A role for excreted quinones in extracellular electron transfer

Abstract

Respiratory processes in bacteria are remarkable because of their ability to use a variety of compounds, including insoluble minerals, as terminal electron acceptors¹. Although much is known about microbial electron transport to soluble electron acceptors, little is understood about electron transport to insoluble compounds such as ferric oxides^2,3. In anaerobic environments, humic substances can serve as electron acceptors and also as electron shuttles to ferric oxides^4,5,6. To explore this process, we identified mutants in Shewanella putrefaciens that are unable to respire on humic substances. Here we show that these mutants contain disruptions in a gene that is involved in the biosynthesis of menaquinone. During growth, the wild type releases a menaquinone-related redox-active small molecule into the medium that complements the mutants. This finding raises the possibility that electron transfer to a variety of oxidants, including poorly soluble minerals, may be mediated by microbially excreted quinones that have yet to be identified.