155
Static, Low-Frequency, and Pulsed Magnetic Fields
predicted to be a functionally active site of the CRY molecule (Solov’yov, Chandler, and
Schulten 2007). The unpaired electron spins (S 1 and S 2 ) precess about a local magnetic
field produced by the addition of the external magnetic field B with contributions I 1 and
I 2 from the nuclear spins on the two radicals. The spin precession continuously alters the
relative spin orientation, causing singlet (antiparallel) to triplet (parallel) interconversion, which underlies the magnetic field effect. Electron back-transfer from tryptophan
to FADH quenches CRY’s active state. However, this back-transfer can take place only
when the electron spins are in the singlet state, and this dependence on spin allows the
external magnetic field, B, to affect CRY activation.
3.4.2 Magnetotaxis
Magnetotactic bacteria are microorganisms that orient and migrate along magnetic
field lines. Magnetotactic bacteria contain membrane-bound intracellular iron crystals
called magnetosomes (Komeili et al. 2006). Magnetosomes comprise a magnetic nanocrystal surrounded by a lipid bilayer membrane. These unique prokaryotic organelles
align inside magnetotactic bacterial cells and serve as an intracellular compass allowing the bacteria to navigate along the geomagnetic field in aquatic environments. Polar
magnetotactic bacteria in vertical chemical gradients are thought to respond to high
oxygen levels by swimming downward into areas with low or no oxygen (toward geomagnetic north in the Northern Hemisphere and geomagnetic south in the Southern
Hemisphere). Simmons, Bazylinski, and Edwards (2006) identified populations of polar
magnetotactic bacteria in the Northern Hemisphere that respond to high oxygen levels
by swimming toward geomagnetic south, which is the opposite of all previously reported
magnetotactic behavior. The percentage of magnetotactic bacteria with south polarity in
the environment is positively correlated with high redox potential. The coexistence of
magnetotactic bacteria with opposing polarities in the same redox environment conflicts with current models of the adaptive value of magnetotaxis.
Two mechanisms of magnetotaxis have been described: (1) Axial magnetotactic cells
swim in both directions along magnetic field lines. (2) In contrast, polar magnetotactic
cells swim either parallel to the geomagnetic field lines toward the North Pole (north
seeking) or antiparallel toward the South Pole (south seeking). Lefèvre et al. (2009)
used a magnetospectrophotometry (MSP) assay to characterize both the axial magnetotaxis of Magnetospirillum magneticum strain AMB-1 and the polar magnetotaxis
of magneto-ovoid strain MO-1. Two pairs of Helmholtz coils were mounted onto the
cuvette holder of a common laboratory spectrophotometer to generate two mutually
perpendicular homogeneous magnetic fields parallel or perpendicular to the light beam.
The application of magnetic fields allowed measurements of the change in light scattering resulting from cell alignment in a magnetic field or in absorbance due to bacteria
swimming across the light beam. The results showed that MSP is a powerful tool for the
determination of bacterial magnetism and the analysis of alignment and swimming of
magnetotactic bacteria in magnetic fields. Moreover, this assay is useful for characterizing south-seeking derivatives and non-magnetosome-bearing strains obtained from
north-seeking MO-1 cultures. These results suggest that oxygen is a determinant factor
that controls magnetotactic behavior.
Static, Low-Frequency, and Pulsed Magnetic Fields
predicted to be a functionally active site of the CRY molecule (Solov’yov, Chandler, and
Schulten 2007). The unpaired electron spins (S 1 and S 2 ) precess about a local magnetic
field produced by the addition of the external magnetic field B with contributions I 1 and
I 2 from the nuclear spins on the two radicals. The spin precession continuously alters the
relative spin orientation, causing singlet (antiparallel) to triplet (parallel) interconversion, which underlies the magnetic field effect. Electron back-transfer from tryptophan
to FADH quenches CRY’s active state. However, this back-transfer can take place only
when the electron spins are in the singlet state, and this dependence on spin allows the
external magnetic field, B, to affect CRY activation.
3.4.2 Magnetotaxis
Magnetotactic bacteria are microorganisms that orient and migrate along magnetic
field lines. Magnetotactic bacteria contain membrane-bound intracellular iron crystals
called magnetosomes (Komeili et al. 2006). Magnetosomes comprise a magnetic nanocrystal surrounded by a lipid bilayer membrane. These unique prokaryotic organelles
align inside magnetotactic bacterial cells and serve as an intracellular compass allowing the bacteria to navigate along the geomagnetic field in aquatic environments. Polar
magnetotactic bacteria in vertical chemical gradients are thought to respond to high
oxygen levels by swimming downward into areas with low or no oxygen (toward geomagnetic north in the Northern Hemisphere and geomagnetic south in the Southern
Hemisphere). Simmons, Bazylinski, and Edwards (2006) identified populations of polar
magnetotactic bacteria in the Northern Hemisphere that respond to high oxygen levels
by swimming toward geomagnetic south, which is the opposite of all previously reported
magnetotactic behavior. The percentage of magnetotactic bacteria with south polarity in
the environment is positively correlated with high redox potential. The coexistence of
magnetotactic bacteria with opposing polarities in the same redox environment conflicts with current models of the adaptive value of magnetotaxis.
Two mechanisms of magnetotaxis have been described: (1) Axial magnetotactic cells
swim in both directions along magnetic field lines. (2) In contrast, polar magnetotactic
cells swim either parallel to the geomagnetic field lines toward the North Pole (north
seeking) or antiparallel toward the South Pole (south seeking). Lefèvre et al. (2009)
used a magnetospectrophotometry (MSP) assay to characterize both the axial magnetotaxis of Magnetospirillum magneticum strain AMB-1 and the polar magnetotaxis
of magneto-ovoid strain MO-1. Two pairs of Helmholtz coils were mounted onto the
cuvette holder of a common laboratory spectrophotometer to generate two mutually
perpendicular homogeneous magnetic fields parallel or perpendicular to the light beam.
The application of magnetic fields allowed measurements of the change in light scattering resulting from cell alignment in a magnetic field or in absorbance due to bacteria
swimming across the light beam. The results showed that MSP is a powerful tool for the
determination of bacterial magnetism and the analysis of alignment and swimming of
magnetotactic bacteria in magnetic fields. Moreover, this assay is useful for characterizing south-seeking derivatives and non-magnetosome-bearing strains obtained from
north-seeking MO-1 cultures. These results suggest that oxygen is a determinant factor
that controls magnetotactic behavior.