The magnetosome chains are membranous prokaryotic organelles present in magnetotactic bacteria. They contain 15 to 20 magnetite crystals that together act like the needle of a compass to orient magnetotactic bacteria in geomagnetic fields, thereby simplifying their search for their preferred microaerophilic environments. Each magnetite crystal within a magnetosome is surrounded by a lipid bilayer, and specific soluble and transmembrane proteins are sorted to the membrane. Recent research has shown that magnetosomes are invaginations of the inner membrane and not freestanding vesicles. Magnetite-bearing magnetosomes have also been found in eukaryotic magnetotactic algae, with each cell containing several thousand crystals.
Overall, magnetosome crystals have high chemical purity, narrow size ranges, species-specific crystal morphologies and exhibit specific arrangements within the cell. These features indicate that the formation of magnetosomes is under precise biological control and is mediated by a mineralization process, which is known as biologically controlled mineralization (biomineralization).
Magnetotactic bacteria usually mineralize either iron oxide magnetosomes, which contain crystals of magnetite (Template:IronO4), or iron sulphide magnetosomes, which contain crystals of greigite (Template:IronTemplate:Sulfur). Several other iron sulphide minerals have also been identified in iron sulphide magnetosomes — including mackinawite (tetragonal Template:IronTemplate:Sulfur) and a cubic Template:IronTemplate:Sulfur — which are thought to be precursors of Template:IronTemplate:Sulfur. One organism is known to produce both iron oxide and iron sulphide magnetosomes.
The particle morphology of magnetosome crystals varies, but is consistent within cells of a single magnetotactic bacterial species or strain. Three general crystal morphologies have been reported in magnetotactic bacteria on the basis: roughly cuboidal, elongated prismatic (roughly rectangular), and tooth-, bullet- or arrowhead-shaped.
Magnetosome crystals are typically 35–120 nm long. Smaller crystals are superparamagnetic–that is, not permanently magnetic at ambient temperature, and domain walls would form in larger crystals. In most magnetotactic bacteria, the magnetosomes are arranged in one or more chains. Magnetic interactions between the magnetosome crystals in a chain cause their magnetic dipole moments to orientate parallel to each other along the length of the chain. The magnetic dipole moment of the cell is usually large enough such that its interaction with the Earth's geomagnetic field overcomes the thermal forces that tend to randomize the orientation of the cell in its aqueous surroundings. Because oxygen gradient is aligned along the vertical gradient of the geomagnetic field, movement along the field would facilitate movement along the oxygen gradient. In this way bacteria can more efficiently search for and reach oxygen rich locations. This process is called magneto-aerotaxis.
While a single magnetosome chain would appear to be ideal for magneto-aerotaxis, a number of magnetotactic bacteria have magnetosomes or magnetosome arrangements that depart from the ideal. One reported example includes large (up to 200 nm) magnetosomes found in coccoid cells in Brazil. These cells have enough magnetosomes so that the calculated magnetic dipole moment of the cell is about 250 times larger than that of a typical cell of Magnetospirillum magnetotacticum. There are also examples of magnetotactic bacteria that contain hundreds of magnetosomes, many more than required for orientation. One large, rod-shaped organism, Magnetobacterium bavaricum, contains up to 1000 bulletshaped magnetosomes arranged in several chains traversing the cell. Some bacteria have magnetosomes that are not arranged in chains, but are clustered on one side of the cell. In such an arrangement, the shape anisotropy of each crystal provides the stability against remagnetization, rather than the overall shape anisotropy in the magnetosome chain arrangement. These non-ideal arrangements may be pointing to additional, currently unknown functions of magnetosomes, possibly related to metabolism.
- Komeili, A., Zhuo Li and D. K. Newman "Magnetosomes Are Cell Membrane Invaginations Organized by the Actin-Like Protein MamK" Science, 311, Jan. 2006, p. 242-245
- R. Frankel "Biological Permanent Magnets" Hyperfine Interactions 151/152: 145–153, 2003