Title: Outer Cell Surface Components Essential for Fe(III) Oxide Reduction by Geobacter

24 Geobacter species are important Fe(III) reducers in a diversity of soils and sediments. 25 Mechanisms for Fe(III) oxide reduction have been studied in detail in Geobacter sulfurreducens, 26 but a number of the most thoroughly studied outer surface components of G. sulfurreducens, 27 particularly c-type cytochromes, are not well conserved among Geobacter species. In order to 28 identify cellular components potentially important in Fe(III) oxide reduction in Geobacter 29 metallireducens, gene transcript abundance was compared in cells grown on Fe(III) oxide or 30 soluble Fe(III) citrate with whole-genome microarrays. Outer-surface cytochromes were also 31 identified. Deletion of genes for c-type cytochromes that had higher transcript abundance during 32 growth on Fe(III) oxides and/ or were detected in the outer-surface protein fraction identified six 33 c-type cytochrome genes, that when deleted removed the capacity for Fe(III) oxide reduction. Several of the c-type cytochromes which were essential for Fe(III) oxide reduction in G. 35 metallireducens have homologs in G. sulfurreducens that are not important for Fe(III) oxide 36 reduction. Other genes essential for Fe(III) oxide reduction included a gene predicted to encode a 37 NHL-repeat containing protein, and a gene potentially involved in pili glycosylation. Genes 38 associated with flagella-based motility, chemotaxis, and pili had higher transcript abundance 39 during growth on Fe(III) oxide, consistent with the previously proposed importance of these 40 components in Fe(III) oxide reduction. These results demonstrate that there are similarities in 41 extracellular electron transfer between G. metallireducens and G. sulfurreducens, but that the 42 outer-surface c-type cytochromes involved in Fe(III) oxide reduction are different.