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 groundwater1,2. 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 cell3,4. 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 environments5,6,7,8.
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We thank L.Yin and the Central Microscopy Facility at the University of Massachusetts for assistance with transmission electron microscopy. Preliminary sequence data was obtained from The Institute for Genomic Research website at http://www.tigr.org. This research was supported with grants from the Office of Biological and Environmental Research of the Department of Energy and the National Science Foundation.
The authors declare that they have no competing financial interests
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Childers, S., Ciufo, S. & Lovley, D. Geobacter metallireducens accesses insoluble Fe(iii) oxide by chemotaxis. Nature 416, 767–769 (2002). https://doi.org/10.1038/416767a
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