The Wonders of the Geomagnetic World

Reading Time – 12 Minutes, Difficulty Level – 3/5

Ever wonder how animals “instinctively” know how to migrate hundreds of kilometers every year? In the world of perception, human senses are nothing special. Many animals have evolved incredible abilities and senses. One fascinating phenomenon is magnetoreception – the ability of animals to sense the magnetic field of the earth.

To see the natural processes behind magnetoreception, we must dive deep below the surface of the earth where a “geodynamo” process creates a magnetic field. The solid inner core under high pressure and the outer core liquid under lower pressure are both rich in iron. The difference in pressure between the hot inner and outer cores leads to convection currents and the movement of iron creates an electrical field, and with it, a magnetic field surrounding the earth (Dobson, 2016; Nordman et al, 2017).

Humans can navigate the magnetic field using a compass, but many animals including marine animals, turbirds, and insects, have the incredible ability to sense the earth’s magnetic field and utilize it as a valuable navigational tool. This is how newly hatched sea turtles on the coast of Florida, who have never swum through the enormous expanse of the ocean, “know” how to migrate the currents across the Atlantic Ocean. Their magnetoreception is crucial to complete their migrations as hatchlings and return years later for nesting.

But how do they perceive the magnetic field?

Sea turtles to spatially orient themselves and navigate based on both the intensity and inclination of the earth’s magnetic field. It can be difficult for us to visualize this geomagnetic map, but imagine the magnetic lines pointing down (entering) at the north magnetic pole, and pointing up (leaving) at the southern magnetic pole. Between the magnetic poles, the magnetic lines run parallel to the surface of the earth.

Secondly, due to the curvature of the earth, the magnetic field varies in intensity; the magnetic lines are denser as they approach the poles. This means, depending on your location on the earth, there is a differing intensity and inclination of the magnetic lines, as pictured in Figure 1.

^{Figure 1. A visual representation of the earth’s magnetic field. Animals can perceive its inclination or intensity (Source: NASA)}

An Enigmatic Ability

Currently, there are 2 main hypotheses for the mechanism of magnetoreception.

1. A magnetite-based magnetoreceptor

This theory posits that the organism’s cells contain a specialized structure on their surface that could “perceive” the earth’s magnetic field as a stimulus and activate molecular processes in the cell. However, since our cells surfaces are made up of primarily carbon, oxygen, and nitrogen, these specialized structures would have to contain an atom or molecule that is more responsive to the magnetic field, such as magnetite or Fe₃O₄, a crystal found in iron ores and often described as a natural magnet (Nordmann et al, 2017).

The magnetoreceptor contains 2 components: magnetite embedded within the cell surface and channel, sensitive to mechanical changes; the magnetite would align with the magnetic field and create a torque force that would alter the channel.

These mechanical exertions would activate the channel, allowing positively charged atoms to enter the cell from the surrounding environment. A sudden influx of positive charge would depolarize the cell, leading to the creation of a nerve impulse or other downstream changes in cell activity.

This type of magnetoreception is found in many animals including birds and insects. Honeybees have small iron granules in their abdomens (Liang et al, 2016) while several bird species have magnetite in their beaks (Wiltschko & Wiltschko, 2013).

Magnetite is also found in magnetotactic bacteria, one of the only living organisms that can form magnetite. The Magnetospirillum gryphiswaldense MSR-1 has been observed to form magnetite chains connected to their cell structures – a direct link between a sensory magnetic signal and mobility (Uebe & Schuler, 2016). These microorganisms live in stratified freshwater, which means that they need to live within a certain range of oxygen concentration. They start forming magnetite through a process called “biomineralization” a complex process in which the outer surface of the cell invaginates and creates small pouches called “magnetosomes”, into which it transports proteins and iron.

These bacteria have over 40 genes that specifically encode proteins that are related to forming the magnetosomes and organizing them into chains and integrating them into the structure of the cell. Thus, when the magnetite crystals align with the magnetic field, the chain creates a directionality that orients a cell towards their preferred oxygen concentration (Figure 2).

