One of the most remarkable “sixth” senses in the animal kingdom is magnetoreception – the ability to detect magnetic fields – but exactly how it works remains a mystery. Now, researchers in Japan may have found a crucial piece of the puzzle, making the first observations of live, unaltered cells responding to magnetic fields.

Many animals are known to navigate by sensing the Earth’s magnetic field, including birds, bats, eels, whales and, according to some studies, perhaps even humans. However, the exact mechanism at play in vertebrates isn’t well understood. One hypothesis suggests it’s the result of a symbiotic relationship between the animals and magnetic field-sensing bacteria.

But the leading hypothesis involves chemical reactions induced in cells through what’s called the radical pair mechanism. Essentially, if certain molecules are excited by light, electrons can jump between them to their neighbors. That can create pairs of molecules with a single electron each, known as a radical pair. If the electrons in those molecules have matching spin states, they will undergo chemical reactions slowly, and if they’re opposites the reactions occur faster. Since magnetic fields can influence electron spin states, they could induce chemical reactions that change an animals’ behavior.

In the living cells of animals with magnetoreception, proteins called cryptochromes are thought to be the molecules that undergo this radical pair mechanism. And now, researchers at the University of Tokyo have observed cryptochromes responding to magnetic fields for the first time.

The team worked with HeLa cells, a lab-grown line of human cervical cancer cells that are often used for these types of experiments. They focused on the cells’ flavin molecules, a subunit of cryptochromes which fluoresce under blue light.

The researchers irradiated the cells with blue light so that they 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.

The team says that this dimming is evidence of the radical pair mechanism at work. Basically, when flavin molecules are excited by light they either produce radical pairs or fluoresce. The magnetic field influences more of the radical pairs to have the same electron spin states, slowing down their chemical reactions and dimming the overall fluorescence.

“We’ve not modified or added anything to these cells,” says Jonathan Woodward, co-lead author of the study. “We think we have extremely strong evidence that we’ve observed a purely quantum mechanical process affecting chemical activity at the cellular level.”

The team says that 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. But interestingly, weaker magnetic fields can actually make it easier for the electron spin states in radical pairs to switch.

That could mean that the radical pair mechanism is at play in animals with magnetoreception, but further work will be needed to know for sure.

The research was published in the journal Proceedings of the National Academy of Sciences.

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