Biologically Controlled Mineralization of Magnetic Iron Minerals by Magnetotactic Bacteria

The biomineralization of several magnetic iron minerals, including the iron oxide magnetite (Fe30 4 ; Fe 2I Fe/+04 ) and the iron sulfides greigite (Fe3S4 ) and pyrrhotite (Fe7SS)' can be mediated by bacteria as well as by other organisms. These minerals are known to be synthesized by procaryotes in one of two fundamentally different modes of biomineralization. The first is uncontrolled mineral formation, called bi­ ologically induced mineralization (BIM) (92), while the second is directed-mineral formation, termed biologically controlled mineralization (BCM), also referred to as organic matrix-mediated mineralization (92) or boundary-organized biominerali­ zation (96). Whereas in BIM the organism does not appear to control the bio­ mineralization process, in BCM the organism exerts a high degree of control over the mineralization process. Different physiological types of bacteria are responsible for BIM and BCM of magnetic minerals. Dissimilatory iron-reducing and dissimilatory sulfate-reducing bacteria cause the formation of magnetite and greigite, respectively, as well as many other nonmagnetic iron minerals, by BIM processes. Among the procm'yotes, only the magnetotactic bacteria biomineralize magnetite and greigite by BCM. There is no apparent function to the magnetite and greigite particles produced via BIM by microorganisms. However, there is ecological, evolutionary, and perhaps physio­ logical significance to the BCM particles produced by the magnetotactic bacteria. Additionally, the biomineralization of magnetic minerals by bacteria can be an important source of tine-grained « I j.lm) magnetic material in sediments and soils, which contributes to the paleomagnetic record of ancient geomagnetic field behav­ ior and to the mineral magnetic record of paleoclimate changes (37, 38, 94, 137). It is important to be able to discern the differences between particle types so as to be able to recogl1lze biogenic magnetic mineral particles and also to determine