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Geobacter is a genus of bacteria. Geobacter species use anaerobic respiration to alter the redox state of minerals and many pollutants, a trait that makes them useful in bioremediation. Geobacter was the first organism described with the ability to completely oxidize organic compounds to carbon dioxide, and transfer these electrons to metals such as Fe(III), Mn(IV), and U(VI). Geobacter species are also found to be able to transfer electrons to conductive surfaces such as graphite electrode. They are found in anaerobic habitats including wetlands, subsurface aquifers, soils, and aquatic sediment.
Assigned to different genera:
Another observed metabolic phenomenon is the cooperation between Geobacter species, in which several species cooperate in metabolizing a mixture of chemicals that neither could process alone. For example, when supplied with ethanol as an electron donor and sodium fumarate, G. metallireducens oxidized the ethanol, generating an excess of electrons that were passed through direct electrical transfer to G. sulfurreducens via nanowires grown between them, enabling G. sulfurreducens to reduce the fumarate .
Microbial biodegradation of recalcitrant organic is of great environmental significance and involves intriguing novel biochemical reactions. In particular, and halogenated compounds have long been doubted to be anaerobically degradable, but the isolation of hitherto unknown anaerobic hydrocarbon-degrading and reductively dehalogenating bacteria documented these processes in nature. Novel biochemical reactions were discovered, enabling the respective metabolic pathways, but progress in the molecular understanding of these bacteria was slowed by the absence of genetic systems for most of them. However, several complete genome sequences later became available for such bacteria. The genome of the hydrocarbon degrading and iron-reducing species G. metallireducens (accession nr. NC_007517) was determined in 2008. The genome revealed the presence of genes for reductive , suggesting a wide dehalogenating spectrum. Moreover, genome sequences provided insights into the evolution of reductive dehalogenation and differing strategies for niche adaptation.
Geobacter species are often the predominant organisms when extracellular electron transfer is an important bioremediation process in subsurface environments. Therefore, a systems biology approach to understanding and optimizing bioremediation with Geobacter species has been initiated with the ultimate goal of developing in silico models that can predict the growth and metabolism of Geobacter species under a diversity of subsurface conditions. The of multiple Geobacter species have been sequenced. Detailed functional genomic/physiological studies on one species, G. sulfurreducens was conducted. Genome-based models of several Geobacter species that are able to predict physiological responses under different environmental conditions are available. Quantitative analysis of gene transcript levels during in situ uranium bioremediation demonstrated that it is possible to track in situ rates of metabolism and the in situ metabolic state of Geobacter in the subsurface.
Electric currents are produced when the transfer of these electrons to anodes is coupled to the oxidation of intracellular organic wastes. Previous research has proposed that the high conductivity of Geobacter biofilms can be used to power microbial fuel cells and to generate electricity from organic waste products. In particular, G. sulfureducens holds one of the highest records for microbial fuel cell current density that researchers have ever measured in vitro. This ability can be attributed to biofilm conductivity, as highly conductive biofilms have been found to be positively correlated with high current densities in microbial fuel cells.
At the moment, the development of microbial fuel cells for power generation purposes is partly restricted by low current density (mA per square centimeter of electrode) compared to industrial sources of power, and an poor understanding of what limits extracellular electron transfer over long distances in biofilms. As such, many researchers are currently studying how we can utilize biofilm conductivity to our advantage to produce even higher current densities. Low pH environments have been proposed to form inside biofilms, limiting electron transfer from microorganisms to cytochromes. The presence of nonconductive filaments such as pili or flagella on Geobacter species has been proposed to affect current generation by interfering with efficient electron transfer. Factors limiting the individual turnover rate of cells, and how many cells can link together in a biofilm, will both need to be understood to maximize electricity production in the future.
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