|MadSci Network: Microbiology|
Dear Alisha –
Wow I wished you had asked me this question sooner! I had never heard of magnetosomes and was really surprised to learn about such an interesting cellular feature. To get everybody on the same page, magnetosomes are small intracellular deposits of crystallized iron enclosed in a phospholipid bilayer found in several evolutionary diverse bacteria.
In order to make magnetosomes, it is thought that bacteria must accumulate fairly incredible levels of iron (in both the ferric and ferrous oxidative states) from their surroundings. Iron metabolism in bacteria is complex; even bacteria that do not form magnetosomes require sufficient iron molecules to act as co-factors for enzymes. While the machinery that most bacteria possess for accumulating iron, such as iron-scavenging siderophores, are present in magnetosome-forming bacteria, it is still unknown how so much iron is absorbed (up to 4% of dry mass).
Once the iron is in the cell, the bacteria somehow manage to create a “seed” crystal of magnetite Fe3O4. This process is also very poorly understood, and iron oxide magnetite is not the only iron-containing compound used for magnetosome production. Once the seed crystal is formed, it is enclosed in a membrane to form the magnetosome…more iron is stored with the seed crystal in the compartment, allowing for the seed crystal to grow into a highly typical size and shape, an itty bitty magnet. Once several magnetosomes have formed, they are lined up to create a chain of magnetosomes. All of these processes are very poorly understood.
This is really were things get interesting. It is thought that the magnetosomes are able to align the bacterial cell like a compass in the magnetic field of the Earth along the North/South axis. In doing this, the bacteria are able to use other behaviors – such as chemotaxis, aerotaxis and phototaxis to hone in on their optimum niche within a water column. Orienting to the poles of the Earth turns an open universe of space into a nice little highway, reducing the complexity of finding proper nutrients. Instead of being able to move anywhere, in any direction for virtually any distance (relative to the size of the bacteria), they only move forward and backward in their search.
I hope this helps – I found this to be a very interesting feature of some bacteria.
PS – Here are some review articles I used to answer this. The first is particular helpful from a genetics standpoint.
Shüler, D. Genetics and cell biology of magnetosome formation in magnetotactic bacteria. FEMS Microbiology Reviews. 32, 654 – 672. 2008.
Jogler, C and Shüler, D. Genomics, Genetics, and Cell Biology of Magnetosome Formation. Annu Rev Microbiol. 63, 501 – 521. 2009.
Faivre, D and Shüler, D. Magnetotactic Bacteria and Magnetosomes. Chemical Reviews. 108, 4875 – 4898. 2008.
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