Navigational Compass in Magnetic Bacteria R. B. Frankel
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چکیده
Several species of bacteria from both fresh water and marine enviornments were observed to orient and swim in a preferred direction in the geomag netic field (magnetotaxis) [1, 2J. Kalmijn and Blakemore [2] observed that most of the magneto tactic bacteria in sediments oriented and swam along magnetic field lines toward the north. Rever sal of the ambient magnetic field with Helmholtz coils caused the cells to make large V-turns within one second and swim in the opposite direction. Killed cells also oriented in uniform fields as low as 0.1 G. In these and other respects [3] the cells behaved like single magnetic dipoles. A magnetotactic spirillum designated strain MS-I was isolated from a fresh water swamp and grown in pure culture (4). Magnetic cells of this new bacterial isolate each had an intracytoplasmic chain of approximately 22 electron opaque par ticles (fig. I). Each particle was roughly octahedral, 500 A along each major axis. Energy dispersive X-ray measurements [1] (and unpublished results) showed that the particles in strain MS-l had high iron content. Mossbauer spectroscopy of 57Fe in freeze dried magnetic and nonmagnetic cells {5] showed conclusively that iron in magnetic cells was present primarily in the form of magnetite, Fe30 4, with a small admixture of y·Fe20 3• Another iron containing material with a room temperature Mossbauer spectrum similar to that of the iron storage protein ferritin was also observed. A population of nonmagnetic cells of MS-I was derived by successively cloning their nonmagneto tactic progeny in media low in iron. The magnetic variant of MS-I had over 10 times the total iron content of the nonmagnetic form. Nonmagne totactic cells lacked the magnetite crystals. Thus, magnetotaxis in MS-I is associated with intracellu lar magnetite. Since the bacteria were grown in a chemically defined media containing soluble (chelated) iron, the presence of intracellular mag netite implies a process of bacterial synthesis. The magnetic properties of magnetite particles depend on their size and shape. For a particle of roughly cubic shape with side dimension d, there is a range of d over which the particle will be a single
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تاریخ انتشار 2010