1.3.2.3 Magnetosome Morphologies
Many different crystal morphologies have been observed in magnetosomes and are
typically combined forms with cubic, octahedral, and dodecahedral faces which can
be distorted and elongated (Devouard et al. 1998) (Fig. 1.3). Furthermore, nonisometric morphologies such as bullet- or tooth-shaped crystals have been
described (Mann et al. 1987a, b; Spring et al. 1993; Taylor and Barry 2004;
Isambert et al. 2007). Shape anisotropy in magnetic single-domain particles
influences their coercivity with elongations along the easy magnetization axis
leading to a decrease in spontaneous reversal of the magnetization (Vereda et al.
2009). Thus, in some cases, MTB have optimized particle morphology with respect
to properties which are advantageous for magnetotaxis.
Generally, isometric cuboctahedral structures, as found in magnetospirilla, are
also obtained by various abiotic syntheses, suggesting that in these organisms no or
little genetically encoded morphological control is necessary. In contrast, symmetry-breaking anisotropic habits, such as elongated forms along one of the equivalent
[111] faces and bullet- and tooth-shaped crystals, are not in agreement with the
symmetry of the crystal structure and their formation is not understood. Iron
transporters could possibly be localized or active at specific sites in the
magnetosome membrane and could favor crystal growth and elongation by a
directed iron flow and thus localized saturation. Another possibility is the existence
of face-selective adhesion by certain biomolecules on the particles, preventing
growth at blocked crystal sites. For neither hypothesis has experimental evidence
been obtained, and this might prove difficult due to resolution problems with
potentially useful microscopy techniques. Indirect evidence could be obtained
Fig. 1.3 Examples of different magnetosome morphology: (a), (b), (c) elongated, (d) cubic,
(e) octahedral, and (f) and (g) irregularly shaped habits. Scale bar represents 50 nm
18
J. Baumgartner and D. Faivre
Many different crystal morphologies have been observed in magnetosomes and are
typically combined forms with cubic, octahedral, and dodecahedral faces which can
be distorted and elongated (Devouard et al. 1998) (Fig. 1.3). Furthermore, nonisometric morphologies such as bullet- or tooth-shaped crystals have been
described (Mann et al. 1987a, b; Spring et al. 1993; Taylor and Barry 2004;
Isambert et al. 2007). Shape anisotropy in magnetic single-domain particles
influences their coercivity with elongations along the easy magnetization axis
leading to a decrease in spontaneous reversal of the magnetization (Vereda et al.
2009). Thus, in some cases, MTB have optimized particle morphology with respect
to properties which are advantageous for magnetotaxis.
Generally, isometric cuboctahedral structures, as found in magnetospirilla, are
also obtained by various abiotic syntheses, suggesting that in these organisms no or
little genetically encoded morphological control is necessary. In contrast, symmetry-breaking anisotropic habits, such as elongated forms along one of the equivalent
[111] faces and bullet- and tooth-shaped crystals, are not in agreement with the
symmetry of the crystal structure and their formation is not understood. Iron
transporters could possibly be localized or active at specific sites in the
magnetosome membrane and could favor crystal growth and elongation by a
directed iron flow and thus localized saturation. Another possibility is the existence
of face-selective adhesion by certain biomolecules on the particles, preventing
growth at blocked crystal sites. For neither hypothesis has experimental evidence
been obtained, and this might prove difficult due to resolution problems with
potentially useful microscopy techniques. Indirect evidence could be obtained
Fig. 1.3 Examples of different magnetosome morphology: (a), (b), (c) elongated, (d) cubic,
(e) octahedral, and (f) and (g) irregularly shaped habits. Scale bar represents 50 nm
18
J. Baumgartner and D. Faivre