Cable bacteria are filamentous

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

that conduct electricity across distances over 1 cm in sediment and groundwater aquifers. Cable bacteria allow for long-distance electron transport, which connects electron donors to electron acceptors, connecting previously separated oxidation and reduction reactions. Cable bacteria couple the reduction of oxygen or nitrate at the sediment's surface to the oxidation of sulfide in the deeper, anoxic, sediment layers.

Discovery

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. The conductivity of single, live filaments was later demonstrated by observing the oxidation state of

cytochromes Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of bi ...

using Raman microscopy. The same phenomenon was later observed in freshwater sediments and groundwater aquifers. Within a 15 cm thick top layer of sediment, cable bacteria densities providing total length of up to 2 km per square centimeter of surface have been observed.

Morphology

Cable bacteria filaments are 0.4—1.7 µm in diameter and up to 15 mm long. Filaments consist of rod-shaped cells with an average length of 3 µm. Filaments are long strings composed of cells stacked together, and can be as long as 30-70mm. Some filaments are composed of upwards of 10,000 cells. Each cell has between 15 and 54 ridges, and ridges span the entire length of the filament. These ridges are hypothesized to contain the cells' conductive structures.

Junctions

Cells in a filament are connected by junctions. The diameter of junctions between cells in the filament varies from being smaller than the cell diameter, the same diameter as the cells on either side of the junction, or bulging out to become wider than the cell diameter. Junctions are able to withstand more force without breaking than the cells themselves. Cells on opposite sides of each junction are separated; if one cell bursts, the cell on the other side of the junction will remain intact.

Strings

Cable bacteria contain structures known as strings. Strings are located inside of ridges on the outer membrane and connect one cell to the next. Strings span the length of the entire filament uninterrupted. The width of the strings is about 20-40 nm. The size and function of a string is similar to that of a microtubule. Strings are thought to serve as a structural foundation for filaments and play a key role in maintaining filament shape, especially during growth.

Distribution

Cable bacteria are generally found in reduced sediments. They can be present as a single filament or as an agglomeration of filaments. Cable bacteria have been identified as being intertwined with the root hairs of aquatic plants and are present in the rhizosphere. Their distribution ranges a gradient of salinities; they are present in freshwater, saltwater lakes, and marine habitats. Cable bacteria have been identified in a diverse range of climatic conditions worldwide, including

Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of Denmark , establish ...

, the

Netherlands ) , anthem = ( en, "William of Nassau") , image_map = , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of the Netherlands , established_title = Before independence , established_date = Spanish Netherl ...

, Japan, Australia, and the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

Motility

Cable bacteria lack flagella, but are capable of

motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

in the form of gliding by propelling themselves forward through the excretion of substances. Cable bacteria have been observed to move as fast as 2.2 µm/s, with an average speed of 0.5 µm/s. Speed of motility in cable bacteria is not related to size of the bacteria. The average distance a cable bacterium glides is approximately 74 µm without interruption. Cable bacteria filaments tend to bend in half, and their movement is led by the apex of the bend as opposed to leading with one tip of the filament. Twisting to move through rotational gliding is rare, but does occur. Cable bacteria likely engage in oxygen chemotaxis, as they are observed to move when in anoxic or hypoxic environments, and cease gliding when contact with oxygen is made. Although motility is important for other microorganisms, once cable bacteria are located in a place that connects oxygen to sulfide, they no longer need to move. The reduced need for motility could explain why the cable bacteria genome contains fewer operons related to chemotaxis than other Desulfobulbaceae. Fewer operons related to chemotaxis results in limited motility.

Taxonomy

The two candidate genera of cable bacteria until now (2016) described: '' Electrothrix'' containing four candidate species, found in marine or brackish sediments, and '' Electronema'' containing two candidate species, found in freshwater sediments, seem to be a monophyletic group. Freshwater and marine cable bacteria have been found to be 88% similar based on

16S ribosomal RNA 16 S ribosomal RNA (or 16 S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome ( SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The genes coding for it are referred to as 16S rR ...

comparisons. These genera are classified within the family Desulfobulbaceae. Cable bacteria are defined by their function rather than their phylogeny, and it is possible that further cable bacteria taxa will be discovered.

Ecological significance

Cable bacteria strongly influence the geochemical properties of the surrounding environment. Their activity promotes the oxidation of

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

at the surface of the sediment, and the resulting iron oxides bind phosphorus-containing compounds and hydrogen sulfide, limiting the amount of

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

and hydrogen sulfide in the water. Phosphorus can cause

eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...

, and hydrogen sulfide can be toxic to marine life, meaning that cable bacteria play an important role in maintaining marine ecosystems in coastal areas.

Methane emissions

The presence of cable bacteria can lead to a decrease in

methane emissions Increasing methane emissions are a major contributor to the rising concentration of greenhouse gases in Earth's atmosphere, and are responsible for up to one-third of near-term global heating. During 2019, about 60% (360 million tons) of methane r ...

from saturated soils. The transfer of electrons through cable bacteria allows the sulfate reduction that occurs in inundated soils to be balanced by sulfate oxidation. Oxidation is possible because of the release of electrons through the cable bacteria filaments. Through this balance, sulfate remains readily available for

sulfate reducing bacteria Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...

, which out compete methanogens. This causes a decrease in production of methane by methanogens.

Practical applications

Cable bacteria have been found associated with benthic microbial fuel cells, devices that convert chemical energy on the ocean floor to electrical energy. In the future, cable bacteria may play a role in increasing the efficiency of microbial fuel cells deployed in sedimentary environments. Cable bacteria have also been found associated with a bioelectrochemical system that enhances the degradation of marine sediment contaminated by hydrocarbons and thus may play a role in future oil spill cleanup technologies.

References

External links

*{{Commons category-inline Microorganisms Bacteria Environmental microbiology