The hallmarks of magnetosomes are their size specificity and distinctive crystal morphologies [ Mann et al., 1990b; Frankel and Bazylinski, 1994]. Although variations exist between species, almost all magnetosomes, regardless of composition, fall within a narrow size range of 35-120 nm when measured along their long axes [ Vali and Kirschvink, 1990; Heywood et al., 1990; 1992; Bazylinski et al., 1994]. This size specificity of magnetosomes is significant because within this size range the particles are uniformly magnetized, permanent single magnetic domains (SD). In addition, the particles are arranged along the chain axis such that the crystallographic magnetic easy axes are also aligned [ Frankel and Bazylinski, 1994]. The size specificity and crystallographic orientation of the chain assembly is optimally designed for magnetotaxis in the geomagnetic field.
For a given cell type, magnetosomes usually have a uniform size, shape, crystal morphology, and arrangement within the cell [ Mann et al., 1990b; Frankel and Bazylinski, 1994]. Magnetosomes occur in at least three different crystal forms determined using transmission electron microscopy. The simplest form, found in M. magnetotacticum, is cubo-octahedral, which preserves the cubic crystal symmetry of magnetite [ Mann et al., 1990b]. A second type, found in coccoid and vibrioid strains, is an elongated hexagonal prism with the axis of elongation parallel to the <111> crystal direction [ Meldrum et al., 1993a,b]. A third type, observed in some uncultured cells, is an elongated cubo-octahedral form producing unique bullet-shaped, tear-drop, and arrowhead particles [ Vali and Kirschvink, 1990; Mann et al., 1990b]. The growth mechanisms for these forms are unknown but particle shapes may be related to anisotropic ion flux through the magnetosome membrane or from constraints imposed by the surrounding membrane structure [ Mann et al., 1990b; Frankel and Mann, 1994]. Whereas the cubo-octahedral form is common in inorganic magnetites, the prevalence of elongated hexagonal forms in magnetosomes appears to be a unique feature of the biomineralization process in MTB [ Frankel and Mann, 1994; Frankel and Bazylinski, 1994]. This aspect of magnetosome morphology forms the basis for distinguishing magnetosomes from detrital or BIM-type magnetite using electron microscopy.
The variability of magnetosome design and chain assembly is apparent in a study by Vali and Kirschvink [1990] on several types of uncultured magnetotactic bacteria. In one microorganism, an estimated 1000 bullet-shaped magnetite crystals were assembled into 5 rope-like bundles traversing the cell's long axis. Another bacterium was found to contain magnetosomes with three different crystal shapes, while yet another produced highly elongated (up to 300nm x 30 nm), but still SD, magnetosomes. Farina et al. [1994] report finding unusual marine MTB that produce chains of ``large'' magnetosomes (200 nm). The magnetosomes are considered large because their crystal dimensions places them outside the theoretical SD size range and within the non-uniformly magnetized two domain (TD) size range for magnetite. If confirmed, the ``large'' magnetosomes raise some interesting questions about their biological function (magnetotaxis?) and may provide an opportunity to study the micromagnetic structure of non-SD particles.