^{Figure 2. The process of mineralization visualized through cryoelectron tomograms.}

Invaginations form through the outer membrane (OM) and line up along the cell’s structural component on the inner cytoplasmic membrane (CM) (Uebe & Schuler, 2016). Thus, although these cells are searching for a chemical signal such as oxygen, they use the inclination and intensity of the magnetic field to reach the depth at which their desired oxygen level is found. Within a stratified aquatic environment, this becomes a highly efficient route of movement. When it comes time for the cell to divide, the magnetosome is bent and divided for the next generation.

2. A light-sensitive chemical compass

This theory posits that cells contain a structure that can biochemically sense the geomagnetic field upon absorbing a photon, a light particle and creating photoinduced radical pairs.

What is a radical pair?

Well, a radical is a molecule that contains an unpaired electron. A radical pair is two radicals, typically created during a chemical reaction. Because electrons typically prefer to be paired, radicals are often incredibly unstable and do not remain in the radical state for long. However, radicals themselves have a magnetic state because the unpaired electrons have spins, which in turn have a minor magnetic effect (Hore & Mouritsen, 2016).

In turn, these tiny magnets are affected by the earth’s magnetic field. When the photon enters the eye, it activates a donor molecule to give up an electron to an acceptor molecule, converting both into radicals. Because the radicals are highly unstable, they exist in a state of fluctuation between different quantum spin states, called “singlet” and “triplet”. However, the proportion of singlet vs triplet product depends on the inclination of the earth’s magnetic field, as pictured in Figure 2 (Wiltschko & Wiltschko, 2006).

^{Figure 2. Within a specialized cell structure called a cryptochrome, a donor molecule transfers an electron to an acceptor molecule and creates free radicals, which can fluctuate between singlet and triplet states (Wiltschko & Wiltschko, 2006).}

Depending on the yield of singlet vs triplet product, animals can interpret the inclination of the magnetic field that they are currently situated in and thus be able to navigate to their targets. This chemical reaction typically occurs inside a cell structure called a cryptochrome and it parallels the way that our eyes see visual stimuli or plant’s chlorophyll produce photosynthesis. This mechanism doesn’t require iron but it’s light-dependent. This model appears to work in many bird species and even in some amphibians such as salamanders.

There are still some mysteries we have yet to uncover.

There are animals such as the Caretta caretta loggerhead sea turtles which are able to orient themselves even in the dark, suggesting that they do not employ the radical pair mechanism – but they also do not show any signs of collecting magnetite. So is there a mechanism for radical pairs that happens without the need for photoreceptors? It’s hard to know, especially since loggerhead sea turtles are massive in size and can be quite difficult to study.

Another mystery is how magnetoreception comes to be – are the proteins simply encoded to detect all magnetic signals and therefore can adapt to local changes in magnetic incline or polarity? Or does a progeny come out of the womb or egg having been conditioned to navigate specifically the magnetic field that they experienced in the womb?

Research has actually shown that it might be the latter; animals are born with magnetoreception – but the development of the senses is also affected by magnetism.

For example, one research group compared two different groups of Caretta caretta eggs; they allowed a control group to develop in the normal magnetic field of northern Portugal, while the second group was developed with magnets placed around them to distort the magnetic field. When the turtles hatched, the second group was unable to orient itself in the correct direction, while the control group oriented itself in the correct direction that matches regular migratory patterns (Fuxjager et al, 2014).

As the researchers point out, this is concerning from a conservation point of view, as man made structures such as resorts and condominiums contain large amounts of metals which can influence magnetic fields and further disorient hatchlings.

The Therapeutic Potential

Although humans cannot detect the magnetic field, magnetotactic bacteria may hold an intriguing and promising potential to help us.

Staphylococcus aureus is a highly pathogenic and contagious bacterial species that often develop antibiotic resistance by creating a protective biofilm. S. aureus is not magnetotactic, but researchers in a 2016 study bioengineered a bridge molecule and attached S. aureus cells to a species of magnetotactic bacteria called MO-1 (Chen et al, 2016). They then treated the sample with an alternating magnetic field for approximately 1 hour; the magnetosomes of MO-1 efficiently absorbed the radiation, and released heat. The accumulating heat formation created a hyperthemic environment which killed over 50% of S. aureus populations.

