Controlled biomineralization of magnetic minerals by magnetotactic bacteria

Magnetotactic bacteria produce two general types of minerals as the mineral phases of their magnetosomes: iron oxides and iron sulfides. The iron oxides include only ferrimagnetic magnetite (Fe3O4) and the iron sulfides, ferrimagnetic greigite (Fe3S4) and non-magnetic pyrite (FeS2). Regardless of composition, the magnetosome particles have a very narrow size range, 35-120 nm, and are single-magnetic-domains. These intracellular particles confer a permanent magnetic dipole moment to the cell resulting in the cell's magnetotactic response, i.e. a motile, biomagnetic compass. The morphology of the magnetosome particles is species- and/or strain-specific. Thus, the organism exerts significant control over the biomineralization process(es) involved in magnetosome formation. However, there is some accumulating evidence that suggests that in certain organisms under specific situations, local environmental conditions appear to affect the magnetosome mineral phase in at least three different ways. These include: (1) variations in the stoichiometry of the metal and non-metal components of the magnetosome mineral phase; (2) the replacement of iron with other transition metal ions in the metal component of the magnetosome mineral phase; and (3) the replacement of oxygen with sulfur or vice versa in the non-metal component of the magnetosome mineral phase. Examples of each type of mineral change or Variation are described and discussed.