Chemistry Reference
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using a temporal sensory mechanism (Segall et al. 1996) that samples the local
environment as they swim and compares the present [O
2
] with that in the recent past
(Taylor 1983). The change in [O
2
] with time determines the sense of flagellar rotation
(Manson 1992). The behavior of individual cells of
M. magnetotacticum
in aerotactic
bands in thin capillaries is consistent with the temporal sensory mechanism (Frankel et
al. 1997). Thus, cells in the band which are moving away from the optimal [O
2
], to
either higher or lower [O
2
], eventually reverse their swimming direction and return to
the band.
Unlike the magnetotactic spirilla, the bilophotrichously-flagellated (possessing two
bundles of flagella on one side of the cell), magnetotactic cocci (Blakemore 1975;
Frankel et al. 1997; Moench 1988; Moench and Konetzka 1978) and some other
magnetotactic bacteria, swim persistently in a preferred direction relative to B when
viewed microscopically in wet mounts (Blakemore 1975; Frankel et al. 1997). The
persistent swimming direction of populations of magnetotactic cocci led to the discovery
of magnetotaxis in bacteria (Blakemore et al. 1975). However, magnetotactic cocci in
oxygen gradients, like cells of
Magnetospirillum magnetotacticum
, can also swim in
both directions along B without turning around (Frankel et al. 1997). The cocci, like
cells of
M. magnetotacticum
, also form microaerophilic, aerotactic bands of cells
seeking a preferred [O
2
] along the concentration gradient (Frankel et al. 1997). However,
while the aerotactic behavior of
M. magnetotacticum
is consistent with the temporal
sensory mechanism, the aerotactic behavior of the cocci is not. Instead their behavior is
consistent with a two-state aerotactic sensory model in which the [O
2
] determines the
sense of the flagellar rotation and hence the swimming direction relative to B. Cells at
[O
2
] higher than optimum swim persistently in one direction relative to B until they
reach a low [O
2
] threshold at which they reverse flagellar rotation, and hence, swimming
direction relative to B. They continue swimming until they reach a high [O
2
] threshold at
which they reverse again. In wet mounts, the [O
2
] is above optimal, and the cells swim
persistently in one direction relative to B. This model, termed polar magneto-aerotaxis
(Frankel et al. 1997), accounts for the ability of the magnetotactic cocci to migrate to
and maintain position at the preferred [O
2
] at the OATZ in chemically-stratified, semi-
anaerobic basins. Frankel et al. (1998) have developed an assay using chemical gradients
in thin capillaries that distinguishes between axial and polar magneto-aerotaxis.
For both aerotactic mechanisms, migration along magnetic field lines reduces a
three-dimensional search problem to a one-dimensional search problem. Magnetotaxis is
advantageous to motile microorganisms in vertical concentration gradients because it
increases the efficiency of finding and maintaining an optimal position in vertical
concentration gradients, in this case, a vertical oxygen gradient. It is possible that there
are other forms of magnetically-assisted chemotaxis to molecules or ions other than
oxygen, such as sulfide, or magnetically-assisted redox- or phototaxis in bacteria that
inhabit the anaerobic zone (e.g., greigite-producers) in chemically-stratified waters and
sediments.
The function of cellular magnetotaxis described above is a consequence of the cell
possessing magnetosomes. There is some conflicting evidence for the role of
magnetosomes in magnetotaxis: many obligately microaerophilic bacteria find and
maintain an optimal position at the OATZ without the aid of magnetosomes and cultured
magnetotactic bacteria form microaerophilic bands of cells in the absence of a magnetic
field. It therefore seems likely that iron uptake and magnetosome production is somehow
also linked to the physiology of the cell and to other cellular functions as yet unknown.
