Logically, magnetotaxis describes an ability to sense a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 and coordinate movement in response. It was applied to the behavior of certain motile, aquatic bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 in 1975 by R. P. Blakemore.

However, it is now known that these bacteria orient to the Earth's magnetic field even when they are dead, just as a compass needle does. Since the orientation is a result of a direct force acting on the bacteria, it has nothing to do with sensation. Thus, a better label for these bacteria is simply magnetic bacteria.

These bacteria (e.g. Magnetospirillum magnetotacticum
Magnetospirillum is a Gram-negative, microaerophilic genus of magnetotactic bacterium, first isolated from pond water by the microbiologist R. P. Blakemore in 1975. It is characterized by a spirillar, or helical, morphology. It is also a motile bacterium owing to the presence of a polar flagellum...

) contain internal structures known as magnetosome
Magnetosome chains are membranous prokaryotic organelles present in magnetotactic bacteria. They contain 15 to 20 magnetite crystals that together act like a compass needle to orient magnetotactic bacteria in geomagnetic fields, thereby simplifying their search for their preferred microaerophilic...

s. They appear as a chain of dark, membrane-bound crystals - often magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

 (Fe3O4). Some extremophile bacteria from sulforous environments have been isolated with greigite
Greigite is an iron sulfide mineral with formula Fe3S4. It is the sulfur equivalent of the iron oxide magnetite . It was first described in 1964 for an occurrence in San Bernardino County, California, and named after the mineralogist and physical chemist Joseph W...

 (an iron-sulfide compound Fe3S4).

It has been suggested that by orienting toward the Earth's poles, marine bacteria are able to direct their movement downwards, towards the sediment. However, these bacteria are found even at the Earth's magnetic equator, where the field is directed horizontally. An alternative explanation is that by keeping the bacteria aligned against Brownian motion, they are more efficient at chemotaxis
Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...


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