Some microorganisms possess extraordinary capabilities. Among these are cable bacteria, a fascinating group of filamentous microbes known for their unique ability to conduct electricity over macroscopic distances. Unlike typical single-celled bacteria, these organisms form long, multicellular chains that function as living electrical wires. Their remarkable conductivity allows them to thrive in environments where other life forms struggle, bridging chemical divides.
Their Discovery and Natural Habitats
Cable bacteria were first discovered in 2010 by microbiologist Lars Peter Nielsen and his team at Aarhus University in Denmark. Initial observations in marine sediments from Aarhus Harbour showed unexpected electrical currents and rapid changes in sulfide levels that could not be explained by known chemical processes alone. Further investigation using electron microscopy revealed thin, long bacterial filaments, which were aptly named “cable bacteria” due to their resemblance to electrical cables. These bacteria belong to the family Desulfobulbaceae, which also includes other sulfate-reducing microbes.
These unique organisms are found globally in a wide array of aquatic sediments, including marine environments like salt marshes, mangroves, and deep-sea sediments. They also inhabit freshwater environments such as lakes, streams, groundwater systems, and even the rhizosphere of aquatic plants like rice paddies. Their growth is favored by specific environmental conditions: the presence of oxygen or nitrate near the sediment surface, coupled with anoxic (oxygen-depleted) conditions deeper down where sulfide is abundant. This creates a redox gradient that cable bacteria exploit for their metabolism.
The Unique Mechanism of Electrical Conductivity
The defining characteristic of cable bacteria is their ability to conduct electricity over distances exceeding one centimeter. This remarkable feat is achieved through their unique multicellular filamentous structure. Each cable bacterium is a long chain of thousands of individual rod-shaped cells, typically 1 to 4 micrometers in diameter and up to several centimeters long.
These cells are interconnected, sharing a common outer cell membrane while each possessing its own inner membrane. Within the periplasm—the space between the inner and outer membranes—are numerous electron-conducting protein fibers, or “wires.” These fibers are visible as parallel, longitudinal ridges along the filament and act as conduits for electron transport.
This internal electrical wiring allows cable bacteria to separate their metabolic processes. Cells located deeper in the anoxic sediment oxidize sulfide, releasing electrons. These electrons are then transported along the conductive fibers through the filament to cells positioned near the sediment surface, where oxygen or nitrate is available. The surface cells then use these electrons to reduce oxygen or nitrate, effectively “breathing” over long distances by coupling distant oxidation and reduction reactions. This metabolic division of labor is a novel strategy in the microbial world, enabling them to access electron donors and acceptors that are spatially separated.
Their Environmental Roles and Significance
Cable bacteria significantly influence their surrounding ecosystems, particularly through their involvement in biogeochemical cycles. Their ability to transport electrons over distances allows them to couple the oxidation of sulfide in deeper sediment layers with the reduction of oxygen or nitrate at the sediment surface. This process directly impacts the sulfur cycle by consuming sulfide and regenerating sulfate, altering sediment chemistry.
Cable bacteria contribute to the cycling of iron and other elements in aquatic environments. Their activity can create a suboxic zone where both oxygen and hydrogen sulfide are depleted, and can also lead to a localized pH peak near the sediment surface. These changes affect nutrient availability for other organisms and can influence the mobility and degradation of pollutants. For instance, by reducing sulfide concentrations, cable bacteria can mitigate hydrogen sulfide toxicity and help in the breakdown of hydrocarbons in contaminated marine sediments.
Future Research and Potential Applications
Ongoing research into cable bacteria continues to uncover their biology and ecological roles. Scientists are working to understand the precise molecular composition of their conductive protein fibers, which remains largely unknown. Efforts are also underway to cultivate pure cultures of cable bacteria, a challenging task that would greatly facilitate detailed study of their physiology and genetics.
The capabilities of cable bacteria suggest several practical applications. Their ability to clean up contaminated sediments by degrading pollutants like hydrocarbons is being explored for bioremediation strategies. By influencing nutrient cycling and mitigating sulfide toxicity, they could also play a role in sustainable agriculture, potentially improving the health and productivity of crops like rice. The natural electrical conductivity of these microbes offers inspiration for the development of novel bio-electronic materials and more efficient microbial fuel cells that convert chemical energy into electrical energy. These possibilities highlight the potential of cable bacteria to contribute to environmental solutions and technological advancements.