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On the Trail of Bacteria That Make Their Own Compasses

Katrina Edwards is no longer surprised by the extraordinary capabilities of microorganisms—but the rest of us can still be amazed. Take, for example, bacteria that make their own magnetic compasses.

Fresh from her research on a previously unknown species of archaea that dissolves metal ores into sulfuric acid, Edwards is now investigating bacteria in Cape Cod salt ponds, in collaboration with Dennis Bayzlinski of Iowa State University. Using iron and sulfate from their environment, these bacteria precipitate crystals of a magnetic mineral, greigite (Fe3S4), within their cells.

The evidence to date suggests that the bacteria use these magnets to align themselves with Earth’s geomagnetic field, Edwards said. They combine this ability with a mechanism that gives them the capacity to sense oxygen levels in surrounding waters. By these means, the bacteria can orient themselves in the water column in places where they find optimal living conditions.

Edwards, an Assistant Scientist at WHOI, is a geomicrobiologist who studies the crucial role that microorganisms play in dissolving or precipitating rocks and minerals at the earth’s surface. Her perspective is simultaneously macro- and microscopic. If the earth were a body, microorganisms would act as the cells and enzymes that catalyze myriad reactions to regulate the planet’s chemistry on land and sea.

Curiously, the greigite crystals the bacteria create are not stable in the oxygen-poor, sulfide-rich coastal waters these organisms inhabit. The bacteria have to work to keep them intact. If bacterial cells did not carefully maintain the composition and structure of greigite, these unstable magnetic minerals might react to form other, more stable sulfide minerals, such as pyrite (FeS2). Edwards is investigating precisely how these microbes maintain these magnets within their cells, and what happens to the crystals when the bacteria die.
 
Not only is this significant for understanding how chemicals cycle through Earth’s atmosphere, land, and ocean, it could be used as a tool to see into Earth’s past. Earth’s magnetic field has flip-flopped many times during its long history—with the north and south poles switching places. Magnetic minerals align with the magnetic field that exists at the time they form. Bacteria-spawned magnetic minerals preserved in coastal sediments should record the geomagnetic field that existed when they were formed. Thus, they can act like stopped pocket watches to help pinpoint the timing of geological events.

Originally published: October 1, 2000