Chemistry Reference
In-Depth Information
cell has maximized its magnetic dipole moment by arranging the magnetosomes in
chains. The magnetic dipole moment of the cell is generally large enough so that its
interaction with the Earth's geomagnetic field overcomes the thermal forces tending to
randomize the cell's orientation in its aqueous surroundings (Frankel 1984). In many
magnetotactic bacteria, the newly-formed crystals appear to be at the end of
magnetosome chains within the cell (Bazylinski and Frankel 2000a) suggesting that the
magnetosome chain increases in size by the precipitation of new magnetosomes at the
ends of the chain following cell division. However, there are some magnetotactic bacteria
that do not show this pattern and have large gaps between magnetosomes where new
magnetosomes could be formed (Bazylinski et al. 1995). Multiple chains of particles
appear to be more common in those bacteria that produce greigite or tooth-, bullet-, and
arrowhead-shaped magnetite crystals (Figs. 1, 3, 6).
There are some magnetotactic bacteria that do not have their magnetosomes arranged
in chains, instead producing a clump at one end of the cell or clumps within partial
chains. These include some magnetite-producing cocci (Moench 1988; Cox et al. 2002),
some greigite-producing, rod-shaped bacteria (Heywood et al. 1990), and the greigite-
producing, many-celled, magnetotactic prokaryote (Mann et al. 1990; Pósfai et al.
1998a,b). Nonetheless, even these organisms clearly show that they have a net magnetic
dipole moment.
A rod-shaped magnetotactic bacterium, collected from the OATZ from the
Pettaquamscutt Estuary (discussed earlier), was found to contain arrowhead-shaped
Figure 6.
Scanning transmission electron
micrograph of: (a) an uncultured
magnetotactic bacterium that produces
multiple parallel chains of greigite; (b)
high magnification image of a portion of
the multiple chains showing that this
organism synthesizes elongated greigite
crystals with a quasi-rectangular projected
shape.
