Magnetotaxis describes an ability to sense a magnetic field and coordinate movement in response. In 1975, Richard P. Blakemore appeared to have observed the phenomena in the behaviour of certain motile aquatic bacteria. However, these bacteria orient to the Earth's magnetic field even after death, without biologically sensing the field. They are now called simply magnetic bacteria.
These bacteria (e.g. Magnetospirillum magnetotacticum) contain internal structures known as magnetosomes. They appear as a chain of dark, membrane-bound crystals - often magnetite (Fe
3O
4). Some extremophile bacteria from sulfurous environments have been isolated with greigite (an iron-sulfide compound Fe
3S
4).
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.