Electric lives of bacteria under the sea

Scientists from Aarhus University in Denmark initially reported the existence of electrical activity in the seabed in 2010, at which point it was unclear exactly what could be conducting these electric currents in the sediment.

Alongside colleagues from the University of Southern California, with whom the Aarhus team have been investigating possible solutions to the enigma, they reported their surprising and intriguing findings in Nature this week.

Dr Nils Risgaard-Petersen, from the Center for Geomicrobiology at Aarhus University and co-author of the paper, spoke to ScienceOmega.com to further explain what he and his colleagues have found. He began by telling me about the origins of the phenomenon that has been puzzling them for almost three years.  

"We were investigating the nitrate transport and sulphur transformations of a sulphur bacterium, Beggiatoa," Dr Risgaard-Petersen said. "We carried out incubations of sediments with and without Beggiatoa and after completing the experiment we left some of the control cores in the lab.

"After some weeks we then analysed the geochemistry of these cores and to our great surprise they showed pH, oxygen (O₂) and hydrogen sulphide (H₂S) signatures that could not be explained within the classical biogeochemical paradigm. It took us quite a while to understand that the only mechanism that could explain these signatures was the presence of electric currents coupling spatially-segregated biogeochemical processes."

Through microscopes, the researchers observed long, multi-cellular bacteria which were always present when an electrical current was measured. It was possible to interrupt the flow of the current by dragging a thin wire horizontally across the sea floor, demonstrating the fragile nature of the cable network.

The study documents the discovery that one type of organism – the Desulfobulbaceae – mediates these electric currents. According to Dr Risgaard-Petersen, the idea of the bacteria carrying the current fell into place when the scientists noticed wire-like strings enclosed by a membrane in the bacteria, which are one hundred times thinner than a hair.

"The filamentous Desulfobulbaceae act like micro-cables; they are multi-cellular, aerobic, sulphide oxidisers in which electrons generated by sulphide oxidation in cells at one end can be passed through internal, insulated wires to cells at the other end, where they are effectively consumed by oxygen reduction," explained Dr Risgaard-Petersen. "This is a unique lifestyle for prokaryotes which has not been reported before."

The bacteria have two membranes between which a number of parallel strings are embedded. The outer membrane is common to all cells in the bacterium and provides complete electrical insulation from the external environment, while strings embedded between the membranes act as ‘electrical’ wires.

The ability to conduct electricity in this way, as part of a living cable, confers certain benefits on the bacteria such that most of the energy from decomposition in the seabed is harnessed by these organisms. They can respire with oxygen whilst simultaneously exploring distant but highly potent energy sources such as H₂S, and a single square metre of sea floor may be covered with tens of thousands of kilometres of cable bacteria.

Many questions remain to be answered as regards the ecology and physiology of this organism. In the long-term, investigations may aid our understanding of the role cable bacteria have played in the Earth’s history and, on a specialist level, the molecular basis of bioelectronics.

"We are now focusing on the mechanism and molecular basis for electron transport from H₂S to O₂ in these organisms, the allocation of energy conservation and growth among the cells, their genetic diversity and their in situ occurrence," Dr Risgaard-Petersen concluded.