This post is the second in a three-part series on how living creatures use the elements of the periodic table. Read the first post here, and learn more about the elements on NOVA's two-hour special, "Hunting The Elements."
Iron (Fe) is the most common element by mass on Earth. But it isn't just the stuff of pots, pans, and fences: It is also one of life's essential nutrients. The iron-containing hemoglobin in our red blood cells carries oxygen, and iron helps plants create chlorophyll. But when iron combines with oxygen to make the magnetic compound magnetite, it becomes a built-in compass for living creatures.
Magnetite has been found in the brains of termites, bees, fish, birds, dolphins and even humans. Creatures like bats, sea turtles, pigeons, and salmon are able to sense the planet's weak magnetic field, which helps guide them on their migrations. Even tiny bacteria (called magnetotactic bacteria) use the Earth's magnetic field to orient themselves.
Magnetotactic bacteria were first discovered in 1975 when a researcher noticed that his cells naturally kept moving north. Curious, he put a magnet near them and, viola, they aligned themselves with the magnet, just like in this video.
When a magnetotactic bacterium develops, it grabs three iron atoms and four oxygen atoms from its environment and uses them to make magnetite crystals. Once the bacterium has created enough magnetite crystals, it links them together in create a magnetosome. This turns the cell into a sensitive living compass. But why would a bacterium need a compass in first place?
Magnetic bacteria are very picky about where they live. They prefer to live in deep water, where there is little oxygen but plenty of the ions they need for their metabolism. Furthermore, there are two varieties of magnetotactic bacteria: one that points north and one that points south, corresponding to the hemisphere they live in. By following Earth's the magnetic field lines, they find their way to the deep water in which they thrive.
The magnetosome automatically directs the movement of the bacteria, even after the bacteria are dead. When magnetotactic bacteria die, their denser-than-water magnetosomes cause them to sink to the seafloor, where they become embedded in marine sediments. Their fossils remain oriented with the magnetic field, leaving a historical record of the Earth's changing magnetic field.
This is just one of many ways that organisms transform the elements, literally bringing chemistry and physics to life. If only we had a large, relatively powerful magnet permanently within us--we'd probably never have to ask for directions again.