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Science 17 September 1999:
Vol. 285. no. 5435, pp. 1892 - 1896
DOI: 10.1126/science.285.5435.1892
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Reports

Oxygen and Iron Isotope Studies of Magnetite Produced by Magnetotactic Bacteria

Kevin W. Mandernack, 1* Dennis A. Bazylinski, 2 Wayne C. Shanks III, 3 Thomas D. Bullen 4

A series of carefully controlled laboratory studies was carried out to investigate oxygen and iron isotope fractionation during the intracellular production of magnetite (Fe3O4) by two different species of magnetotactic bacteria at temperatures between 4° and 35°C under microaerobic and anaerobic conditions. No detectable fractionation of iron isotopes in the bacterial magnetites was observed. However, oxygen isotope measurements indicated a temperature-dependent fractionation for Fe3O4 and water that is consistent with that observed for Fe3O4 produced extracellularly by thermophilic Fe3+-reducing bacteria. These results contrast with established fractionation curves estimated from either high-temperature experiments or theoretical calculations. With the fractionation curve established in this report, oxygen-18 isotope values of bacterial Fe3O4 may be useful in paleoenvironmental studies for determining the oxygen-18 isotope values of formation waters and for inferring paleotemperatures.

1 Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401, USA.
2 Department of Microbiology, Iowa State University, Ames, IA 50011, USA.
3 U.S. Geological Survey, 973 Denver Federal Center, Denver, CO 80225, USA.
4 Water Resources Division, U.S. Geological Survey, MS 420, 345 Middlefield Road, Menlo Park, CA 94025, USA.
*   To whom correspondence should be addressed. E-mail: kmandern{at}mines.edu

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Fe Isotope Variations in the Modern and Ancient Earth and Other Planetary Bodies.
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Science. ISSN 0036-8075 (print), 1095-9203 (online)