Magnetotactic Bacteria
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Magnetotaxis is movement along magnetic field lines, from Latin magnes (magnet) and Greek taxis (arrangement). The Latin magnes is itself from Greek ho Magnes lithos “the Magnesian stone,”(This is what my etymology research turns up, if you know more, let me know so I can pass it on!)
Where Do They Live?
Magnetotactic bacteria on Earth are found in a layer of water called the oxic-anoxic transition zone: where the water goes from having oxygen to not having oxygen. The bacteria often occur within sediments (like the muck at the bottom of a pond – but that’s a simplified example). They don’t occur in all water at this transition zone, the water must be exactly the right composition.
There are many types of magnetotactic bacteria on Earth. The shapes include: coccoid (connected bubbles), ovoid, rods, vibrios (curved rods), & spirilla (spirals).
Movement
All know magnetotactic bacteria on Earth have flagella (tails) for movement. They orient along magnetic field lines and actively swim along those lines. Dead magnetotactic bacteria do not move along magnetic field lines.
Some northern hemisphere magnetotactic bacteria swim along the geomagnetic field towards our North Pole. Some southern hemisphere magnetotactic bacteria swim along the geomagnetic field towards our South Pole. Since our magnetic field is centered on the center of the earth, these magnetotactic bacteria are swimming along the field line in the direction of “down.” It was originally thought that this was to allow the bacteria to burrow down deeper into the sediment. It has since been discovered that magnetotactic bacteria also use aerotaxis. Wahhh! Too many big words! Aerotaxis: move to or away from oxygen. So really, the bacteria not preferentially going “down” the magnetic field line, they’re preferentially going away from the oxygen and also happen to be moving along the magnetic field line.
How Do They Work?
Magnetotactic bacteria have structures inside them, magnetosomes, which contain crystals of iron. Those crystals are magnetic: magnetite. There’s a “skin” (membrane) around the magnetosomes – it probably helps the bacterium control the growth of the magnetite crystals. Most freshwater magnetotactic bacteria create magnetite (iron & oxygen), but many marine and salt-water species create greigite (iron & sulfur) instead.
Here’s what matters in terms of finding magnetite in that Martian meteorite: “The narrow size range and consistent morphologies of the magnetosome crystals in each species or strain are clear indications that the magnetotactic bacteria exert a high degree of control over the processes of magnetosome formation.” (Frankel) In other words, the magnetite crystals are very, very similar between bacteria, and that’s the type of magnetite crystal found in the Martian meteorite ALH84001.
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