7 Nature Driven Magnetic Nanoarchitectures
163
only a few representatives have been isolated in axenic culture and deposited in bioresource centers. This explains why most of the knowledge rests on the two first species
isolated and easy-cultured, Magnetospirillum magneticum AMB-1 and Magnetospirillum gryphiswaldense MSR-1. They were isolated from freshwater sediment in the
early 1990s [28, 29]. Both strains are spirilla and biomineralize cubooctahedral magnetite crystals arranged in a single chain. In Fig. 7.3 we show TEM and cryoelectron
tomography images of M. gryphiswaldense and the isolated magnetosomes.
7.2 Biomineralization Process of the Magnetosome
The biomineralization of magnetosomes is a complex biochemical process genetically controlled. Up to date, more than 30 specific genes implicated in magnetosome
biomineralization have been identified [18, 30]. Even though the biomineralization
process is not well understood yet, different steps have been well described [21, 23,
31].
First, the magnetosome vesicles are formed in the cell by invagination of the
cytoplasmic membrane. The vesicle acts as a ‘nano-reactor’ in which the conditions
of the nanocrystal nucleation and growth (pH, redox, etc.) can be controlled, and at
the same time, protects the cell from harmful byproducts [16, 32]. The vesicle will
grow up to a certain size before the magnetite nucleation process starts. This seems to
allow supersaturation of Fe to facilitate nucleation [33]. Second, once the vesicles are
formed, the magnetosome membrane is targeted by several proteins (MamA, MamP,
MamY, etc.), most of which are encoded in a conserved genomic segment named
the magnetosome island (MAI) [34], although it is still not well known how the
process works [23]. These proteins will control, among other things, the size, shape
and morphology of the biomineralized nanoparticles. Third, iron is transported into
the vesicle and mineralized as a nanocrystal. Finally, magnetosomes are aligned into
chains through the interaction of the magnetosomes with a cytoskeletal filament that
traverses the cell. All four steps are regulated by a complex genetic machinery which
has been thoroughly described in the literature [18]. A schematic representation of
the different steps can be seen in Fig. 7.4.
The intracellular formation of the magnetite nanocrystal inside the magnetosomes
requires first the transport of iron from the surrounding environment into the cell.
Three possible routes have been proposed for the iron uptake: (i) iron is introduced
into the vesicle when the magnetosome is still attached to the cell membrane, (ii)
(alternative to i) but not mutually exclusive) iron is first taken up by cellular iron
transport systems and then introduced into the magnetosomes through specific transporters, and (iii) iron is transported from the cytoplasmic membrane to the magnetosome membrane by ligation to unknown organic substrates [18]. Previous studies
seem to indicate that in some species the Fe is stored as a compound inside the cytoplasm before being introduced into the magnetosomes [35]. It has also been shown
that magnetotactic bacteria are capable of taking up either Fe
2+ or Fe
3+ , and this
process involves in some cases the use of iron chelators called siderophores [36].
163
only a few representatives have been isolated in axenic culture and deposited in bioresource centers. This explains why most of the knowledge rests on the two first species
isolated and easy-cultured, Magnetospirillum magneticum AMB-1 and Magnetospirillum gryphiswaldense MSR-1. They were isolated from freshwater sediment in the
early 1990s [28, 29]. Both strains are spirilla and biomineralize cubooctahedral magnetite crystals arranged in a single chain. In Fig. 7.3 we show TEM and cryoelectron
tomography images of M. gryphiswaldense and the isolated magnetosomes.
7.2 Biomineralization Process of the Magnetosome
The biomineralization of magnetosomes is a complex biochemical process genetically controlled. Up to date, more than 30 specific genes implicated in magnetosome
biomineralization have been identified [18, 30]. Even though the biomineralization
process is not well understood yet, different steps have been well described [21, 23,
31].
First, the magnetosome vesicles are formed in the cell by invagination of the
cytoplasmic membrane. The vesicle acts as a ‘nano-reactor’ in which the conditions
of the nanocrystal nucleation and growth (pH, redox, etc.) can be controlled, and at
the same time, protects the cell from harmful byproducts [16, 32]. The vesicle will
grow up to a certain size before the magnetite nucleation process starts. This seems to
allow supersaturation of Fe to facilitate nucleation [33]. Second, once the vesicles are
formed, the magnetosome membrane is targeted by several proteins (MamA, MamP,
MamY, etc.), most of which are encoded in a conserved genomic segment named
the magnetosome island (MAI) [34], although it is still not well known how the
process works [23]. These proteins will control, among other things, the size, shape
and morphology of the biomineralized nanoparticles. Third, iron is transported into
the vesicle and mineralized as a nanocrystal. Finally, magnetosomes are aligned into
chains through the interaction of the magnetosomes with a cytoskeletal filament that
traverses the cell. All four steps are regulated by a complex genetic machinery which
has been thoroughly described in the literature [18]. A schematic representation of
the different steps can be seen in Fig. 7.4.
The intracellular formation of the magnetite nanocrystal inside the magnetosomes
requires first the transport of iron from the surrounding environment into the cell.
Three possible routes have been proposed for the iron uptake: (i) iron is introduced
into the vesicle when the magnetosome is still attached to the cell membrane, (ii)
(alternative to i) but not mutually exclusive) iron is first taken up by cellular iron
transport systems and then introduced into the magnetosomes through specific transporters, and (iii) iron is transported from the cytoplasmic membrane to the magnetosome membrane by ligation to unknown organic substrates [18]. Previous studies
seem to indicate that in some species the Fe is stored as a compound inside the cytoplasm before being introduced into the magnetosomes [35]. It has also been shown
that magnetotactic bacteria are capable of taking up either Fe
2+ or Fe
3+ , and this
process involves in some cases the use of iron chelators called siderophores [36].