Cochise College           Student Papers in Geology

Geology Home Page                   physical geology  historical geology  planetary  gems           

Roger Weller, geology instructor

wellerr@cochise.edu

Biomagnetism
by Adam Bolton
Physical Geology
Spring 2007
        

 Biogenic Magnetite and Biomagnetism

What is Biogenic Magnetite?

 Through a process known as biomineralization, organisms have synthesized chains of magnetite, a black, ferromagnetic mineral form of Iron (II) Iron (III) Oxide (Fe3O4) which crystallizes in isomeric system (or, all crystals have crystallographic axes of equal length, situated 90 degrees to each other). It is one of the major compounds of iron which is strongly magnetic and some are natural magnets, or lodestones. Its ferromagnetic crystals interact more strongly with external magnetic forces than other magnetic materials, which are diamagnetic or paramagnetic and are only generated by cells when needed. In most cases, biogenic magnetite has been found in membrane bound chains or layers, in close proximity to neural dendrites. The relationship between bundles of magnetite and the nerve endings supposes a navigational function within the majority of magnetite bearing organisms.
 

Above: A linear chain of biogenic magnetite crystals, extracted from tissues in the frontal region of the sockeye salmon, Oncorhynchus nerka, a close relative of the rainbow trout, Oncorhynchus mykiss. These are also single magnetic domains, with crystal alignments similar to those in magnetotactic bacteria. (Photo credit: S. Mann).

Biogenic magnetite is produced through the reduction of ferric iron within cells. However, the exact process of magnetite synthesis is unknown. Below is a proposal diagram of how a magetotactic bacterium would generate magnetite crystals:

            As seen in the model for magnetite biomineralization in Magnetospirillum species above, Fe(III) is actively taken up by the cell, possibly via a reductive step. Iron is then thought to be reoxidized and magnetite is produced within the magnetosome vesicle. The magnetosome membrane contains specific proteins, which are thought to have crucial functions in the accumulation of iron, nucleation of minerals and redox and pH control. (Credit for diagram: Junior Group at the MPI for Marine Microbiology , Bremen. Head of the Group: Dr. Dirk Schüler).

 

Above: Electron micrograph of purified magnetosomes from Magnetospirillum gryphiswaldense. Individual magnetosome particles are enclosed by the magnetosome membrane, which prevents agglomeration. (Junior Group at the MPI for Marine Microbiology , Bremen Head of the Group: Dr. Dirk Schüler).

 Above: One example of a "magnetotactic" (magnetite-producing) bacterium from Earth. Note the line of slightly-elongated magnetite crystals down the bacterium's center. These crystals act as a compass, aligning the bacterium with the Earth's magnetic field. Image courtesy of Dr. Dennis Bazylinski of Iowa State University.

           

Actual TEM photo of a freshwater magnetotactic bacterium. The chain of dark objects are crystals of the mineral magnetite (Fe3O4), which have the proper size and shape to behave as perfect, single magnetic domains. The largest crystals are about 70 nm in length. (Photo credit: A. Kobayashi).

            The ultra-fine-grained, single-domain magnetite formed by magnetotactic bacteria have been shown to contribute significantly to the natural remnant magnetization of carbonates and limestones, hemipelagic and deep-sea marine sediments. Magnetite, produced by dissimilatory iron reducing bacteria, have a distinctive morphology and size range; but it is controversial as to whether these can be distinguished from certain chemically precipitated magnetites. However, the presence of dissimilatory iron reducing bacteria can be detected using miro biological techniques and sediments geochemistry. Biogenic magnetites are trace fossils ans potentially useful environmental indicators and are considered to have significant input to the magnetization of most desiments, both modern and ancient (Stolz, Lovely, Haggerty. J. Geophys. Res. 1990)

            What Stolz et al, are telling us is that the magnetization of the ocean floor, primarily in banded iron formations or BIFs, and other places could be due to the magneto-fossils of Precambrian magnetotactic bacteria.

  

Biomagnetism and Navigation

            As melted material moves about in the Earth’s core, a magnetic field is created around the globe. The magnetic field is faint, yet some how, some animals are able to sense and in rare occasions even see the Earth’s magnetic field. The ability to detect, or otherwise be effected by the natural magnetic field is called biomagnetism. Intensive research has been conducted on the correlation between biogenic magnetite bearing creatures and their ability to navigate seemingly without any other cue than biomagnetism. Floating crystals of magnetite in receptor cells, normally found in the front/snout portion of organisms, will react to magnetic fields. The torque placed upon the magnetite causes the depolarization of nearby nerve cells. The action is analogous. The Earth’s magnetic field is weaker near the equator (around 27,000 nT) and strengthens nearer to the poles (roughly 80,000 nT). An animal would be able to sense its latitude along the varying field. Another theory holds, that a cetacean, or a migratory bird perhaps, would be able to memorize magnetic anomalies and use them as way points on long migrations when no other cues are present. Although magnetite-based magnetoreceptors have not yet been found in any animal, its presence can be verified deductively. Empirical evidence is limited to electron microscopic images which do not support any hypotheses as far as function is concerned.

 

Biogenic magnetite has been found in the following:

  1. Homing pigeons and other migratory birds

  2. Dolphins and Whales

  3. Insects such as Bees

  4. Salamanders

  5. Salmon and Trout

  6. Bacteria

  7. Human Beings

 

Biogenic Magnetite in Humans

            Biogenic magnetite was first discovered in Human Beings in 1992. Although the function of the ferromagnetic material in Humans is largely unknown, the presence of the magnetite in human brain tissue also provides plausible theoretical mechanisms for the interaction of environmental magnetic fields with the human central nervous system.
 

Above: Transmission electron micrographs of magnetite and/or maghemite particles extracted from a human hippocampus (well defined crystal faces can seen in some particles). Second image: particles showing some dissolution at the edges. Credit: Dobson and St. Pierre.

 

 

 

Above: High-resolution TEM image of a single-domain magnetite (Fe3O4) crystal from the human cerebellum. This image shows the pattern of intersecting {111} and {022} crystal lattice fringes, with particle elongation in the [111] lattice direction. The morphology and structure of these crystals resemble strongly those produced by the magnetotactic bacteria and salmon. Although biogenic magnetite is present in trace levels (1-100 ppb) in most human tissue samples, we do not yet know what biological function it has, if any.

 Martian Magnetite

          In 1984, a Martian meteorite was found in Antarctica. The meteorite was dubbed ALH84001. Strands of magnetite were found in ALH84001, similar to those found in magnetotactic bacteria on Earth. The scientific struggle ebbs and flows between evidence that supports the fossilized magnetite as being of organic origins, and evidence which refutes it. Below is a side by side comparison of the magnetite particles from ALH84001 and crystals from Earthly bacteria known as MV-1.

Above is a photo of the Martian Meteorite.

Conclusion

            The study of biogenic magnetite and biomagnetism has provided theoretical answers to how organisms are able to navigate over massive amounts of terrain without any other cues. The derivation of the biogenic magnetite is still some what of a mystery, although a movement to exploit megnetotactic bacteria commercially has helped shed some light on the subject.