(Faivre et al. 2007). Both iron species would have to be supplied to the process in
the correct stoichiometry, i.e., in the ratio Fe
III
/Fe
II
¼ 2. This could be obtained
through precise tuning of transport rates of the respective species into the
magnetosome vesicle or controlled intravesicular redox reactions. In any scenario,
iron has to be concentrated to supersaturating conditions in order to form nuclei that
can then grow into mature crystals. For a coprecipitation-like process, results
obtained from abiotic chemical synthesis indicate that a lower concentration limit
exists in the range of 30 mM iron to form the magnetite phase. Lower
concentrations favored formation of poorly crystalline iron oxides, hydroxides,
and goethite (Faivre et al. 2004). Supersaturating conditions could be provided by
two different means, regulated iron transport into the magnetosome vesicle by
proteins such as MamB and MamM, and/or localized supersaturation by ironbinding entities such as the lipid membrane or attached proteins.
Growth kinetics has been studied to some extent by induction experiments.
Some MTB can be grown in culture with very low iron supply and/or high oxygen
concentrations, suppressing magnetite biomineralization. Addition of iron under
microaerobic conditions then leads to induced crystal formation and gives insights
into magnetosome growth behavior (Komeili et al. 2004; Faivre et al. 2007).
M. magneticum has been shown to form arranged magnetite crystals within 2 h
after iron addition with increasing size and number over 21 h. Magnetite
crystallized simultaneously within several vesicles of the same chain (Komeili
et al. 2004). In induction experiments with iron-starved M. gryphiswaldense,
transmission electron microscopy showed magnetite crystals approximately an
hour after iron addition. Particles reached full size and number over a period of
6 h. M€ ossbauer spectroscopy detected the first magnetite-related signals only
20 min after induction, indicating an even faster nucleation. Analysis of cell
fractions suggests that very small (<5 nm) magnetite particles are formed at the
cytoplasmic membrane, in accordance with the model that proposes nucleation in
undetached magnetosomes and subsequent growth in detached vesicles (Faivre
et al. 2007; Faivre et al. 2008).
In a recent paper by Staniland et al., the authors proposed rapid formation of full
chains with mature crystals within 15 min after iron induction, challenging the
slower growth observed in M. magneticum and M. gryphiswaldense before
(Staniland et al. 2007). X-ray magnetic circular dichroism results suggested the
existence of an a-Fe 2 O 3 precursor phase during the first 30 min after induction. It is
still unclear which experimental details led to these substantial reported differences.
However, experiments were conducted with different iron sources in the medium,
potentially influencing uptake rates. Furthermore, reported C mag values, a measure
of cell magnetization based on a light scattering assay are significantly different.
C mag is zero for nonmagnetic cells and increases with magnetization. Thus, in
induction experiments, C mag should begin at zero and increase over time with
progressing magnetosome formation. Reported C mag values in the latter experiment
were marginally above zero at t ¼ 0, indicating slight magnetization of the cells
already before induction and possibly explaining differences in described
magnetosome formation rates (Staniland et al. 2007).
14
J. Baumgartner and D. Faivre
the correct stoichiometry, i.e., in the ratio Fe
III
/Fe
II
¼ 2. This could be obtained
through precise tuning of transport rates of the respective species into the
magnetosome vesicle or controlled intravesicular redox reactions. In any scenario,
iron has to be concentrated to supersaturating conditions in order to form nuclei that
can then grow into mature crystals. For a coprecipitation-like process, results
obtained from abiotic chemical synthesis indicate that a lower concentration limit
exists in the range of 30 mM iron to form the magnetite phase. Lower
concentrations favored formation of poorly crystalline iron oxides, hydroxides,
and goethite (Faivre et al. 2004). Supersaturating conditions could be provided by
two different means, regulated iron transport into the magnetosome vesicle by
proteins such as MamB and MamM, and/or localized supersaturation by ironbinding entities such as the lipid membrane or attached proteins.
Growth kinetics has been studied to some extent by induction experiments.
Some MTB can be grown in culture with very low iron supply and/or high oxygen
concentrations, suppressing magnetite biomineralization. Addition of iron under
microaerobic conditions then leads to induced crystal formation and gives insights
into magnetosome growth behavior (Komeili et al. 2004; Faivre et al. 2007).
M. magneticum has been shown to form arranged magnetite crystals within 2 h
after iron addition with increasing size and number over 21 h. Magnetite
crystallized simultaneously within several vesicles of the same chain (Komeili
et al. 2004). In induction experiments with iron-starved M. gryphiswaldense,
transmission electron microscopy showed magnetite crystals approximately an
hour after iron addition. Particles reached full size and number over a period of
6 h. M€ ossbauer spectroscopy detected the first magnetite-related signals only
20 min after induction, indicating an even faster nucleation. Analysis of cell
fractions suggests that very small (<5 nm) magnetite particles are formed at the
cytoplasmic membrane, in accordance with the model that proposes nucleation in
undetached magnetosomes and subsequent growth in detached vesicles (Faivre
et al. 2007; Faivre et al. 2008).
In a recent paper by Staniland et al., the authors proposed rapid formation of full
chains with mature crystals within 15 min after iron induction, challenging the
slower growth observed in M. magneticum and M. gryphiswaldense before
(Staniland et al. 2007). X-ray magnetic circular dichroism results suggested the
existence of an a-Fe 2 O 3 precursor phase during the first 30 min after induction. It is
still unclear which experimental details led to these substantial reported differences.
However, experiments were conducted with different iron sources in the medium,
potentially influencing uptake rates. Furthermore, reported C mag values, a measure
of cell magnetization based on a light scattering assay are significantly different.
C mag is zero for nonmagnetic cells and increases with magnetization. Thus, in
induction experiments, C mag should begin at zero and increase over time with
progressing magnetosome formation. Reported C mag values in the latter experiment
were marginally above zero at t ¼ 0, indicating slight magnetization of the cells
already before induction and possibly explaining differences in described
magnetosome formation rates (Staniland et al. 2007).
14
J. Baumgartner and D. Faivre