155
which are determined by the number of magnetic domains contained within the
particle (the domain state). The domain state determines the magnetic properties
which will, in turn, be selected according to physiological requirements and the role
the magnetite particles play in the organism, (Davila 2005 ).
Magnetic particle with uniform direction of spontaneous magnetization are the
basic elements of corresponding magnetic domains. However, situations where
different domains within a single particle may have different directions are also
known. Correspondingly, an MD particle can show zero remanence if the magnetization directions cancel each other. On the other hand, a magnetically stable SD
is always magnetized to saturation and has a remanent magnetization at room
temperature. It must be noted that because to their small volume, SP particles loose
“their remanence in time spans of seconds to nano-seconds, and can be considered,
for practical reasons, to acquire only an induced magnetization in the presence of a
magnetic fi eld,” (Davila 2005 ).
Biogenic magnetite can be generated by a wide spectrum of mechanisms, which
differ in the degree of control the organism has over the mineralization process
(Schüler 2006 ). Thus, Lowenstam clearly defi ned biologically controlled (BCM)
and biologically induced (BIM) mineralization. He describes BIM as the process
with the least biological control. In the case of magnetite, BIM is often related to
dissimilatory iron-reducing bacteria. During respiration, these absorb ferric ions
(Fe
3+ ) in the form of amorphous ferric oxy-hydroxide and export ferrous ions
(Fe
2+ ) into the environment, where they interact with excess ferric oxy-hydroxide
resulting in extracellular magnetite precipitates that usually resemble those formed
inorganically (Frankel and Blakemore 1991 ). Thus BIM of magnetite occurs
through chemical changes in the environment due to biological activity. The mechanisms governing the formation of such particles are seemingly altered by evolution
and natural selection to a low extent; hence BIM of magnetite has likely remained
practically invariant throughout geological time (Davila 2005 ).
BCM of magnetite, on the other hand, is to be found both in bacterial and eukaryote
cells with “a remarkable level of control of the particle size, shape, composition and
structure” (Davila 2005 ; see for review Schüler 2006 ). Contrary to what occurs in
BIM processes, the degree of control that organisms exhibit in BCM processes is
only achievable as a result of long time of evolution of the biological functions in
which these particles are involved.
Single biogenic magnetite nanoparticles (Wei et al. 2011 ) seem to be the basic
structural unit in prokaryote. In 1975, Richard Blakemore discovered a new type of
bacterium with an unusual preference to move along the local geomagnetic fi eld
lines (Blakemore 1975 ). These magnetotactic bacteria (MTB) have since then been
thoroughly studied (DeLong et al. 1993 ) and the nature of their magnetotactic
behavior is now fairly well understood, and linked to the presence of intracellular
chains of magnetically stable single-domain (SD) magnetic crystals. Cells of
magnetotactic bacteria include of one or more chains of magnetically stable SD
crystals of magnetite (iron-oxide type) or greigite (iron-sulfi de type), the so-called
magnetosomes, which often appear enveloped by a phospholipid membrane (see for
review Davila 2005 ; Schüler 2006 ) (Fig. 3.28 ).
3.5 Biomagnetite in Marine Vertebrates
which are determined by the number of magnetic domains contained within the
particle (the domain state). The domain state determines the magnetic properties
which will, in turn, be selected according to physiological requirements and the role
the magnetite particles play in the organism, (Davila 2005 ).
Magnetic particle with uniform direction of spontaneous magnetization are the
basic elements of corresponding magnetic domains. However, situations where
different domains within a single particle may have different directions are also
known. Correspondingly, an MD particle can show zero remanence if the magnetization directions cancel each other. On the other hand, a magnetically stable SD
is always magnetized to saturation and has a remanent magnetization at room
temperature. It must be noted that because to their small volume, SP particles loose
“their remanence in time spans of seconds to nano-seconds, and can be considered,
for practical reasons, to acquire only an induced magnetization in the presence of a
magnetic fi eld,” (Davila 2005 ).
Biogenic magnetite can be generated by a wide spectrum of mechanisms, which
differ in the degree of control the organism has over the mineralization process
(Schüler 2006 ). Thus, Lowenstam clearly defi ned biologically controlled (BCM)
and biologically induced (BIM) mineralization. He describes BIM as the process
with the least biological control. In the case of magnetite, BIM is often related to
dissimilatory iron-reducing bacteria. During respiration, these absorb ferric ions
(Fe
3+ ) in the form of amorphous ferric oxy-hydroxide and export ferrous ions
(Fe
2+ ) into the environment, where they interact with excess ferric oxy-hydroxide
resulting in extracellular magnetite precipitates that usually resemble those formed
inorganically (Frankel and Blakemore 1991 ). Thus BIM of magnetite occurs
through chemical changes in the environment due to biological activity. The mechanisms governing the formation of such particles are seemingly altered by evolution
and natural selection to a low extent; hence BIM of magnetite has likely remained
practically invariant throughout geological time (Davila 2005 ).
BCM of magnetite, on the other hand, is to be found both in bacterial and eukaryote
cells with “a remarkable level of control of the particle size, shape, composition and
structure” (Davila 2005 ; see for review Schüler 2006 ). Contrary to what occurs in
BIM processes, the degree of control that organisms exhibit in BCM processes is
only achievable as a result of long time of evolution of the biological functions in
which these particles are involved.
Single biogenic magnetite nanoparticles (Wei et al. 2011 ) seem to be the basic
structural unit in prokaryote. In 1975, Richard Blakemore discovered a new type of
bacterium with an unusual preference to move along the local geomagnetic fi eld
lines (Blakemore 1975 ). These magnetotactic bacteria (MTB) have since then been
thoroughly studied (DeLong et al. 1993 ) and the nature of their magnetotactic
behavior is now fairly well understood, and linked to the presence of intracellular
chains of magnetically stable single-domain (SD) magnetic crystals. Cells of
magnetotactic bacteria include of one or more chains of magnetically stable SD
crystals of magnetite (iron-oxide type) or greigite (iron-sulfi de type), the so-called
magnetosomes, which often appear enveloped by a phospholipid membrane (see for
review Davila 2005 ; Schüler 2006 ) (Fig. 3.28 ).
3.5 Biomagnetite in Marine Vertebrates