Humic Substances and Extracellular Electron Transfer

Abstract

Humic substances (HS) are redox-active organic molecules that are present in virtually all environments. A wide variety of bacteria including Fe(III)-reducers, sulfate reducers, methanogens, and fermenting bacteria can reduce HS and in a second, abiotic step, the reduced HS can transfer their electrons to terminal electron acceptors such as poorly soluble Fe(III) minerals, in summary a process called humic substance electron shuttling. Electron shuttling between HS-reducing bacteria and Fe(III) minerals can increase the rate of Fe(III) reduction compared to direct Fe(III) reduction and, furthermore, enables the indirect reduction of Fe(III) minerals by some bacterial groups that are not able to reduce the Fe(III) minerals directly. This chapter will first summarize the knowledge about the redox properties of humic substances including a discussion of their redox-active functional groups. We then focus on the mechanism of electron shuttling and evaluate the advantages and disadvantages of electron shuttling versus direct contact Fe(III) mineral reduction. The role of solid-phase humics and other extracellular electron shuttles is discussed as well as the environmental consequences for long-range electron transfer via humic substances. The chapter concludes by illustrating some remaining open questions and by providing suggestions for future research.