Magnetite formation by a sulphate-reducing bacterium

BACTERIAL production of magnetite (Fe₃O₄)¹ makes an important contribution to iron biomineralization and rcmanent magnetization of sediments^2,3. Accurate magnetostratigraphy, reconstruction of the Earth's past magnetic-field behaviour and extraction of environmental information from the geomagnetic record depend on an understanding of the conditions under which bacterial magnetite is formed. In aquatic sediments, the process is thought to be restricted to a zone between the levels at which nitrate and iron reduction occur⁴. In sulphate-reducing habitats, deeper in the sediment, the presence of H₂S reduces iron oxyhydroxides to iron sulphides^5,6. Thus magnetite would not be expected to form under such reducing conditions^5,7. We report here the isolation and pure culture of a dissimilatory sulphate-reducing bacterium, designated RS-1, which can synthesize intracellular magnetite particles. RS-1 is a freshwater anaerobe which is also capable of extracellular iron sulphide precipitation. This isolate illustrates the wider meta-bolic diversity of magnetic bacteria and suggests the presence of a novel mechanism of magnetic biomineralization. The discovery of such bacteria may also explain why large quantities of magnetite have been observed in sulphate-rich, oil-bearing, sedimentary deposits^8-11. In addition, these results significantly enlarge the environments in which biogenic magnetite may be expected to occur and have important implications regarding the evolution of the ability to synthesize magnetite.