obtained by high-resolution synchrotron X-ray diffraction show that both cultivated
M. gryphiswaldense and M. magneticum form stoichiometric magnetite and
magnetosome particles seem to be protected by the cells from oxidation (Fischer
et al. 2010, submitted manuscript). Most magnetosomes are also free of
imperfections except for frequently observed twinning along the [111] direction.
However, this kind of twinning does not necessarily affect the magnetization of
magnetosome crystals, as it is oriented along the easy axis (<111>) of magnetite
(Winklhofer 2007). Also, no dislocation lines which might indicate a screw dislocation mechanism for growth in elongated directions have been observed by
transmission electron microscopy (Devouard et al. 1998). Thus, MTB seem to
have optimized iron usage and crystal growth with respect to magnetosome formation and their magnetic properties on the atomic structural level. Additionally,
magnetosome crystals were initially thought to be chemically pure, i.e., no traces
of other elements are normally incorporated. However, recent literature has shown
that in certain cases, elements such as manganese (Keim et al. 2009) and cobalt
(Staniland et al. 2008) might be incorporated under specific conditions when highly
abundant in the growth medium. This has created enormous interest as doping of
magnetite can be used to increase the coercivity and hence its magnetic hardness.
Co-doped magnetosomes were shown to have an up to 49% higher coercivity than
their non-doped counterparts (Staniland et al. 2008). However, it remains to be
shown whether cobalt or other metals can be incorporated within the whole
magnetosome crystal or if doping is more prominent on its surface. It also remains
to be verified whether cobalt uptake is an active process or unspecific diffusion.
Thus, future experiments will show whether MTB can be used to form other ferrite
phases with interesting properties and how other metals than iron affect the biology
of MTB.
1.3.2.2 Magnetosome Dimensions
Mature magnetosome crystals are typically in the single-magnetic-domain (SD)
size range of 35–120 nm in diameter and have narrow asymmetric size distributions
with sharp cutoffs toward larger sizes (Devouard et al. 1998). Particles of smaller
size are superparamagnetic at ambient temperature, meaning that due to thermal
fluctuations they have no remanent magnetization and cannot provide cells with a
magnetic dipole sufficient for alignment in Earth’s magnetic field. Larger particles
form multiple domains with separate magnetic moments orienting antiparallel. This
results in less magnetic remanence per unit volume and therefore less efficiency
with respect to iron usage (Dunlop and O ¨ zdemir 1997; Muxworthy and Williams
2006; Winklhofer 2007; Muxworthy and Williams 2009). It is thus thought that
MTB have evolved highly species-specific means to genetically control the size
of magnetosome crystals to tune their magnetic properties. Typical sizes for
magnetospirilla are in the 30–50 nm size range (Devouard et al. 1998).
D. magneticus crystals are about 40 nm long (Po ´sfai et al. 2006), MV-1 and MC1 produce elongated particles with sizes of 40–60 nm and 80–120 nm, respectively
16
J. Baumgartner and D. Faivre
M. gryphiswaldense and M. magneticum form stoichiometric magnetite and
magnetosome particles seem to be protected by the cells from oxidation (Fischer
et al. 2010, submitted manuscript). Most magnetosomes are also free of
imperfections except for frequently observed twinning along the [111] direction.
However, this kind of twinning does not necessarily affect the magnetization of
magnetosome crystals, as it is oriented along the easy axis (<111>) of magnetite
(Winklhofer 2007). Also, no dislocation lines which might indicate a screw dislocation mechanism for growth in elongated directions have been observed by
transmission electron microscopy (Devouard et al. 1998). Thus, MTB seem to
have optimized iron usage and crystal growth with respect to magnetosome formation and their magnetic properties on the atomic structural level. Additionally,
magnetosome crystals were initially thought to be chemically pure, i.e., no traces
of other elements are normally incorporated. However, recent literature has shown
that in certain cases, elements such as manganese (Keim et al. 2009) and cobalt
(Staniland et al. 2008) might be incorporated under specific conditions when highly
abundant in the growth medium. This has created enormous interest as doping of
magnetite can be used to increase the coercivity and hence its magnetic hardness.
Co-doped magnetosomes were shown to have an up to 49% higher coercivity than
their non-doped counterparts (Staniland et al. 2008). However, it remains to be
shown whether cobalt or other metals can be incorporated within the whole
magnetosome crystal or if doping is more prominent on its surface. It also remains
to be verified whether cobalt uptake is an active process or unspecific diffusion.
Thus, future experiments will show whether MTB can be used to form other ferrite
phases with interesting properties and how other metals than iron affect the biology
of MTB.
1.3.2.2 Magnetosome Dimensions
Mature magnetosome crystals are typically in the single-magnetic-domain (SD)
size range of 35–120 nm in diameter and have narrow asymmetric size distributions
with sharp cutoffs toward larger sizes (Devouard et al. 1998). Particles of smaller
size are superparamagnetic at ambient temperature, meaning that due to thermal
fluctuations they have no remanent magnetization and cannot provide cells with a
magnetic dipole sufficient for alignment in Earth’s magnetic field. Larger particles
form multiple domains with separate magnetic moments orienting antiparallel. This
results in less magnetic remanence per unit volume and therefore less efficiency
with respect to iron usage (Dunlop and O ¨ zdemir 1997; Muxworthy and Williams
2006; Winklhofer 2007; Muxworthy and Williams 2009). It is thus thought that
MTB have evolved highly species-specific means to genetically control the size
of magnetosome crystals to tune their magnetic properties. Typical sizes for
magnetospirilla are in the 30–50 nm size range (Devouard et al. 1998).
D. magneticus crystals are about 40 nm long (Po ´sfai et al. 2006), MV-1 and MC1 produce elongated particles with sizes of 40–60 nm and 80–120 nm, respectively
16
J. Baumgartner and D. Faivre