HR: 17:15h
AN: B24A-06    [Abstracts]
TI: Magnetic properties of magnetosomes in cultured magnetotactic bacteria and in sediments: relationships with the environment
AU: Moskowitz, B
EM: bmosk@umn.edu
AF: Institute for Rock Magnetism, 291 Shepherd Laboratories 100 Union Street SE, Minneapolis, MN 55455 United States
AU: * Egli, R
EM: eglix007@umn.edu
AF: Institute for Rock Magnetism, 291 Shepherd Laboratories 100 Union Street SE, Minneapolis, MN 55455 United States
AB: Since the discover of magnetotactic bacteria in 1975, their occurrence has been recognized in a wide range of environments. In freshwater and marine sediments, the magnetic crystals produced inside these bacteria (magnetosomes) account typically for 20-60% of the bulk magnetization, and up to 80% in some special cases. Therefore, magnetic measurements can be used to quantify the total amount of magnetosomes in a sample, and, in some cases, to identify differences in their magnetic properties that can be related to the environment. The identification of magnetosomes with magnetic measurements is based on unique properties, that are due to the very narrow grain size distribution in the single-domain range, and the arrangement in chains. One identification method is based on the properties of the Verwey transition of magnetite magnetosomes. Another method is based on the very narrow distribution of coercivities that characterizes cultures of magnetotactic bacteria as well as magnetosomes identified in natural sediments. Two distinct groups of magnetosomes with median remanence coericivities of 41 and 66 mT, respectively, could be identified systematically in the sediments of different lakes by the analysis of their coercivity distributions. Interestingly, the relative abundance of these two groups in the sediment is clearly correlated to well-known changes of the environmental conditions. The high-coercivity magnetosomes prevail over the other group in anoxic environments. The reason for the systematic occurrence of two groups of magnetosomes characterized by different coercivities is not clear. We suppose that the difference between the two groups is related to the elongation of the magnetosomes or their spatial arrangement within a cell. This hypothesis is supported by the observation that elongated magnetosomes prevail in anoxic marine sediments. We summarize and compare all known magnetic properties of cultured and uncultured magnetosomes, and the temporal stability of some properties. We also show how it is possible to identify and eventually quantify the occurrence of magnetosomes in natural samples with magnetic techniques on the example of lacustrine and marine sediments.
DE: 1505 Biogenic magnetic minerals
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
SC: Biogeosciences [B]
MN: Fall Meeting 2005