Cable bacteria conduct electricity over centimeters through a sulfur-ligated nickel cofactor — a non-redox mechanism with no known precedent in biology

Cable bacteria are centimeter-long filaments of the Desulfobulbaceae family, first identified in Aarhus Harbour sediment in 2010, that carry electric current end-to-end along their length like an insulated wire — coupling sulfide oxidation at depth to oxygen reduction at the sediment surface across a distance thousands of times the size of a single cell. The open question was how a protein structure conducts over such a distance, since every well-characterised biological electron-transport pathway (redox hopping through cytochromes, or the conductive-pilin models proposed for other microbes) degrades badly over millimetres, let alone centimetres.

A 2021 Nature Communications study reported that the periplasmic conductive fibers are built around "a sulfur-ligated nickel cofactor," and concluded that "the involvement of nickel as the active metal in biological conduction is remarkable, and suggests a hitherto unknown form of electron transport that enables efficient conduction in centimeter-long protein structures." The described conduction is metal-like and non-redox — reported as largely insensitive to humidity and electrolyte — which distinguishes it from the redox-active cytochrome and pilin mechanisms invoked elsewhere in electromicrobiology. Nickel as the load-bearing conductive metal is itself unusual: biology uses nickel in a handful of enzymes, but not as a wire.

The wider dispute over whether other electroactive microbes (notably Geobacter) conduct through protein pili or stacked cytochromes is a separate, still-unresolved question tracked at question-geobacter-nanowire-pili-vs-cytochrome-controversy; the cable-bacteria nickel mechanism is distinct from both of the models contested there. The in-situ geophysical detectability of microbial electrical activity is a companion claim at claim-spectral-induced-polarization-detects-bacterial-cells-in-sand.