Many animals are known to navigate by sensing the Earth’s magnetic field, including bacteria, birds, bats, eels and whales. Some observations suggests that even dogs and cows sense the Earth's magnetic field. But exactly how magnetoreception – the ability to detect magnetic fields – works remains a mystery. Some bacteria use magnetite crystals, a magnetic iron-oxide, like a compass needle to follow the magnetic field lines. However, vertebrates don’t possess such structures inside their cells. Another leading hypothesis involves chemical reactions induced in cells by a magnetic field. If certain molecules are excited, electrons can jump between them to their neighbors. Magnetic fields can influence the speed of this exchange, affecting the chemical behavior of the molecule. This effect could slow down or speed up certain chemical reactions that change an animals’ behavior.
In the living cells of animals with magnetoreception, proteins called cryptochromes are thought to be such magnetic field sensitive molecules. Cryptochromes are a class of photoactive pigments found in plants and animals. When they absorb light, they emit an electromagnetic signal. Based on this property, researchers believe that they are also sensitive to magnetic fields.
Now, for the first time ever, a team of researchers at the University of Tokyo directly observed cryptochromes responding to magnetic fields in a living cell. The research was published in the journal Proceedings of the National Academy of Sciences.
Using a special optical microscope, sensitive to faint flashes of light, the team watched a culture of human cells containing a special pigment used to dye proteins like cryptochromes. The researchers irradiated the cells with blue light under the microscope so that their dyed proteins fluoresced, then swept a magnetic field over them every four seconds. And each time it swept over them, the fluorescence of the cells dropped by about 3.5 percent in response to the protein's activity. The magnetic field used in the experiments was about the same as a regular fridge magnet, which is much stronger than the Earth’s natural field.
The Earth's magnetic field is a result of the movement or convection of liquid iron in the outer core. As the liquid metal in the outer core moves, it generates electric currents, which lead to a magnetic field. Earth's magnetic field plays a vital role in protecting the Earth from the Sun's harsh solar wind, and provides a world-spanning grid useful for navigation. So it's not too surprising that organisms developed a sense to detect it.
In human cells, cyrptochromes act as a molecular clock, using sunlight to synchronize the body's function with the solar day. In species of migratory birds, cyrptochromes levels are especially high in specialized cells found in the retina, the light receptive part of the eye. Biologists already suspected that these cells react to changes in the electromagnetic field, and birds use the variability and orientation of Earth's magnetic field to navigate. This new study provides first direct evidence on how birds and other migratory species may do it. How magnetic fields could indirectly affect other biological processes, or even humans, remains to be seen.