| A freshwater magnetotactic bacterium. The chain of dark
objects are crystals of the mineral magnetite (Fe3O4),
which have the proper size and shape to behave as perfect, single magnetic
domains. The largest crystals are about 70 nm in length. (Photo credit:
A. Kobayashi). Originally discovered by Blakemore[1],
these bacteria are know known to leave these crystals of biologically-precipitated
magnetite in marine and freshwater sediments as 'magnetofossils', and
are responsible for much of the remanent magnetization of marine sediments[2-8].
They may also be present in the carbonate blebs of the ALH84001 Martian
Meteorite[9], and as such may be one
of the strongest remaining bits of evidence for ancient life on Mars.
[1] R. P. Blakemore, "Magnetotactic bacteria,"
Science, vol. 190, pp. 377-379, 1975.
[2] J. L. Kirschvink and H. A. Lowenstam, "Mineralization and magnetization
of chiton teeth: Paleomagnetic, sedimentologic, and biologic implications
of organic magnetite," Earth & Planetary Science Letters,
vol. 44, pp. 193-204, 1979.
[3] J. L. Kirschvink, "Paleomagnetic evidence for fossil biogenic
magnetite in western Crete," Earth & Planetary Science Letters,
vol. 59, pp. 388-392, 1982.
[4] J. L. Kirschvink and S.-B. R. Chang, "Ultra fine-grained magnetite
in deep-sea sediments: possible bacterial magnetofossils," Geology,
vol. 12, pp. 559-562, 1984.
[5] S.-B. R. Chang and J. L. Kirschvink, "Magnetofossils, the magnetization
of sediments, and the evolution of magnetite biomineralization,"
Annual Reviews of Earth & Planetary Sciences, vol. 17, pp. 169-195,
1989.
[6] H. Vali, O. Forster, G. Amarantidis, and N. Petersen, "Magnetotactic
bacteria and their magnetofossils in sediments," Earth and Planetary
Science Letters, vol. 86, pp. 389-400, 1987.
[7] H. Vali and J. L. Kirschvink, "Magnetofossil Dissolution in
a Paleomagnetically Unstable Deep-Sea Sediment," Nature,
vol. 339, pp. 203-206, 1989.
[8] H. Vali, T. von Dobeneck, O. Amarantidis, O. Forster, G. Morteani,
L. Bachmann, and N. Petersen, "Biogenic and Lithogenic Magnetic
Minerals in Atlantic and Pacific Deep Sea Sediments and Their Paleomagnetic
Significance," Geologische Rundschau, vol. 78, pp. 753-764,
1989.
[9] D. McKay, E. Gibson, K. Thomaskeprta, H. Vali, C. Romanek, S. Clemett,
X. Chillier, C. Maechling, and R. Zare, "Search for past life on
Mars - Possible relic biogenic activity in Martian Metoerite ALH84001,"
Science, vol. 273, pp. 924-930, 1996.
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