Fe(III) Oxide Reduction Bypassing Outer-Surface Cytochromes in a Marine Respiratory Anaerobe

-type cytochromes homologous to proteins previously implicated in EET. Deletion of these cytochrome genes did not impair growth on Fe(III) oxide. Cultures produced 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ), a soluble electron shuttle that enabled EET activity when added to cell suspensions. These results demonstrate that respiratory Fe(III) reducers can rely on internally reduced electron shuttles even when expressing outer-surface cytochromes. This previously unrecognized mechanism for Fe(III) oxide reduction in a respiratory anaerobe that is abundant in sedimentary environments provides a key concept for improving biogeochemical models and for interpreting extracellular electron transfer mechanisms inferred from omics-based studies of anaerobic ecosystems.