Chemistry Reference
In-Depth Information
crystals of magnetite and rectangular prismatic crystals of greigite co-organized within
the same chains of magnetosomes (Fig. 1) (Bazylinski et al. 1993b, 1995). The magnetite
and greigite crystals in this organism occur with different, mineral-specific morphologies
and sizes and are positioned with their long axes oriented along the chain direction (Fig.
1). Both particle morphologies have been found in organisms with single mineral
component chains (Mann et al. 1987a,b; Heywood et al. 1990) suggesting that the
magnetosome membranes surrounding the magnetite and greigite particles contain
different nucleation templates and that there are differences in magnetosome vesicle
biosynthesis. Thus, it is likely that two separate sets of genes control the
biomineralization of magnetite and greigite in this organism (Bazylinski et al. 1995).
The magnetic dipole moment resulting from the presence of magnetosomes causes
the cell to passively align along geomagnetic field lines while it swims (magnetotaxis).
Cells are neither attracted nor pulled toward either geomagnetic pole. Dead cells
,
like
living cells
,
align along geomagnetic field lines but do not move along them. Living cells
behave like very small, self-propelled, magnetic compass needles (Frankel 1984).
Function and physics of magnetotaxis
Magnetotactic bacteria were originally thought to have one of two magnetic
polarities, north- or south-seeking, depending on the magnetic orientation of the cell's
magnetic dipole with respect to their direction of motion (Blakemore et al. 1980). The
vertical component of the inclined geomagnetic field seems to select for a predominant
polarity in each hemisphere by favoring those cells whose polarity causes them to
migrate down towards the microaerobic sediments and away from potentially high, toxic
concentrations of oxygen in surface waters. This hypothesis appears to be partially true:
north-seeking magnetotactic bacteria predominate in the Northern hemisphere while
south-seeking cells predominate in the Southern hemisphere (Blakemore et al. 1980). At
the Equator, where the vertical component of the geomagnetic field is zero, both
polarities co-exist in approximately equal numbers (Frankel et al. 1981). However, the
discovery of stable populations of magnetotactic bacteria existing at specific depths in
the water columns of chemically-stratified aquatic systems at higher latitudes
(Bazylinski et al. 1995) and the observation that virtually all magnetotactic bacteria in
pure culture form microaerophilic bands of cells below the meniscus of the growth
medium (Frankel et al. 1997) are not consistent with this model of magnetotaxis. For
example, according to this model, persistent, north-seeking, magnetotactic bacteria in
the Northern hemisphere should always be found in the sediments or at the bottom of
culture tubes.
Most free-swimming bacteria including magnetotactic species propel themselves
forward in their aqueous surroundings by rotating their helical flagella (Silverman and
Simon 1974). Unlike cells of
Escherichia coli
and several other chemotactic bacteria,
magnetotactic bacteria do not display “run and tumble” motility (Bazylinski et al. 1995).
Because of their magnetic dipole moment, magnetotactic bacteria align and migrate
along the local magnetic field B. Some species, especially the magnetotactic spirilla
(e.g.,
Magnetospirillum magnetotacticum
) swim parallel or antiparallel to B and form
aerotactic bands (Blakemore et al. 1979; Spormann and Wolfe 1984) at a preferred
oxygen concentration [O
2
]. In a homogeneous medium, approximately equal numbers of
cells swim in either direction along B. Cells use the magnetic field as an axis for
migration, with aerotaxis determining the direction of migration along the axis. This
behavior has thus been termed axial magneto-aerotaxis (Frankel et al. 1997). The
distinction between north-seeking and south-seeking does not apply to axial magneto-
aerotactic bacteria. Most flagellated microaerophilic bacteria form aerotactic bands at a
preferred or optimal [O
2
] where the proton motive force is maximal (Zhulin et al. 1996),
