Oxygen and iron isotope studies of magnetite produced by magnetotactic bacteria

A series of carefully controlled laboratory studies was carried out to inve stigate oxygen and iron isotope fractionation during the intracellular prod uction of magnetite (Fe3O4) by two different species of magnetotactic bacte ria at temperatures between 4 degrees and 35 degrees C under microaerobic a nd anaerobic conditions. No detectable fractionation of iron isotopes in th e bacterial magnetites was observed. However, oxygen isotope measurements i ndicated a temperature-dependent fractionation for Fe3O4 and water that is consistent with that observed for Fe3O4 produced extracellularly by thermop hilic Fe3+-reducing bacteria. These results contrast with established fract ionation curves estimated from either high-temperature experiments or theor etical calculations. With the fractionation curve established in this repor t, oxygen-18 isotope Values of bacterial Fe3O4 may be useful in paleoenviro nmental studies for determining the oxygen-18 isotope values of formation w aters and for inferring paleotemperatures.