Magnetosome
Encyclopedia
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
environments. Each magnetite crystal within a magnetosome is surrounded by a lipid
bilayer, and specific soluble and transmembrane protein
s are sorted to the membrane
. Recent research has shown that magnetosomes are invaginations of the inner membrane and not freestanding vesicle
s. 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 biomineralization.
Magnetotactic bacteria usually mineralize either iron oxide magnetosomes, which contain crystals of magnetite
, or iron sulfide magnetosomes, which contain crystals of greigite
. Several other iron sulfide minerals have also been identified in iron sulfide magnetosomes — including mackinawite
(tetragonal ) and a cubic — which are thought to be precursors of . One type of magnetotactic bacterium present at the oxic-anoxic transition zone (OATZ) of the southern basin of the Pettaquamscutt River
Estuary, Narragansett, Rhode Island
, USA is known to produce both iron oxide and iron sulfide 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, which makes them single-domain
. Single-domain crystals have the maximum possible magnetic moment per unit volume for a given composition. 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 Earth's magnetic field overcomes thermal forces that tend to randomize the orientation of the cell in its aqueous surroundings. Magnetotactic bacteria also use aerotaxis, a response to changes in oxygen concentration that favors swimming toward a zone of optimal oxygen concentration. In lakes or oceans the oxygen concentration is commonly dependent on depth. As long as the Earth's magnetic field has a significant downward slant, the orientation along field lines aids the search for the optimal concentration. 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
.
Magnetotactic bacteria
Magnetotactic bacteria are a polyphyletic group of bacteria discovered by Richard P. Blakemore in 1975, that orient along the magnetic field lines of Earth's magnetic field. To perform this task, these bacteria have organelles called magnetosomes that contain magnetic crystals...
. They contain 15 to 20 magnetite
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...
crystals that together act like a compass
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
needle to orient magnetotactic bacteria in geomagnetic
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
fields, thereby simplifying their search for their preferred microaerophilic
Microaerophile
A microaerophile is a microorganism that requires oxygen to survive, but requires environments containing lower levels of oxygen than are present in the atmosphere . Many microphiles are also capnophiles, as they require an elevated concentration of carbon dioxide. In the laboratory they can be...
environments. Each magnetite crystal within a magnetosome is surrounded by a lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
bilayer, and specific soluble and transmembrane protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s are sorted to the membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
. Recent research has shown that magnetosomes are invaginations of the inner membrane and not freestanding vesicle
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
s. 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 biomineralization.
Magnetotactic bacteria usually mineralize either iron oxide magnetosomes, which contain crystals of magnetite
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...
, or iron sulfide magnetosomes, which contain crystals of greigite
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...
. Several other iron sulfide minerals have also been identified in iron sulfide magnetosomes — including mackinawite
Mackinawite
Mackinawite is an iron nickel sulfide mineral with formula 1 + xS . It crystallizes in the tetragonal crystal system and occurs as opaque bronze to grey-white tabular crystals and anhedral masses. It has a Mohs hardness of 2.5 and a specific gravity of 4.17.-Occurrence:It occurs in serpentinized...
(tetragonal ) and a cubic — which are thought to be precursors of . One type of magnetotactic bacterium present at the oxic-anoxic transition zone (OATZ) of the southern basin of the Pettaquamscutt River
Pettaquamscutt River
The Pettaquamscutt River is a tidal extension of the Mattatuxet River in the U.S. state of Rhode Island. It flows approximately . There are no dams along the river's length.-Course:...
Estuary, Narragansett, Rhode Island
Narragansett, Rhode Island
Narragansett is a town in Washington County, Rhode Island, United States. The population was 15,868 at the 2010 census, but there is a greater population in the summer. The nickname for the town is "Gansett". The town of Narragansett occupies a narrow strip of land running along the eastern bank...
, USA is known to produce both iron oxide and iron sulfide 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, which makes them single-domain
Single domain (magnetic)
Single domain, in magnetism, refers to the state of a ferromagnet in which the magnetization does not vary across the magnet. A magnetic particle that stays in a single domain state for all magnetic fields is called a single domain particle . Such particles are very small...
. Single-domain crystals have the maximum possible magnetic moment per unit volume for a given composition. 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
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
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 Earth's magnetic field overcomes thermal forces that tend to randomize the orientation of the cell in its aqueous surroundings. Magnetotactic bacteria also use aerotaxis, a response to changes in oxygen concentration that favors swimming toward a zone of optimal oxygen concentration. In lakes or oceans the oxygen concentration is commonly dependent on depth. As long as the Earth's magnetic field has a significant downward slant, the orientation along field lines aids the search for the optimal concentration. This process is called magneto-aerotaxis
Magnetotaxis
Logically, magnetotaxis describes an ability to sense a magnetic field and coordinate movement in response. It was applied to the behavior of certain motile, aquatic bacteria in 1975 by R. P. Blakemore....
.
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
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
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
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
.