HR: 17:15h
AN: GP34A-06    [Abstracts]
TI: A Possible Magnetite/Maghemite Electrochemical Battery in the Magnetotactic Bacteria
AU: * Kopp, R E
EM: rkopp@caltech.edu
AF: California Institute of Technology, Geological and Planetary Sciences, MC 170-25, Pasadena, CA 91125 United States
AU: Nash, C Z
EM: cody@caltech.edu
AF: California Institute of Technology, Geological and Planetary Sciences, MC 170-25, Pasadena, CA 91125 United States
AU: Kirschvink, J L
EM: kirschvink@caltech.edu
AF: California Institute of Technology, Geological and Planetary Sciences, MC 170-25, Pasadena, CA 91125 United States
AU: Leadbetter, J R
EM: jleadbetter@caltech.edu
AF: California Institute of Technology, Environmental Science and Engineering, MC 138-78, Pasadena, CA 91125 United States
AB: Magnetotactic bacteria produce membrane-bound chains of magnetic minerals (magnetosomes) within their cells. The purposes of magnetosomes, which include orientation in a redox gradient (1), are not fully characterized. A better understanding will assist in interpreting the magnetofossil record. Magnetotactic bacteria are known to make magnetosomes of either magnetite or greigite, both of which are mixed valence minerals with cubic inverse spinel structures. Vali and Kirschvink (2) proposed that magnetite producers might transform their magnetosomes into a third ferrimagnetic spinel, maghemite, to generate energy: a sort of electrochemical battery. Simple calculations suggest that, by transforming magnetite to maghemite in oxidizing waters and back to magnetite in reducing waters, a bacterium could access ~90 mV of additional redox potential. The energy necessary to traverse a cm-scale redox gradient and tap this energy is less than the additional energy provided by complete oxidation of a single magnetosome. We are currently testing the battery hypothesis by several methods. Initial experiments involved innoculation of dense suspensions of Magnetospirillum magneticum AMB-1 into an oxygen gradient medium. After several days, samples were collected from the high-oxygen top and low-oxygen bottom of the medium and freeze-dried. Low temperature magnetic properties were measured to test for magnetosome oxidation, the presence of magnetite chains, and the size of the magnetic particles. Variation in the strength of the Verwey transition indicated that magnetosomes from high-oxygen conditions were more oxidized than those from low-oxygen conditions, as the magnetosome battery hypothesis would predict. The Moskowitz test (4, 5) was used to test for the presence of chains. While the strength of the test weakens with magnetosome oxidation, results from both high- and low-oxygen conditions suggest the presence of intact chains. Both low-temperature and hysteresis properties indicate that the magnetosomes remained as single-domain grains, rather than forming the superparamagnetic rims typical of abiotic oxidation. Initial results therefore support the magnetosome battery hypothesis. If a magnetosome battery does exist, then magnetofossils may indicate the presence of sharp redox gradients and provide information about chemical stratification in ancient environments. Magnetite/maghemite batteries would reflect more oxidizing conditions than greigite/mackinawite batteries; assuming the magnetofossil record can be extended through most of the Precambrian, greigite-derived magnetofossils may be expected to dominate the magnetofossil record of the Archean, before the rise of free oxygen. Some of the maghemitization observed in many magnetofossils may reflect biologically-controlled oxidation rather than diagenetic oxidation. 1. R. B. Frankel, D. A. Bazylinski, M. S. Johnson, B. L. Taylor, Biophys. J. 73, 994 (1997). 2. H. Vali, J. L. Kirschvink, in Iron biominerals R. B. Frankel, R. P. Blakemore, Eds. (Plenum Press, New York, 1990) pp. 97-115. 3. A. R. Muxworthy, E. McClelland, Geophys. J. Intl. 140, 101 (2000). 4. B. M. Moskowitz, R. B. Frankel, D. A. Bazylinski, EPSL 120, 283 (1993). 5. B. P. Weiss et al., EPSL 224, 73 (2004).
DE: 1505 Biomagnetism
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
DE: 1615 Biogeochemical processes (4805)
DE: 0400 Biogeosciences
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting