Magnetosomes In Magnetotactic Bacteria Research Articles

Magnetic microstructure of iron sulfide crystals in magnetotactic bacteria from off-axis electron holography

Transmission electron microscopy, off-axis electron holography and energy-selected imaging were used to study the crystallography, morphology, and magnetic microstructure of nanoscale greigite (Fe 3S 4) magnetosomes in magnetotactic bacteria from a sulfidic habitat. The greigite magnetosomes were organized in chains, but were less ordered than magnetite magnetosomes in other bacteria. Nevertheless, the magnetosomes comprise a permanent magnetic dipole, sufficient for magnetotaxis.

Biomineralization and magnetism of bacterial magnetosomes

Magnetosomes of magnetotactic bacteria are of great interest in understanding biomineralization and possible links between organisms and geomagnetic field. Fossil magnetosomes are ubiquitous in marine and lake sediments and may significantly contribute to magnetic signals. In this review, we firstly introduce some characteristics of magnetotactic bacteria, followed by considering recent progress in magnetosome formation, magnetic measurements, and identification of bacterial magnetites in bulk sediments as well as their paleoenvironmental implications. Finally, we briefly discuss potential future breakthroughs in magnetosome studies and its applications.

Instrument for the measurement of hysteresis loops of magnetotactic bacteria and other systems containing submicron magnetic particles

An electronic control system for the measurement of hysteresis curves of microscopically observed magnetic structures such as chains of magnetosomes in magnetotactic bacteria suspended or swimming in water is described. Using continuous magnetic fields generated by four coils for guidance or orientation of the bacteria or other magnetic structures, and pulsed magnetic fields in two additional coils for changing the degree of magnetization in small steps, hysteresis curves can be traversed. The circuits described can be constructed with readily available components. The guiding- and pulsed-field coils can be fashioned in any standard machine shop. The typical sensitivity of the system is better than 10−12 ergs/G, which makes a wide range of bacteria accessible to quantitative measurement of their magnetosome chains. The electronic circuits as well as the coil systems are described in detail.

Elastic stability of chains of magnetosomes in magnetotactic bacteria

Electron micrographs of magnetotactic bacteria reveal that chains of magnetosomes are often bent. This is surprising inasmuch as straight chains are actually the most favourable arrangement for magnetonavigation achieving the maximum value of the bacterial net magnetic moment. In order to answer the question of what causes the chains to bend, we calculated the stability limit of straight magnetosome chains by taking into account elastic and magnetic forces. For several scenarios, the threshold values of external forces leading to elastic instability were computed. From our calculations and observations on freeze-dried cells, we conclude that, under normal conditions, magnetosome chains are straight or only slightly bent, whereas shrinkage during preparation may cause severe artifacts such as kinks or zig-zag structures in the chains.

Spatial arrangement of chains of magnetosomes in magnetotactic bacteria

Scanning and transmission electron microscopy (SEM/TEM) were used to investigate the spatial arrangement of chains of magnetosomes in cells of two morphologically different types of magnetotactic bacteria which possess at least two chains: Magnetobacterium bavaricum, and wildtype magnetic cocci. The TEM pictures show apparently very different arrangements of the chains within the cells. Stereo-micrographs obtained by tilting individual bacteria in the TEM, however, revealed that magnetosome chains in magnetic cocci always lie on opposite sides of the cell body and in close proximity to the cell envelope. The rod-shaped cells of M. bavaricum contain up to 1000 bullet-shaped magnetosomes forming 3–5 rope-shaped bundles of magnetosomes arranged such that they are separated by the maximum possible distance from each other and positioned adjacent to the cell envelope. These observations can be understood in terms of repulsion forces between parallel magnetic dipoles driving the chains apart from each other and forcing them to be in direct mechanical contact to the cell envelope. Thus, the magnetic torque acting on the chains under the influence of the geomagnetic field can be transferred very effectively to the whole cell body. Moreover, the formation of two or more chains inside the cell body is an effective means of achieving a magnetic moment large enough for a given cell size to overcome the viscous resistance with respect to the surrounding medium, which is required to ensure an alignment with the geomagnetic field as fast as possible. Based on these two arguments and on the electron optical observations, we hypothesize that all coccoid magnetotactic bacteria contain at least two chains of magnetosomes.

Electron microscopic studies of magnetosomes in magnetotactic bacteria.

Electron microscopic studies on magnetosomes in magnetotactic bacteria have revealed much information on their composition, structure, and even the formation of their mineral phase. The mineral phases of the magnetosomes are of two general types: iron oxides and iron sulfides. Iron oxide-type magnetosomes contain particles of the ferrimagnetic mineral magnetite (Fe3O4) while the iron sulfide-type contain ferrimagnetic greigite (Fe3S4), greigite and non-magnetic pyrite (FeS2), or possibly ferrimagnetic pyrrhotite (Fe7S8). Regardless of their composition, the crystalline particles in magnetosomes have a narrow size range: approximately 35 to 120 nm. Magnetite crystals in this size range are single-magnetic-domains and confer a permanent magnetic dipole moment to the cell. The single-domain size range for greigite is not known but is probably similar to that for magnetite. The morphology of the particles in the bacterial magnetosomes appears to be species-specific. Morphologies of magnetite crystals in different species of magnetotactic bacteria include cubo-octahedra, parallelepipedal (truncated hexahedral or octahedral prisms), and tooth- or bullet-shaped (anisotropic). Morphologies of greigite particles include cubo-octahedra and rectangular prismatic. The greigite-pyrite particles are generally pleomorphic with no consistent crystalline morphology. A membrane has been shown to surround the particles in some organisms and may be involved in the formation of the crystalline phase while also providing physical constraints on the size and the shape of the crystal. These results clearly indicate that the biomineralization process involved in the bacterial magnetosome, a good example of a self-assembled structure on a nanometer scale, is highly controlled by the organism.