TUESDAY, 15 JUNE 2021As spring takes hold, I enjoy the warmth of sunlight on my skin, the sound of birdsong, and the fresh scent of grass outside. But I am entirely insensible to an enormous grid of energy that runs right through me on its path between the poles: the Earth’s magnetic field. Many mammals, birds, fishes, insects, plants, and even some bacteria can detect the Earth’s magnetic field. It is clear how this sense serves migratory species like birds and fish, though the advantage for plants is less obvious. Exactly how it works though, is widely debated. Determining the mechanisms of natural magnetoreception would not only increase our understanding of animal physiology and behaviour, but also pave the way for technological advances from the laboratory to the clinic: possible applications include biological manufacture of extremely precise magnets, and the development of non-invasive tools to monitor and control cells in the human body.
Challenges of Magnetoreception
The Earth’s magnetic field strength is about 20-60 microtesla (less than 100 times the strength of a fridge magnet). The energy of a molecule interacting with this field is less than a millionth of its thermal energy at body temperature, and therefore far too small to affect normal chemical reactions. Even if the Earth’s field were stronger, most biological tissue is unaffected by magnetic fields, and so cannot detect them at all. This makes potential magnetosensory organs almost impossible to find, as they could be distributed throughout an animal’s body, rather than being restricted to the surface. To detect the tiny signal of the Earth’s magnetic field, an organism must either have hypersensitive detectors, be able to amplify the signal, or somehow circumvent the noise of thermal energy in the body. Three main mechanisms have been proposed for biological magnetoreception, each answering to one of these conditions. The first one has been observed in some marine animals, the second in bacteria, and the third has never been proven, but is theoretically appealing, and the hunt is on as we speak.
As some of us may remember from high school physics, a conductive wire moving through a magnetic field induces a current in that wire. Sharks and rays (together termed ‘elasmobranchs’), among others, exploit this principle to build exquisitely sensitive organs capable of picking up the tiny signal of the Earth’s magnetic field. These organs consist of hundreds of long tubes running from tiny skin pores into their body, filled with a conductive jelly. These act as wires, and at the end of each are the ‘ampullae of Lorenzini’ — collections of cells sensitive to voltage changes. These tubes are sufficiently sensitive to detect the voltage generated by a shark swimming through the Earth’s magnetic field, but that alone is insufficient for functional magnetoreception. One complication is that swimming forward would generate a DC current (electrons flowing one way) in these jelly tubes, but elasmobranch electroreceptors can only detect AC currents (where electrons flow back and forth). They might solve this by swaying their heads back and forth as they swim, thereby reversing the electron flow at every head turn, to generate AC currents. This might also filter out noise from ocean currents (which are also fundamentally conductive ‘wires’ moving through a magnetic field). To test these theories, scientists stuck small magnets (and nonmagnetic brass as a control) up the noses of captive rays, which gave some indications to the nature of their magnetoreception but was not definitively conclusive; as one review (Johnsen and Lohmann, 2008) sardonically noted, ‘sharks and rays are not ideal experimental animals’. Regardless, this mechanism is unworkable for most magnetoreceptive insects and animals, as it would require internal organs filled with conductive liquid that have not yet been observed outside marine animals.
Phytoplankton and bacteria have gone another way, building signal amplifiers that augment the Earth’s field strength. These amplifiers consist of chains of magnetic material (magnetite or greigite) that amplify the Earth’s magnetic field until it’s large enough to rotate the entire organism. The individual ‘magnets’ must be about 0.1–1 µm in diameter to fulfil their purpose, but one magnet that size cannot amplify the Earth’s field sufficiently: hence daisy-chains of exquisitely-sized magnetic particles. There are also reports of microbes acquiring magnetoreception by consuming magnetoreceptive bacteria, and of sustainable symbioses between magnetoreceptive bacteria and unicellular flagellates.
Could this mechanism be used by insects and larger animals? While magnetite has been detected in many magnetoreceptive species, including honeybees, salmon, sea turtles, and some birds, specific magnetoreceptors have so far only been conclusively found in microorganisms. The discovery of these ‘mini-magnets’ in animals would be extremely technically challenging, if they do exist: they are too small to be seen with light microscopy, are dissolved by many common tissue preservatives, and their constituent iron is commonly found outdoors, in labs, and in organs.
The final option for sensing the Earth’s magnetic field is so far only theoretical, but the circumstantial evidence is undeniably attractive. The theory is this: when two transient radicals (atoms or molecules with an unpaired outer electron) are created at the same time, the spins of the two electrons are correlated. The chemical properties of these kinds of radicals are strongly affected by magnetic fields, and largely insensitive to ambient temperature. This is an exceptional workaround to the problem of detecting a weak magnetic signal amidst the energetic noise of the body. Now this kind of a reaction would only detect the axis of a magnetic field, not its polarity. Intriguingly, most magnetosensory animals tested seem to be insensitive to field polarity, often simply defining ‘polewards’ as the direction which gives the smallest angle between the field and Earth’s surface. The exceptions shown to detect field polarity include lobsters, salamanders, and mole rats.
Now the hunt is on for radical pair formation in animals with magnetoreception senses. Cryptochromes, proteins involved in timing and biological rhythms in plants and animals, were the first vertebrate pigments shown to generate radical pairs, though only when exposed to blue light. Surprisingly, some of the best evidence for cryptochrome function in magnetoreception comes from plants. Plants grown in 500 microtesla magnetic fields grow much more slowly than plants grown in 50 microtesla magnetic fields – but only under blue light. When grown in the dark or under red light, these plants all grow at the same speed. In addition, when the gene for this cryptochrome was removed, the effect of the magnetic field vanished, proving that cryptochromes are required for plants to react to magnetic fields.
In birds and mammals, however, the search for magnetoreceptors has been more inconclusive — including an interlude where it was suggested that birds’ magnetic compasses were located only in the right eye, and not the left (it has now been proven to exist in both). Determining the mechanisms of animal magnetoreception remains a key goal not only for understanding the natural world around us, but also as potential blueprints to new inventions. Already scientists are working to apply the lessons learned from magnetotactic bacteria to building tools to monitor and control cell fate with microscopic magnets. How much more might not be learned from finally determining the basis for exquisitely sensitive animal magnetoreception?
Susanne is a fourth year PhD student at Girton College studying Biochemistry. Artwork by Debbie Ho.