MAGNETIC-PROPERTIES OF MAGNETOTACTIC BACTERIA

This paper reports on the magnetic properties of magnetosomes in the freshwater magnetotactic bacterium Aquaspirillum magnetotacticum. The magnetosomes are crystallized particles of magnetite with dimensions of 40 to 50 nm, which are arranged within cells in a single linear chain and are within the single-magnetic-domain (SD) size range for magnetite. Magnetic properties have been measured for two samples of dispersions of freeze-dried cells consisting of (1) whole cells (M-1) and (2) magnetosome chains separated from cells (M-2). An important result is that the acquisition and demagnetization of various types of remanent magnetizations are markedly different for the two samples and suggest that remanence is substantially affected by magnetostatic interactions. Interactions are likely to be more important in M-2 because the extracted magnetosome chains are no longer separated from one another by the cell membrane and cytoplasm. Other experimental data for whole cells agree with predictions based on the chain of spheres model for magnetization reversal. This model is consistent with the unique linear arrangement of equidimensional particles in A. magnetotacticum. The magnetic properties of bacterial and synthetic magnetites are compared and the paleomagnetic implications are discussed.