Exoelectrogen

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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. [1] 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 (O2) 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. [2] 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). [3] [4] [5] As oxygen is a strong oxidizer, cells are able to do this strictly in the absence of oxygen. [6]

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. [1] [7]

Figure 1. Alternative Electron Transport Chain to move electrons to outer membrane of Geobacter Sulfurreducens. Geobacter Sulfurreducens Pathway.jpg
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. [3] 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). [3] [4] [5] [7] [8]

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. [1]

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. [1]

Extracellular electron transport mechanisms

Figure 2. Proposed methods of exoelectrogen electron transport: Direct Transfer, Transfer through Electron Shuttle, Transfer through Conductive Biofilm, Transfer through Conductive Pili. Exoelectrogen Electron Transport Mechanisms.jpg
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, [9] through a conductive biofilm, and through conductive pili (Figure 2). However, the possibility exists that these methods are not mutually exclusive, [8] 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. [9] 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. [10]

Additionally, the presence of electron shuttles dramatically increases the direct transfer rate. [4] As an example in Shewanella oneidensis MR-1, transport is characterized through a series of redox and structural proteins [11] 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. [9] Furthermore, since cytochromes generally recognize specific surfaces on the substrate metal, [10] soluble flavins may act as a universal bridge allowing for electron donation to a variety of different metal shapes and sizes, [4] 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. [11]

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, [12] 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, [10] and delivering it to the cell surface or within the cell. The components used in each pathway are phylogenetically diverse, [11] 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. [10]

Related Research Articles

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Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.

<i>Geobacter</i> Genus of anaerobic bacteria found in soil

Geobacter is a genus of bacteria. Geobacter species are anaerobic respiration bacterial species which have capabilities that make them useful in bioremediation. Geobacter was found to be the first organism with the ability to oxidize organic compounds and metals, including iron, radioactive metals, and petroleum compounds into environmentally benign carbon dioxide while using iron oxide or other available metals as electron acceptors. Geobacter species are also found to be able to respire upon a graphite electrode. They have been found in anaerobic conditions in soils and aquatic sediment.

Microbial metabolism is the means by which a microbe obtains the energy and nutrients it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics. The specific metabolic properties of a microbe are the major factors in determining that microbe's ecological niche, and often allow for that microbe to be useful in industrial processes or responsible for biogeochemical cycles.

Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system that generates electric current by diverting electrons produced from the microbial oxidation of reduced compounds on the anode to oxidized compounds such as oxygen on the cathode through an external electrical circuit. MFCs produce electricity by using the electrons derived from biochemical reactions catalyzed by bacteria. MFCs can be grouped into two general categories: mediated and unmediated. The first MFCs, demonstrated in the early 20th century, used a mediator: a chemical that transfers electrons from the bacteria in the cell to the anode. Unmediated MFCs emerged in the 1970s; in this type of MFC the bacteria typically have electrochemically active redox proteins such as cytochromes on their outer membrane that can transfer electrons directly to the anode. In the 21st century MFCs have started to find commercial use in wastewater treatment.

<i>Shewanella</i> Genus of bacteria

Shewanella is the sole genus included in the marine bacteria family Shewanellaceae. Some species within it were formerly classed as Alteromonas. Shewanella consists of facultatively anaerobic Gram-negative rods, most of which are found in extreme aquatic habitats where the temperature is very low and the pressure is very high. Shewanella bacteria are a normal component of the surface flora of fish and are implicated in fish spoilage. Shewanella chilikensis, a species of the genus Shewanella commonly found in the marine sponges of Saint Martin's Island of the Bay of Bengal, Bangladesh.

<i>Shewanella oneidensis</i> Species of bacterium

Shewanella oneidensis is a bacterium notable for its ability to reduce metal ions and live in environments with or without oxygen. This proteobacterium was first isolated from Lake Oneida, NY in 1988, hence its name.

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Geobacter sulfurreducens is a gram-negative metal and sulphur-reducing proteobacterium. It is rod-shaped, aerotolerant anaerobe, non-fermentative, has flagellum and type four pili, and is closely related to Geobacter metallireducens. Geobacter sulfurreducens is an anaerobic species of bacteria that comes from the family of bacteria called Geobacteraceae. Under the genus of Geobacter, G. sulfurreducens is one out of twenty different species. The Geobacter genus was discovered by Derek R. Lovley in 1987. G. sulfurreducens was first isolated in Norman, Oklahoma, USA from materials found around the surface of a contaminated ditch.

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OmcS nanowires are conductive filaments found in some species of bacteria, including Geobacter sulfurreducens, where they catalyze the transfer of electrons. They are multiheme c-Type cytochromes localized outside of the cell of some exoelectrogenic bacterial species, serving as mediator of extracellular electron transfer from cells to Fe(III) oxides and other extracellular electron acceptors.

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Microbial electrochemical technologies (METs) use microorganisms as electrochemical catalyst, merging the microbial metabolism with electrochemical processes for the production of bioelectricity, biofuels, H2 and other valuable chemicals. Microbial fuel cells (MFC) and microbial electrolysis cells (MEC) are prominent examples of METs. While MFC is used to generate electricity from organic matter typically associated with wastewater treatment, MEC use electricity to drive chemical reactions such as the production of H2 or methane. Recently, microbial electrosynthesis cells (MES) have also emerged as a promising MET, where valuable chemicals can be produced in the cathode compartment. Other MET applications include microbial remediation cell, microbial desalination cell, microbial solar cell, microbial chemical cell, etc.,.

References

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  2. ↑ Willey J.; et al. (2011). Prescott's Microbiology. McGraw Hill. pp. 228–245. ISBN   978-0-07-337526-7.
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  6. ↑ Logan B. (2008). Microbial Fuel Cells. John Wiley & Sons Inc. pp. 4–6. ISBN   978-0470239483.
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