Magnetotactic bacteria

Magnetotactic bacteria are bacteria that use the magnetic field of Earth to orient themselves. This phenomenon is known as magnetotaxis. Magnetotaxis is another means by which bacteria can actively respond to their environment. Response to light (phototaxis) and chemical concentration (chemotaxis) exist in other species of bacteria.

The first magnetotactic bacterium, Aquasprilla magnetotactum was discovered in 1975 by Richard Blakemore. This organism, which is now called Magnetospirillum magnetotacticum, inhabits swampy water, where because of the decomposition of organic matter, the oxygen content in the water drops off sharply with increasing depth. The bacteria were shown to use the magnetic field to align themselves. By this behavior, they were able to position themselves at the region in the water where oxygen was almost depleted, the environment in which they grow best. For example, if the bacteria stray too far above or below the preferred zone of habitation, they reverse their direction and swim back down or up the lines of the magnetic field until they reach the preferred oxygen concentration. The bacteria have flagella, which enables them to actively move around in the water. Thus, the sensory system used to detect oxygen concentration is coordinated with the movement of the flagella.

Magnetic orientation is possible because the magnetic North Pole points downward in the Northern Hemisphere. So, magnetotactic bacteria that are aligned to the fields are also pointing down. In the Northern Hemisphere, the bacteria would move into oxygen-depleted water by moving north along the field. In the Southern Hemisphere, the magnetic North Pole points up and at an angle. So, in the Southern Hemisphere, magnetotactic bacteria are south-seeking and also point downward. At the equator, where the magnetic North Pole is not oriented up or down, magnetotactic bacteria from both hemispheres can be found.

Since the initial discovery in 1975, magnetotactic bacteria have been found in freshwater and salt water, and in oxygen rich as well oxygen poor zones at depths ranging from the near-surface to 2000 meters beneath the surface. Magnetotactic bacteria can be spiral-shaped, rods and spheres. In general, the majority of magnetotactic bacteria discovered so far gather at the so-called oxic-anoxic transition zone; the zone above which the oxygen content is high and below which the oxygen content is essentially zero.

Magnetotaxis is possible because the bacteria contain magnetically responsive particles inside. These particles are composed of an iron-rich compound called magnetite, or various iron and sulfur containing compounds (ferrimagnetite greigite, pyrrhotite, and pyrite). Typically, these compounds are present as small spheres arranged in a single chain or several chains (the maximum found so far is five) in the cytoplasm of each bacterium. The spheres are enclosed in a membrane. This structure is known as a magnetosome. Since many bacterial membranes selectively allow the movement of molecules across them, magnetosome membranes may function to create a unique environment within the bacterial cytoplasm in which the magnetosome crystal can form. The membranes may also be a means of extending the chain of magnetosome, with a new magnetosome forming at the end of the chain.

Magnetotactic bacteria may not inhabit just Earth. Examination of a 4.5 billion-year-old Martian meteorite in 2000 revealed the presence of magnetite crystals, which on Earth are produced only in magnetotactic bacteria. The magnetite crystals found in the meteorite are identical in shape, size and composition to those produced in Magnetospirillum magnetotacticum. Thus, magnetite is a "biomarker," indicating that life may have existed on Mars in the form of magnetotactic bacteria. The rationale for the use of magnetotaxis in Martian bacteria is still a point of controversy. The Martian atmosphere is essentially oxygen-free and the magnetic field is nearly one thousand times weaker than on Earth.

Magnetotactic bacteria are also of scientific and industrial interest because of the quality of their magnets. Bacterial magnets are much better in performance than magnets of comparable size that are produced by humans. Substitution of man-made micro-magnets with those from magnetotactic bacteria could be both feasible and useful.

See also Bacterial movement