Chemistry Reference
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are thought to be precursors to greigite (Pósfai et al. 1998a,b). Magnetic, monoclinic,
pyrrhotite (Fe
7
S
8
) (Farina et al. 1990) and non-magnetic iron pyrite (FeS
2
) (Mann et al.
1990) were also identified in a many-celled magnetotactic prokaryote that produces iron-
sulfide magnetosomes but likely represent misinterpretations of electron diffraction patterns.
An unusual rod-shaped bacterium, collected from the OATZ in the Pettaquamscutt Estuary,
produces both iron-oxide and iron-sulfide magnetosomes (Bazylinski et al. 1993b, 1995),
but has not been isolated and grown in pure culture. Figure 1 shows magnetite and greigite
crystals co-aligned in a double chain within a cell of this organism.
The mineral composition of the magnetosome appears to be under strict chemical
control since cells of several cultured magnetotactic bacteria continue to synthesize
magnetite, not greigite, even when hydrogen sulfide is present in the growth medium
(Meldrum et al. 1993a,b). Moreover, magnetite crystals in magnetosomes are of high
chemical purity (Bazylinski 1995; Bazylinski and Frankel 2000b) and reports of impurities
such as other metal ions within the particles are rare. Gorby (1989) showed that iron could
not be replaced by other transition metal ions, including titanium, chromium, cobalt,
copper, nickel, mercury, and lead, in the magnetite crystals of
Magnetospirillum
magnetotacticum
when cells were grown in the presence of these ions. Towe and Moench
(1981) reported trace amounts of titanium in the magnetite particles of an uncultured
freshwater magnetotactic coccus collected from a wastewater treatment pond. Significant
amounts of copper have been found in the greigite particles of a many-celled,
magnetotactic microorganism (Bazylinski et al. 1993a). The concentration of copper was
extremely variable and ranged from about 0.1 to 10 atomic % relative to iron. The copper
appeared to be mostly concentrated on the surface of the particles and was only present in
those organisms collected from a salt marsh in Morro Bay (CA, USA) and not in those
collected from other sites. The presence of copper in the magnetosomes did not appear to
affect their function since the organisms were still magnetotactic. Interestingly,
magnetosome crystals in greigite-producing, rod-shaped magnetotactic bacteria collected
from the same site did not contain copper. However, copper was present in the greigite-
particles of rod-shaped magnetotactic bacteria collected from other sites (Pósfai et al.
1998b). These observations might indicate that the mineral phase of the magnetosomes in
Figure 1.
Brightfield scanning-transmission electron micrograph of tooth-shaped magnetite (m at
arrows) and pleomorphic greigite (g at arrows) magnetosome crystals co-organized within the same
chains in an uncultured magnetotactic bacterium collected from the oxic-anoxic transition zone of the
Pettaquamscutt Estuary, RI.
