Oxygen and Iron Isotope Studies of Magnetite Produced by Magnetotactic Bacteria

A series of carefully controlled laboratory studies was carried out to investigate oxygen and iron isotope fractionation during the intracellular production of magnetite (Fe$_3$O$_4$) 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 Fe$_3$O$_4$ and water that is consistent with that observed for Fe$_3$O$_4$ produced extracellularly by thermophilic Fe$^{3+}$-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 Fe$_3$O$_4$ may be useful in paleoenvironmental studies for determining the oxygen-18 isotope values of formation waters and for inferring paleotemperatures.