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Magnetosome are membranous structures present in magnetotactic bacteria (MTB). They contain iron-rich magnetic particles that are enclosed within a lipid bilayer membrane. Each magnetosome can often contain 15 to 20 magnetite crystals that form a chain which acts like a compass needle to orient magnetotactic bacteria in geomagnetic fields, thereby simplifying their search for their preferred microaerophilic environments. Recent research has shown that magnetosomes are invaginations of the inner membrane and not freestanding vesicles.[2] 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 (Fe3O4), or iron sulfide magnetosomes, which contain crystals of greigite (Fe3S4). Several other iron sulfide minerals have also been identified in iron sulfide magnetosomes—including mackinawite (tetragonal FeS) and a cubic FeS—which are thought to be precursors of Fe3S4. 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, United States is known to produce both iron oxide and iron sulfide magnetosomes.[3][4]

Formation of the Magnetosome:

Magnetotactic Bacteria (MBT) use a process known as Biomineralization to exert an incredible degree of control on the formation of the mineral crystals within the magnetosomes. The process of biomineralization allows the MBT to control the shape and size along with the alignment of each individual magnetite crystal. These specific magnetite crystals are all identical within a species but between species they can vary in size, structure, formation, amount, but not purpose. They are always used to follow geomagnetic pulls to more agreeable climates for the bacteria.

These magnetite crystals are contained within an organelle envelope. This envelope is referred to as a magnetosome. Within the organelle there can either ferrimagnetic crystals of magnetite (Fe3O4) or the iron sulfide greigite (Fe3S4). Recently there have been a few other magnetic compounds found but these are far less common and do not change the purpose of the organelle.

Around twenty proteins have been found in Magnetotactic bacteria that are specifically used for the creation of magnetosomes. These proteins are responsible for the control of vesicle formation, magnetosome ion transport, and the crystallization of the magnetites and their arrangement with in the particular vesicle. The arrangement of the magnienties is critical because individually they are not very strong but when linked in an ordered chain they increase significantly in strength. There is another set of acidic proteins in the magnetosome that are used to create a link between the vesicle and the cytoskeletal structure in the cell to help the magnetosome hold shape. - Genetics and cell biology of magnetosome formation in magnetotactic bacteria.

Bibliography:


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