Magnetotactic Bacteria

Magnetotactic bacteria (MTB) orient and migrate along the geomagnetic field towards favorable habitats, a behavior known as magnetotaxis. Since the first report of magnetotactic bacteria by Blakemore [1975], subsequent studies have shown that MTB are a morphologically diverse and cosmopolitan group of aquatic microorganisms inhabiting freshwater and marine environments ranging from aerobic to anoxic. Three significant findings since the last IUGG report on biomineralization are (1) an increase in the types of environments where MTB are found besides microaerobic to include anaerobic and aerobic [ Bazylinski, 1990; Matsunaga et al., 1991; Sakaguchi et al., 1993]; (2) an increase in the number of identified phases besides magnetite (FeO) to include ferrimagnetic greigite (FeS), possible pyrrhotite (FeS), and non-magnetic pyrite (FeS) [ Mann et al., 1990a; Farina et al., 1990; Bazylinski, 1990]; and (3) the discovery of non-magnetotactic magnetite producing bacteria [ Lovley, 1990].

In natural habitats, large populations of MTB are usually found near the oxic-anoxic transition zone, which is usually located at the sediment-water interface in freshwater environments or displaced upward into the water column in marine semi-anaerobic environments [ Stolz, 1992; Bazylinski and Frankel, 1992]. In the latter types of environments, magnetite and greigite producing MTB exist in horizontal zones at specific water depths depending on the vertical chemical and redox gradients [ Stolz, 1992; Bazylinski, 1991]. Petermann and Bleil [1993] identify several different types of living MTB in deep sea sediments from the South Atlantic, some at water depths of 2000 m. Magnetic bacteria containing intercellular magnetite particles have also been identified in the uppermost horizon of a waterlogged soil in Germany [ Fassbinder et al., 1990]. Although population densities of the magnetic bacteria were too low at the time of sampling to contribute significantly to the magnetism of the soil, the authors suggest that population densities can change dramatically depending on microenvironmental conditions. However, the question of a biogenic magnetic component in soil must await further confirmation and study.

Magnetotactic bacteria from reducing environments with high concentrations of HS contain Fe-sulphide particles instead of Fe-oxides [ Mann et al., 1990a; Farina et al., 1990; Bazylinski and Frankel, 1992]. The sulphide minerals were identified by indexing single crystal electron diffraction patterns [ Mann et al., 1990a; Heywood et al., 1990, 1991]. Several types of bacteria have been studied including a rod shaped bacterium containing only greigite particles [ Bazylinski, 1990], a multicellular magnetotactic prokaryote (MMP) containing a mixture of greigite and pyrite particles [ Mann et al., 1990a], and another bacterium containing both magnetite and greigite particles [ Bazylinski et al., 1993a]. In another study, ferrimagnetic pyrrhotite (FeS) was proposed as the iron-sulfide mineral in a similar (or perhaps the same) MMP from Brazil [ Farina et al., 1990]. No single crystal diffraction patterns have yet been obtained that unequivocally identify the phase as pyrrhotite, so the report of FeS in MTB remains problematical. Finally, Sakaguchi et al. [1993] report a magnetotactic sulphate-reducing bacterium that produces intercellular magnetite and extracellular magnetic iron-sulfide particles. This discovery extends the range of magnetite producing microorganisms to sedimentary levels where sulphate reduction occurs.