Cable bacteria are filamentous bacteria, found in aqueous sediments, which form chains that conduct electricity. They transfer electrons across distances over 1cm (possibly more) in sediment and groundwater aquifers. They enable the reduction of oxygen and/or nitrates at the sediment's surface to the oxidation of sulphides in the deeper, oxygen-free, sediment layers.
Their electrical activity is predicted to have profound impacts on mineral deposition - and they very probably play an important role in maintaining marine ecosystems in coastal areas. Recycling key elements such as carbon, nitrogen, and phosphorus.
Laboratory cultures have shown that there can be more than 2km of conductive 'cables' in a single cubic cm. of sediment. 1)
“Long-distance electrical conductance in sediment was first observed in 2010 as a spatial separation of sulfide oxidation and oxygen reduction in marine sediment that was interrupted and re-established at a rate faster than could be explained by chemical diffusion. It was later found that this electrical conductance could be observed across a non-conductive layer of glass microspheres, where the only possible conductive structures were filamentous bacteria belonging to the family Desulfobulbaceae.”
A 2019 report in Nature Communications detailed findings that the electrical conductivity works via 17 to 60 parallel conductive fibres embedded in the cell envelope.
“We have succeeded into depositing intact cable bacterium filaments onto micro-fabricated electrodes, in such a way that the filaments remain conductive. These results unequivocally demonstrate that electrical currents are running through cable bacteria.”
The 'current density' (i.e. the ability to conduct) was found to be roughly the same as for household copper wiring. The conductive fibres are believed to be protein-based - but its exact composition is unknown.
See: A highly conductive fibre network enables centimetre-scale electron transport in multicellular cable bacteria Nature Communications, volume 10, Article number: 4120 (2019).
How the 'cables' evolved is also currently unexplained.