electrons may serve a role in the communication as a quorum signal in biofilms.<ref name=LOG/>
In addition to ''S. oneidensis MR-1'', exoelectrogenic activity has been observed in the following strains of bacteria without an exogenous mediator: ''Shewanella putrefaciens IR-1'', ''Clostridium butyricum'', ''Desulfuromonas acetoxidans'', ''Geobacter metallireducens'', ''Geobacter sulfurreducens'', ''Rhodoferax ferrireducens'', ''Aeromonas hydrophilia (A3)'', ''Pseudomonas aeruginosa'', ''Desulfobulbus propionicus'', ''Geopsychrobacter electrodiphilus'', ''Geothrix fermentans'', ''Shewanella oneidensis DSP10'', ''Escherichia coli'', ''Rhodopseudomonas palustris'', ''Brucella anthropi YZ-1'', ''Desulfovibrio desulfuricans'', ''Acidiphilium sp.3.2Sup5'', ''Klebsiella pneumoniae L17'', ''Thermincola sp.strain JR'', ''Pichia anomala''.<ref name=LOG/>
==Extracellular electron transport mechanisms==
thumb|right|alt=Proposed methods of exoelectrogen electron transport|Figure 2. Proposed methods of exoelectrogen electron transport: Direct Transfer, Transfer through Electron Shuttle, Transfer through Conductive Biofilm, Transfer through Conductive Pili.
Reduced oxidoreductase enzymes at the extracellular membrane have been shown to use the following methods in transferring their electrons to the exogenous final acceptor: direct contact, shuttling via excreted mediators, iron chelating agents,<ref name="Richter">{{cite journal|doi=10.1128/AEM.06803-11|title=Dissimilatory Reduction of Extracellular Electron Acceptors in Anaerobic Respiration|year=2012|last1=Richter|first1=Katrin|last2=Schicklberger|first2=Marcus|last3=Gescher|first3=Johannes|journal=Applied and Environmental Microbiology|volume=78|issue=4|pages=913–921|pmid=22179232|pmc=3273014|bibcode=2012ApEnM..78..913R }}</ref> through a conductive biofilm, and through conductive pili (Figure 2). However, the possibility exists that these methods are not mutually exclusive,<ref name=LOV /> and the method used may depend on environmental conditions. Under low microbial population densities, usage of electron shuttles and chelators synthesized by the exoelectrogen may be energetically costly due to insufficient concentrations of such molecules required for recovery and reuse.<ref name="Richter" /> Under these circumstances, direct transfer would be favored; however, energy benefits would outweigh energy demands when the microbial community is of sufficient size.
Direct reduction of an exogenous acceptor is achieved through contact between the cell’s oxidoreductases and the terminal electron acceptor (i.e. an electrode or external metal compound). While these proteins are diverse (taking on both membrane-bound or soluble forms), their common locations in the outer membrane or periplasm in Gram-negative and Gram-positive bacteria provide intimate contact for electron transfer.<ref name="Gralnick2007">{{cite journal|last1=Gralnick|first1=Jeffrey A.|last2=Newman|first2=Dianne K.|title=Extracellular respiration|journal=Molecular Microbiology|date=July 2007|volume=65|issue=1|pages=1–11|doi=10.1111/j.1365-2958.2007.05778.x|pmc=2804852|pmid=17581115}}</ref>
Additionally, the presence of electron shuttles dramatically increases the direct transfer rate.<ref name=BAR /> As an example in ''Shewanella oneidensis MR-1'', transport is characterized through a series of redox and structural proteins<ref name="Shi2016" /> extending from the cytoplasmic membrane to the outer cell surface (similar to Figure 1). Flavins are secreted which are thought to bridge the “gap” between cell surface protein(s) and the external metal, which may alleviate the need for immediate contact and facilitate transfer at a distance.<ref name="Richter"/> Furthermore, since cytochromes generally recognize specific surfaces on the substrate metal,<ref name="Gralnick2007" /> soluble flavins may act as a universal bridge allowing for electron donation to a variety of different metal shapes and sizes,<ref name=BAR /> which may be useful in microbial fuel cell applications. Flavins have also been hypothesized to bind to terminal electron transfer proteins as co-factors to increase oxidation rates.<ref name="Shi2016">{{cite journal|last1=Shi|first1=Liang|last2=Dong|first2=Hailiang|last3=Reguera|first3=Gemma|last4=Beyenal|first4=Haluk|last5=Lu|first5=Anhuai|last6=Liu|first6=Juan|last7=Yu|first7=Han-Qing|last8=Fredrickson|first8=James K.|s2cid=20626915|title=Extracellular electron transfer mechanisms between microorganisms and minerals|journal=Nature Reviews Microbiology|date=30 August 2016|volume=14|issue=10|pages=651–662|doi=10.1038/nrmicro.2016.93|pmid=27573579}}</ref>
In the case of ''Geobacter sulferreducens'', the electron carrier riboflavin is used; however, the electron carrier is not entirely freely soluble and can be loosely bound in the culture's biofilm, resulting in a highly conductive biofilm. Furthermore, ''G. sulferreducens'' produces electrically conductive pili (nanowires) with OmcS oxidoreductase enzymes embedded on its surface,<ref name=LEA>{{cite journal |author=Leang C. |title=Alignment of the ''c''-Type Cytochrome OmcS along Pili of ''Geobacter sulfurreducens'' |journal=Applied and Environmental Microbiology |volume=76 | issue = 12 |pages=4080–4084 |year= 2010 |doi=10.1128/AEM.00023-10|display-authors=etal|pmc=2893476 |pmid=20400557|bibcode=2010ApEnM..76.4080L }}</ref> demonstrating the usage of multiple exoelectrogenic transfer methods.
In iron chelation, insoluble ferric oxide compounds are solubilized in aqueous solutions. As bioavailability of iron is scarce, many microbes secrete iron chelating compounds to solubilize, uptake, and sequester iron for various cellular processes. Certain exoelectrogens have shown capability of using such compounds for electron transport by solubilizing iron extracellularly,<ref name="Gralnick2007" /> and delivering it to the cell surface or within the cell. The components used in each pathway are phylogenetically diverse,<ref name="Shi2016" /> thus some chelating agents may reduce iron outside the cell acting as electron shuttles, while others may deliver iron to the cell for membrane bound reduction.<ref name="Gralnick2007" />
== See also ==
* Electric bacteria * Cable bacteria
==References== {{Reflist}}
Category:Microbiology terms Category:Thermodesulfobacteriota