The researchers recreated this effect in mice and as a bonus, they noticed that the heat released from MO-1 actually significantly accelerated wound-healing (Chen et al, 2016). This can be incredibly useful as staph often opportunistically infects organisms through wounds.

Similarly, many cancers also confer resistance towards drug treatments and chemotherapy. Cancers are highly invasive and aggressive; they consumer all nutrients quickly and often create biologically hostile environments with high acidity and low oxygen, creating a barrier for drug entry or immune cell entry. However, if an agent is introduced that can be externally controlled – such as, say, a magnetically influenced carrier – then scientists can penetrate this hostile environment.

Magnetotactic bacteria are already well-versed in navigating environments with oxygen gradients. Felfoul et al (2016) achieved this by binding nanoparticles containing drugs onto a magnetotactic bacterial species called MC-1, and injecting these carriers near tumour masses in mice. They were able to magnetically guide the bacteria into the hypoxic core of the tumours and target the tumour significantly more effectively. Another study from 2019 attached two different types of anthracyclines to magnetosomes and targeted liver cancer cells in mice; not only did these transporters effectively penetrate hypoxic environments while not harming the healthy cells, but they had a significantly greater lethality on cancer cells than using the anthracyclines alone (Geng et al, 2019).

Magnetoreception is an incredible phenomenon that helps animals detect a seemingly invisible navigational tool. It’s what helps sea turtles and birds make their tremendous migrations every year, or honeybees to find their way home. But the more we learn about the underlying mechanisms of this mysterious ability, the more tools we can uncover to help us combat human diseases like antibiotic resistance and drug-resistant cancers!

References

• Chen, C., Chen, L., Yi, Y., Chen, C., Wu, L. F., & Song, T. (2016). Killing of Staphylococcus aureus via Magnetic Hyperthermia Mediated by Magnetotactic Bacteria. Applied and environmental microbiology, 82(7), 2219–2226. https://doi.org/10.1128/AEM.04103-15
• Dobson, D. (2016). Earth’s core problem. Nature, 534(7605), 45-45.
• Fuxjager, M. J., Davidoff, K. R., Mangiamele, L. A., & Lohmann, K. J. (2014). The geomagnetic environment in which sea turtle eggs incubate affects subsequent magnetic navigation behaviour of hatchlings. Proceedings of the Royal Society B: Biological Sciences, 281(1791), 20141218.
• Geng, Y., Wang, J., Wang, X., Liu, J., Zhang, Y., Niu, W., … Jiang, W. (2019). Growth-inhibitory effects of anthracycline-loaded bacterial magnetosomes against hepatic cancer in vitro and in vivo. Nanomedicine. doi:10.2217/nnm-2018-0296
• Hore, P. J., & Mouritsen, H. (2016). The Radical-Pair Mechanism of Magnetoreception. Annual Review of Biophysics, 45(1), 299–344. doi:10.1146/annurev-biophys-032116-094545
• Liang, C. H., Chuang, C. L., Jiang, J. A., & Yang, E. C. (2016). Magnetic sensing through the abdomen of the honey bee. Scientific reports, 6(1), 23657.
• Nordmann, G. C., Hochstoeger, T., & Keays, D. A. (2017). Magnetoreception-A sense without a receptor. PLoS biology, 15(10), e2003234. https://doi.org/10.1371/journal.pbio.2003234
• Uebe, R., & Schüler, D. (2016). Magnetosome biogenesis in magnetotactic bacteria. Nature Reviews Microbiology, 14(10), 621–637. doi:10.1038/nrmicro.2016.99
• Wiltschko, R., & Wiltschko, W. (2006). Magnetoreception. BioEssays, 28(2), 157–168. doi:10.1002/bies.20363
• Wiltschko, R., & Wiltschko, W. (2013). The magnetite-based receptors in the beak of birds and their role in avian navigation. Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology, 199(2), 89–98. https://doi.org/10.1007/s00359-012-0769-3