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GEOBACTER-METALLIREDUCENS GEN-NOV SP-NOV, A MICROORGANISM CAPABLE OF COUPLING THE COMPLETE OXIDATION OF ORGANIC-COMPOUNDS TO THE REDUCTION OF IRON AND OTHER METALS
The gram-negative metal-reducing microorganism, previously known as strain GS-15, was further characterized. This strict anaerobe oxidizes several short-chain fatty acids, alcohols, and monoaromatic compounds with Fe(III) as the sole electron acceptor. Furthermore, acetate is also oxidized with the reduction of Mn (IV), U (VI), and nitrate. In whole cell suspensions, the c-type cytochrome(s) of this organism was oxidized by physiological electron acceptors and also by gold, silver, mercury, and chromate. Menaquinone was recovered in concentrations comparable to those previously found in gram-negative sulfate reducers. Profiles of the phospholipid ester-linked fatty acids indicated that both the anaerobic desaturase and the branched pathways for fatty acid biosynthesis were operative. The organism contained three lipopolysaccharide hydroxy fatty acids which have not been previously reported in microorganisms, but have been observed in anaerobic freshwater sediments. The 16S rRNA sequence indicated that this organism belongs in the delta proteobacteria. Its closest known relative is Desulfuromonas acetoxidans. The name Geobacter metallireducens is proposed
Geobacter Metallireducens Accesses Insoluble Fe(III) Oxide by Chemotaxis
Microorganisms that use insoluble Fe(III) oxide as an electron acceptor can have an important function in the carbon and nutrient cycles of aquatic sediments and in the bioremediation of organic and metal contaminants in groundwater. Although Fe(III) oxides are often abundant, Fe(III)-reducing microbes are faced with the problem of how to access effectively an electron acceptor that can not diffuse to the cell. Fe(III)-reducing microorganisms in the genus Shewanella have resolved this problem by releasing soluble quinones that can carry electrons from the cell surface to Fe(III) oxide that is at a distance from the cell. Here we report that another Fe(III)-reducer, Geobacter metallireducens, has an alternative strategy for accessing Fe(III) oxides. Geobacter metallireducens specifically expresses flagella and pili only when grown on insoluble Fe(III) or Mn(IV) oxide, and is chemotactic towards Fe(II) and Mn(II) under these conditions. These results suggest that G. metallireducens senses when soluble electron acceptors are depleted and then synthesizes the appropriate appendages to permit it to search for, and establish contact with, insoluble Fe(III) or Mn(IV) oxide. This approach to the use of an insoluble electron acceptor may explain why Geobacter species predominate over other Fe(III) oxide-reducing microorganisms in a wide variety of sedimentary environments