 |
| Titel |
Investigation of magnetotaxis of magnetotactic bacteria in water and sediment |
| VerfasserIn |
X. Mao, R. Egli |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250059749
|
|
|
|
|
|
| Zusammenfassung |
| Magnetotactic bacteria contain chains of magnetic particles which allow them to align and
move along the magnetic field to search for optimal habitats in chemically stratified
environments. This phenomenon is known as magnetotaxis. The alignment is passive,
and driven by the torque mÃB between the magnetic dipole m of the chain and
the external magnetic field B. The alignment is counteracted by the randomizing
effect of Brownian motion, and the average alignment of free bacteria in water,
expressed by cos(θ), where θ is the angle between m and B, is expected to follow the
Langevin law cos(θ) = L(mB/kT) where k is the Boltzmann constant and T the absolute
temperature [Frankel, 1980]. This law implies that a minimum strength for the
Earth’s field is required for a bacterium with given magnetic moment to have a
certain alignment with the field. The resulting motion is a biased random walk whose
“efficiency” for a displacement along B is equal to the average aligmnent. Calculated
values for cos(θ) in the present Earth’s field give good alignments >0.9. Direct
experimental observations of the Langevin law on living bacteria in various field
intensity are limited to one study by [Kalmijn, 1981]. Here we report systematic
observations for two types of wild-type bacteria: magnetic cocci containing two to five
chains of ~100 magnetosomes, and magnetobacterium Bavaricum , a rod-shaped
bacterium containing several hundreds magnetosomes [Hanzlik et al., 1996]. We also
investigated the aligment of those bacteria in natural sediment, where physical
constraints imposed by the pore volume are expected to have a great influence on their
capability of to align with an external field. Our results show that the alignment of
free swimming magnetotactic bacteria in water obeys Langevin’s law, with >0.8
alignments in external fields of 3μT for Bavaricum and 30μT for big cocci (the
present earth’s field is at avrage 50μT). This result shows that magnetotactic bacteria,
and in particular Bavaricum, synthesize more than 10 times magnetosomes than
they need for magnetotaxis in present field. This fact could leat to the suggestion
that magnetosomes could have other unknown functions besides navigation in a
magnetic field. On the other hand, typical sedimentary environments pose drastically
different conditions for magnetotaxis. This is shown by our measurements of bacteria’s
alignment in acquaria containing natural lake sediments. Because direct observation of
bacteria in sediment is not possible, we applied strong pulsed magnetic field to
reverse the magnetic moment of bacteria whose magnetic moment forms a >90Ë
angle to the pulse field. These bacteria become South-seeking and can be counted.
By applying pulses along different directions with respect to the external field it
is possible to calculate their alignment from the ratio between North- and South
seeking.
Our results indicate that the bacteria’s alignment in sediment is |
|
|
|
|
|
|