Magnetotactic Bacteria at Cal Poly

By Richard B. Frankel
Department of Physics, Cal Poly State University, San Luis Obispo, CA 93407
(rfrankel@calpoly.edu)

Magnetotactic behavior in bacteria was discovered over 30 years ago by microbiologist Richard P. Blakemore. The discovery was based on the fact that certain motile, aquatic bacteria orient and migrate along magnetic field lines when subjected to a magnetic field of the order of the geomagnetic field, or greater. For magnetotactic bacteria (mtb) from Northern Hemisphere collection sites, the predominant direction of migration in drops of water and sediment on a microscope slide is parallel to the magnetic field corresponding to northward migration in the geomagnetic field. The migration speed of individual bacteria along the magnetic field lines depends on the field strength, but can be 90% or more of the forward swimming speed of the cell (which can be up to 150 microns per second). If the direction of the local magnetic field is reversed while mtb are swiming, they execute a "U-turn" and continue migrating in the same direction relative to the local magnetic field. The predominant migration direction of mtb in the magnetic field can be reversed by subjecting the cells to a strong (several hundred gauss) magnetic field pulse, oriented opposite to the ambient field. Mtb that spontaneously migrate southward along geomagnetic field lines are found in aquatic sediments and waters from the Southern Hemisphere.

 

[For a more complete description of magnetotaxis, see Magnetotaxis in Bacteria (http://www.calpoly.edu/~rfrankel/magbac101.html)]

 

Magnetosome Mineral Particles

All mtb contain magnetosomes, which are nanometer-sized, magnetic, mineral crystals enclosed in a membrane. In most cases the magnetosomes are arranged in a chain or chains, apparently fixed within the cell adjacent to the plasma membrane. In many mtb strains, the magnetosome crystals are magnetite, Fe3O4, characterized by a narrow size distribution and uniform, species-specific, crystal habits. The crystal sizes typically range from ca. 40 to 100 nm, which are within the permanent, single-magnetic-domain size range for magnetite.

In mtb from marine, sulfidic environments, the magnetosome crystals are the iron-sulfide mineral greigite, Fe3S4, which is isostructural with magnetite and is also ferrimagnetically ordered at room temperature. The greigite crystals are also characterized by a narrow size distribution and species-specific crystal habits. However, whereas the magnetite crystals in a magnetosome chain are usually oriented so that a [111] crystallographic axis of each particle lies along the chain direction, the greigite particles in a magnetosome chain are usually oriented so that a [100] crystallographic axis of each particle is oriented along the chain direction. While most mtb strains have either magnetite or greigite magnetosomes, there is an organism which has both magnetite and greigite magnetosomes co-organized in chains.

Constructing the Cellular Magnetic Dipole

For cells with either magnetite or greigite magnetosomes, the chain of magnetosomes constitutes a permanent magnetic dipole fixed within the bacterium. The magnetic dipole moment is generally sufficiently large so that it, and consequently the bacterium, is oriented in the geomagnetic field at ambient temperatures. Thus magnetotaxis is a passive process in which the orientation of the magnetic dipole in the ambient magnetic field as the organism swims causes it to migrate along the magnetic field lines. Killed cells align along the field but do not migrate. Thus motile mtb behave like self-propelled, magnetic, compass needles.

Mtb have two possible magnetic polarities, depending on the orientation of the magnetic dipole within the cell. The polarity can be reversed by a magnetic pulse which is greater than the coercive force of the chain of particles (several hundred gauss). Bacteria with reversed polarity migrate along magnetic field lines in the direction opposite to that of bacteria with the original polarity. In natural habitats, the predominant polarity type in a population of a given bacterial species is determined by the sign of the inclination of the geomagnetic field.

Chemically Stratified Sites

It has been reported that high concentrations of mtb occur in a horizontal "plate" at the oxic-anoxic transition zone (OATZ) in chemically stratified marine environments. In these environments, downward oxygen diffusion from the surface and upward sulfide diffusion, resulting from bacterial sulfate reduction in the anaerobic sediment, create a double vertical chemical concentration gradient system, with a concomitent redox gradient. Salt Pond, a 5 meter deep coastal pond in Falmouth, MA on Cape Cod, stratifies in the summer with the OATZ at about 3 meters. The mtb concentration in the plate is greater than 105 cells per cc. At least seven morphologically-distinct, magnetotactic-bacterial types occur at the OATZ, some containing Fe3O4 particles, and some containing Fe3S4 particles [21]. The magnetite-containing cells tend to be more abundant at the top of the plate, in the relatively oxygen-rich portion of the OATZ, while the greigite-containing cells tend to be more abundant at the bottom of the plate, in the relatively sulfide-rich portion of the OATZ.


Publications on magnetotatic bacteria by Richard B. Frankel (http://www.calpoly.edu/~rfrankel/mtbrbf.html)


Additional publications on magnetotactic bacteria               (http://www.calpoly.edu/~rfrankel/mtbother.html)

 

Download a recent review of magnetotactic bacteria, (http://www.calpoly.edu/~rfrankel/NatRevMicro.pdf)