Structure, Morphology and Growth of Biogenic Greigite (Fe₃S₄)

Abstract

Several species of aquatic bacteria are known to exploit the earth's geomagnetic field as a means of directing their motion towards suitable habitats. A feature common to these bacteria is the presence of discrete intracellular magnetic inclusions, magnelosomes, aligned in chains along the long axis of the organism. The size and orientation of the individual magnetic particles imparts a permanent magnetic dipole moment to the cell which is, in turn, responsible for the magnetotactic response. In all species examined to date the magnetic particles have been found to be well-ordered, single domain, membrane-bounded crystals with reproducible, species-specific morphologies. Until recently, however, only crystals of the mixed valence iron oxide, magnetite (Fe304), were identified in these magnetotactic bacteria. We have now identified three species of bacteria from sulphidic environments which contain crystals of the mixed valence ferrimagnetic iron sulphide, greigite (Fe3S4)' High resolution electron microscopical studies of the biogenic greigite crystals showed that they also exhibit the narrow size range (50-90nm) and unique crystallographic habits (e.g. cubo-octahedral, rectangular prismatic) which characterized and distinguished the inclusions in other magnetotactic species. Thus, it would appear that the bio-precipitation of iron sulphides in magnetotactic bacteria is a highly regulated process which is directed and controlled at the molecular level. These findings are not only important to our understanding of biomineralization in unicellular organisms but may also be significant to studies of paleomagnetism. Furthermore, the controlled synthesis of greigite presents an interesting challenge to material scientists and solid state chemists.

Electron microscopic studies of magnetosomes in magnetotactic bacteria

“…The iron sulfide-type magnetosomes have been shown to contain particles of ferrimagnetic greigite (Heywood et al, 1990(Heywood et al, , 1991 or a mixture of greigite and non-magnetic pyrite particles (Mann et al, 1990a). Two morphologies of greigite particles have been observed in magnetotactic bacteria: (a) cubo-octahedral (the equilibrium form of face-centered cubic greigite); and (b) rectangular prismatic (Heywood et al, 1990(Heywood et al, , 1991.…”

“…Two morphologies of greigite particles have been observed in magnetotactic bacteria: (a) cubo-octahedral (the equilibrium form of face-centered cubic greigite); and (b) rectangular prismatic (Heywood et al, 1990(Heywood et al, , 1991. Examples of both morphological types are shown in Figure 6.…”

“…Conventional microscopy was carried out either with a JEOL Model 200CX microscope operating a t 200 kV or a JEOL Model lOOX instrument operating at 100 kV. High resolution analytical electron microscopy (HRTEM) of microorganisms and magnetosomes was performed using a JEOL Model 2000FX TEM operating at 200 kV as previously described (Heywood et al, 1991).…”

“…Selected area electron diffraction was also performed on magnetosomes using the JEOL Model 2000FX instrument. Operating parameters and calibration methods for SAED using this microscope are published elsewhere (Heywood et al, 1991).…”

Crystal-size distributions and possible biogenic origin of Fe sulfides

“…Magnetotactic bacteria generally produce either magnetite or greigite with only one morphological type described to date that produces both (Bazylinski, 1999). Although most studies on magnetotactic bacteria were performed on mag netite-producing species, several studies addressed the physical and chemical properties and forma tion mechanisms of greigite magnetosomes (Heywood et al, 1991;Pósfai et al, 1998a and b;Lins et al, 2000); from these we know that BCM greigite crystals are characterized by a size range from about 30 to 150 nm, an abundance of defects, and a typical spotty contrast in transmission elec tron microscope (TEM) images.…”