Chemistry Reference
In-Depth Information
Figure 8.
Transmission electron micrograph of: (a) purified magnetite magnetosomes released from
cells of strain MV-1 negatively stained with 0.5% uranyl acetate; “halo” around crystals represents
magnetosome membrane while material at arrows might indicate additional membranes holding chains
together; (b) the same magnetosomes after treatment with 1% sodium deodecyl sulfate, a detergent that
removes lipids.
Magnetite magnetosomes
Iron uptake in magnetotactic bacteria.
Magnetotactic bacteria consist of up to 3%
iron on a dry weight basis (Blakemore 1982), several orders of magnitude higher than
non-magnetotactic species. However, there is no evidence that they possess unique iron
uptake systems. Fe
2+
, which is soluble up to 100 mM at neutral pH (Neilands 1984), is
generally thought to be taken up by bacteria by non-specific means. In contrast, Fe
3+
is so
insoluble that most microbes produce and rely on iron chelators, called siderophores,
which bind and solubilize Fe
3+
for uptake. Siderophores are defined as low molecular
weight (<1 kDa) specific ligands usually produced by bacterial cells under iron-limited
conditions (Guerinot 1994). Their synthesis is repressed under high iron conditions.
There are a number of studies directed at iron uptake in the magnetotactic bacteria.
Cells of
Magnetospirillum magnetotacticum
were thought to take up iron via a non-
specific transport system (Frankel et al. 1983). Although iron is supplied as Fe
3+
quinate
to the cells, the growth medium also contains chemical reducing agents (e.g., ascorbic
acid) potent enough to reduce Fe
3+
to Fe
2+
. Thus, both reduced and oxidized iron are
present in the growth medium and it is not known which form is taken up by cells.
However, cells of
M. magnetotacticum
were reported to produce a hydroxamate
