Microbes, cables, and an electrical touch

Abstract

In nature, highly efficient and diverse consortia of microbes cycle carbon and other elements while generating energy for growth. Driving these reactions are organisms with the ability to extract electrons from the chemical substrates and transfer them to insoluble and soluble electron acceptors. One bacterial group in particular, Geobacter spp., can couple their respiratory metabolism to the reduction of insoluble minerals, such as iron and manganese oxides, and soluble toxic metals such as uranium. Key to these activities is the ability of the cells to transfer respiratory electrons extracellularly using an electroactive cell envelope containing abundant metalloproteins, including c-cytochromes, and conductive protein appendages or pili (known as nanowires). Thus, in addition to been ecological drivers of the cycling of carbon and metals in nature, these organisms show promise for the bioremediation of environments impacted with toxic metals. The electrical activity of Geobacter can also be mimicked in electrochemical reactors equipped with an electrode poised at a metabolically oxidizing potential, so that the electrode functions as an unlimited sink of electrons to drive the oxidation of electron donors and support cell growth. Electrochemical reactors are promising for the treatments of agricultural, industrial, and human wastes, and the electroactivity of these microbes can be used to develop materials and devices for bioenergy and bioremediation applications.

Keywords: Geobacter; c-cytochromes; electrochemical reactors; microbial fuel cells; nanowires; type IV pili.