1.1 Introduction
Iron biominerals are formed by a broad range of terrestrial and aquatic organisms,
in which they serve various functions. The best known function is magnetoreception, i.e., the ability to detect a magnetic field (Johnsen and Lohmann 2005).
Magnetite-based magnetoreception has been recognized in birds and fish, a typical
example being the homing pigeon (Winklhofer et al. 2001). Further functions
typically include the strengthening of tissues (Frankel and Blakemore 1991) and
hardening of teeth (Lowenstam 1967).
One of the most intriguing examples for the aquatic biomineralization of iron
oxides and biomineral formation, in general, is represented by the synthesis of
magnetic minerals in prokaryotes. Biomineralization has been divided into two
distinct fields: Extracellularly bio-induced formation (Frankel and Bazylinski 2003)
or intracellularly bio-controlled formation, as in the bacterial magnetosomes
(Faivre and Sch€ uler 2008).
It is remarkable that such simple organisms can induce the extracellular formation of minerals, but that this can also be performed under highly controlled
conditions with high reproducibility and perfection is even more impressive. The
formation of magnetosomes is a fascinating example of how supposedly primitive
organisms can translate genetic blueprint information into complex inorganic and
cellular structures. Because many of the fundamental mechanisms of biomineralization are found in bacterial magnetosome formation, magnetotactic bacteria
(hereafter referred to as MTB) may serve as an accessible and relatively simple
model for studying and understanding biomineralization processes in general.
Magnetite crystals from MTB are formed in the so-called magnetosomes (Gorby
et al. 1988), which are specialized organelles synthesized by the cells for the
purpose of geomagnetic navigation in their aquatic habitats (Bazylinski and Frankel
2004). The magnetosomes comprise membrane-enveloped, nano-sized crystals of
either the magnetic iron oxide magnetite, Fe 3 O 4 (Frankel et al. 1979) or the
magnetic iron sulfide greigite, Fe 3 S 4 (Farina et al. 1990; Mann et al. 1990). The
magnetosomes are arranged in one or more intracellular chains by a recently
discovered molecular mechanism (Komeili 2007), which enables the cell to align
and swim along external magnetic fields, a behavior known as “magnetotaxis”
(Blakemore 1975).
Magnetotaxis facilitates the search for growth-favoring microoxic zones within
chemically stratified natural waters (Frankel et al. 2007). The synthesis of bacterial
magnetosomes involves iron uptake from an environmental source, iron transport to
the deposition site via possible storage and precursor compounds, and finally
mineral formation in the dedicated magnetosome organelle, which is achieved by
a high degree of control over the biomineralization of perfectly shaped and sized
magnetic crystals. Moreover, in MTB this process also includes the assembly into
hierarchically structured chains to serve most efficiently as a magnetic field actuator.
The unique characteristics of magnetosome biomineralization have attracted multidisciplinary interest and might be exploited for a variety of applications, specifically
in bio- and nanotechnologies (Lang et al. 2007; Matsunaga and Arakaki 2007).
4
J. Baumgartner and D. Faivre
Iron biominerals are formed by a broad range of terrestrial and aquatic organisms,
in which they serve various functions. The best known function is magnetoreception, i.e., the ability to detect a magnetic field (Johnsen and Lohmann 2005).
Magnetite-based magnetoreception has been recognized in birds and fish, a typical
example being the homing pigeon (Winklhofer et al. 2001). Further functions
typically include the strengthening of tissues (Frankel and Blakemore 1991) and
hardening of teeth (Lowenstam 1967).
One of the most intriguing examples for the aquatic biomineralization of iron
oxides and biomineral formation, in general, is represented by the synthesis of
magnetic minerals in prokaryotes. Biomineralization has been divided into two
distinct fields: Extracellularly bio-induced formation (Frankel and Bazylinski 2003)
or intracellularly bio-controlled formation, as in the bacterial magnetosomes
(Faivre and Sch€ uler 2008).
It is remarkable that such simple organisms can induce the extracellular formation of minerals, but that this can also be performed under highly controlled
conditions with high reproducibility and perfection is even more impressive. The
formation of magnetosomes is a fascinating example of how supposedly primitive
organisms can translate genetic blueprint information into complex inorganic and
cellular structures. Because many of the fundamental mechanisms of biomineralization are found in bacterial magnetosome formation, magnetotactic bacteria
(hereafter referred to as MTB) may serve as an accessible and relatively simple
model for studying and understanding biomineralization processes in general.
Magnetite crystals from MTB are formed in the so-called magnetosomes (Gorby
et al. 1988), which are specialized organelles synthesized by the cells for the
purpose of geomagnetic navigation in their aquatic habitats (Bazylinski and Frankel
2004). The magnetosomes comprise membrane-enveloped, nano-sized crystals of
either the magnetic iron oxide magnetite, Fe 3 O 4 (Frankel et al. 1979) or the
magnetic iron sulfide greigite, Fe 3 S 4 (Farina et al. 1990; Mann et al. 1990). The
magnetosomes are arranged in one or more intracellular chains by a recently
discovered molecular mechanism (Komeili 2007), which enables the cell to align
and swim along external magnetic fields, a behavior known as “magnetotaxis”
(Blakemore 1975).
Magnetotaxis facilitates the search for growth-favoring microoxic zones within
chemically stratified natural waters (Frankel et al. 2007). The synthesis of bacterial
magnetosomes involves iron uptake from an environmental source, iron transport to
the deposition site via possible storage and precursor compounds, and finally
mineral formation in the dedicated magnetosome organelle, which is achieved by
a high degree of control over the biomineralization of perfectly shaped and sized
magnetic crystals. Moreover, in MTB this process also includes the assembly into
hierarchically structured chains to serve most efficiently as a magnetic field actuator.
The unique characteristics of magnetosome biomineralization have attracted multidisciplinary interest and might be exploited for a variety of applications, specifically
in bio- and nanotechnologies (Lang et al. 2007; Matsunaga and Arakaki 2007).
4
J. Baumgartner and D. Faivre