Biomedical Engineering Reference
In-Depth Information
dispelling the proposed dual evolution.
D. magneticus
respires on sulfates and
thiosulfates, is strictly anaerobic, but is also chemo-organoheterotrophic and uti-
lizes organic acids as carbon and electron sources. In order to further dispel any
discrete relationship between mineral and phylogenetics, an uncharacterized mag-
netic bacterium has been observed to contain both magnetite and greigite particles
within the same cell (and even the same chain), with the ratio of magnetite to
greigite depending on the available oxygen and sulfi de in the environment [39, 40].
The diversity of magnetic bacteria was further illustrated by the discovery of an
exotic strain from the
Nitrospira phylum
prokaryotic group.
Magnetobacterium bava-
ricum
is a very large magnetic bacterium that has a number of chains of several
hundred tooth-shaped magnetite magnetosomes [40]. The recent sampling of hot
springs and saline lakes has revealed yet more diversity, with magnetic bacteria
being observed in extreme environments, and the suggested tentative discovery of
magnetotactic archaea [42]. Thus, as more magnetic bacteria are discovered and
isolated from more diverse environments, it can be seen that magnetotaxis does
not have a simple evolutionary lineage. Rather, with its very wide phylogenetic
distribution, magnetotaxis is most likely the result of horizontal gene transfer, or
might even be an ancient underlying trait. The details of some of these different
bacteria are listed in Table 11.2.
In recent years there has been much debate over the purpose of magnetosomes
and magnetotaxis. It appears that magnetotaxis is a deliberate trait [1], as the
magnetism of the magnetosomes is optimized by the single-domain crystals in
the chain formation providing the maximum magnetic moment. Initially, mag-
netotaxis was thought to have a simple navigational role, since swimming towards
the pole takes the bacteria down the inclination angle of the Earth's magnetic fi eld
towards their preferred environment [43]. It was also noted that magnetic bacteria
found in the Northern Hemisphere were exclusively north- seeking [31] , and
vice-
versa
for the Southern Hemisphere [44]. Unfortunately, however, this theory has
several fl aws: fi rst, many magnetic bacteria orientate axially, swimming in either
direction along the fi eld line; and second, magnetic bacteria show an aerotaxial
dominance, leading to a modifi ed magneto-aerotaxis model. Here, it is suggested
that aerotaxis is aided by magnetic navigation, although in practical terms this is
not the case. While magnetic bacteria orientate at the correct oxygen tension faster
in a magnetic fi eld than do their nonmagnetic counterparts [45], this difference is
not very signifi cant and is negligible under the Earth's magnetic fi eld. Indeed, it
appears that this navigational role of the magnetosomes is advantageous, but not
benefi cial enough to warrant the cells' large biological investment into magneto-
some synthesis; this suggests that navigation has an advantageous and coinciden-
tal secondary role. The third - and most important - fl aw is seen when it is
considered that magnetosome synthesis does not occur under aerobic conditions
(
2% oxygen) [46]. The question then is that, if the purpose of magnetosomes is
to navigate to microaerobic environments, why are they absent when they are
needed the most? This again suggests navigation as a secondary feature, and
therefore the primary question of why magnetosomes are synthesized remains
unclear. It has been proposed that magnetosomes might have a detoxifying
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