Credit: NPG

Anoxic marine sediments were thought to house only those microorganisms that depend on energetically less favourable anaerobic processes. However, in 2010, a study revealed that electrical currents can connect oxygen reduction at the surface with sulphide oxidation in the underlying sediment over centimetre distances, but the identity of the components involved in this electrical circuit was unclear. Writing in Nature, Nielsen, Risgaard-Petersen and colleagues now report the unexpected identification of cable-like filamentous bacteria that support the long-distance transport of electrons in anoxic sediments.

within the ridges are strings that transport electrons along the filament in a continuous periplasmic space that is insulated from the external medium by the outer membrane

In agreement with previous work, when the authors incubated sulphidic marine sediment in the dark with overlying oxic sea water, they observed the formation of an electrical current linking oxygen reduction at the surface to sulphide oxidation in the anoxic sediment layers below. Surprisingly, when sediment from the top 20 mm was washed, they observed tufts of filamentous bacteria, fragments of which were up to 1.5 cm long. Analysis of the 16S rRNA from the dissected filaments identified the bacteria as previously undiscovered members of the Desulfobulbaceae family of deltaproteobacteria. Importantly, fluorescence in situ hybridization analysis revealed a high density of filaments present in the oxic and suboxic layers, but none in the deeper sulphidic zones.

To confirm that the Desulfobulbaceae sp. filaments were important for electron transport, the authors inserted filters with varying pore sizes into the sediment cores. They found that electron transport was supported by filters with a pore size large enough to allow the passage of bacterium-sized objects, but it was prevented by filters with smaller pore-sizes, despite these pores being large enough to allow free diffusion of dissolved or colloidal compounds. In addition, when the filaments were suspended in a layer of non-conductive glass microspheres (rather than in sediment), electron transport still occurred, suggesting that the bacterial filaments are both required and sufficient to form the circuit. Using electron microscopy, the authors observed that the filaments exhibited a unique structure with uniform ridges running along their entire length. The ridges consisted of a filled channel of periplasm that was tightly wrapped by the outer membrane and spanned the gaps between adjacent cells.

The author suggest that the channel fillings within the ridges are strings that transport electrons along the filament in a continuous periplasmic space that is insulated from the external medium by the outer membrane. However, the precise molecular and electronic mechanisms at play in these living electrical cables remain to be determined.