Light Receptor May Be Key In How Animals Use Earth's Magnetic Field

The receptor -- cryptochrome -- is known to play a prominent role regulating an animal's day-and-night rhythm. Now, UI scientists report in the February issue of the Biophysical Journal that chemical experiments and computational modeling indicate that cryptochrome may be the site of a neurochemical reaction that lets birds, for example, process visual clues from the magnetic field and stay on course.

"Animals as diverse as migratory birds, salamanders, salmon, or hamsters use the geomagnetic field for orientation," said Klaus Schulten, holder of the UI Swanlund Chair in Physics and professor at the Beckman Institute for Advanced Science and Technology. "We know how such a compass works in bacteria, and we know that the magnetic compass ability is widespread in animals. But it has been a mystery how magnetoreception is achieved in higher animals."

Typical biomolecules interact with Earth's magnetic field too weakly to alter the course of their chemical reactions. In earlier experiments, Schulten had shown that certain chemical reactions involving so-called radical pairs can be influenced by weak magnetic fields, like that of a door magnet. Previous research had identified only cases in which bacteria as a whole, filled with magnetic particles, are being oriented like compass needles for swimming in the right direction.

Schulten's team, including doctoral student Thorsten Ritz, found theoretical evidence that a biochemical reaction involving cryptochromes can be influenced by an Earth-strength magnetic field. The computations were based on fundamental physics as described by the complex equations of quantum mechanics. The National Institutes of Health and the Roy J. Carver Charitable Trust funded the work.

Migratory birds and other animals, in many cases, cannot distinguish between north and south based on magnetic information alone. They can only detect the angle of the magnetic field lines with the horizon, which, Schulten said, is explained through symmetries in visual modulation patterns.

If radical-pair reactions in cryptochromes were connected by photoreception to the vision of animals, the magnetic field would modulate visual sensitivity, Schulten theorized. Animals would "see" the geomagnetic field by superimposing onto its visual images information about the field's direction.

Behavioral biologists tested Schulten's theory. They found that many magnetic responses require light, and that the orientation of some animals was erratic when exposed to monochromatic red light. Such findings strengthened the theory, Schulten and Ritz said, because radical-pair reactions require light above a certain energy threshold.

"The visual modulation patterns that we found show surprising agreement," Schulten said. "The hunt for the elusive magnetoreceptor is not over, but we have provided a new, promising track."