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Magnetoreception Research
[a] Claus Nielsen, et al. "Absorption Spectra of FAD Embedded in Cryptochromes" 2018 9 (13), 3618-3623 DOI: 10.1021/acs.jpclett.8b01528

[b] Atticus Pinzon-Rodriguez, et al. "Expression patterns of cryptochrome genes in avian retina suggest involvement of Cry4 in light-dependent magnetoreception" J. R. Soc. Interface 2018 15 20180058; DOI: 10.1098/rsif.2018.0058. Published 28 March 2018

[c] Alexander Pakhomov, et al. "Very weak oscillating magnetic field disrupts the magnetic compass of songbird migrants" J. R. Soc. Interface 2017 14 20170364; DOI: 10.1098/rsif.2017.0364. Published 9 August 2017

Proposed mechanisms
An unequivocal demonstration of the use of magnetic fields for orientation within an organism has been in a class of bacteria known as magnetotactic bacteria. These bacteria demonstrate a behavioural phenomenon known as magnetotaxis, in which the bacterium orients itself and migrates in the direction along the Earth's magnetic field lines. The bacteria contain magnetosomes, which are nanometer-sized particles of magnetite or iron sulfide enclosed within the bacterial cells. The magnetosomes are surrounded by a membrane composed of phospholipids and fatty acids and contain at least 20 different proteins They form in chains where the magnetic moments of each magnetosome align in parallel, causing each bacterium cell to essentially act as a magnetic dipole, giving the bacteria their permanent-magnet characteristics.

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For animals the mechanism for magnetoreception is unknown, but there exist two main hypotheses to explain the phenomenon. According to one model, magnetoreception is possible via the radical pair mechanism. The radical-pair mechanism is well-established in spin chemistry,  and was speculated to apply to magnetoreception in 1978 by Schulten et al.. In 2000, cryptochrome was proposed as the "magnetic molecule", so to speak, that could harbor magnetically sensitive radical-pairs. Cryptochrome, a flavoprotein found in the eyes many species of bird[b] (such as European robins) and other animal species, is the only protein known to form photoinduced radical-pairs in animals. The function of cryptochrome is diverse across species, however, the photoinduction of radical-pairs occurs by exposure to blue light, which excites an electron in a chromophore. The Earth's magnetic field is only 0.5 gauss and so it is difficult to conceive of a mechanism, other than phase shift, by which such a field could lead to any chemical changes other than those affecting the weak magnetic fields between radical pairs. Weak magnetic fields have been shown to affect songbirds generally[c]. Cryptochromes are therefore thought to be essential for the light-dependent ability of the fruit fly Drosophila melanogaster to sense magnetic fields. This light-dependency of cryptochromes has been observed to play an important role in daily orientation as well as migration[b]. Additional research into the absorption spectra that leads to effective magnetoreception in the cryptochromes[a] may lead to further cement the cryptochromes as the "magnetic molecule."

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[magnetism-start]

The second proposed model for magnetoreception relies on Fe3O4, also referred to as iron (II, III) oxide or magnetite, a natural oxide with strong magnetism. Iron (II, III) oxide remains permanently magnetized when its length is larger than 50 nm and becomes magnetized when exposed to a magnetic field if its length is less than 50 nm. In both of these situations the Earth's magnetic field leads to a transducible signal via a physical effect on this magnetically sensitive oxide.

The second proposed model for magnetoreception relies on clusters composed of iron, a natural mineral with strong magnetism. The idea is favorable as it builds up on the magnetoreceptive abilities of magnetotactic bacteria. These iron clusters have been observed mainly in homing pigeons in the upper beak[44], but also in other taxa.

These iron clusters have been observed in two types of compounds: magnetite (Fe3O4) and maghemite (γ-Fe2O3). Both are believed to play a part in the magnetic sense, particularly for the magnetic map sense. These concentrations are believed to be connected to the central nervous system to form a sensing system. Research has focused on magnetite concentrations, however, magnetite alone has been shown to not be in magnetosensitive neurons .

Maghemite has been observed in platelets concentrated along the dendrites of the upper beak, consistently in the nanoscale. When in the nanoscale, iron oxides will remain permanently magnetized at lengths greater than 50 nm and will become magnetized at lengths smaller than 50 nm[11]. Since these platelets have been observed in collections of 5-10, they are thought to form dipoles local to the dendrite they are present in. These local magnetic changes then cause a mechanical response along the membrane of the nerve cell, leading to a change in ion concentrations. This ion concentration, with respect to the other dendrite clusters is believed to form the magnetic sense.

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[conclusion-start]

Despite more than 50 years of research, a sensory receptor in animals has yet to be identified for magnetoreception. While research is ongoing, it is possible that the entire receptor system could fit in a one-milimeter cube and have a magnetic content of less than one ppm. As such, even discerning the parts of the brain where the information is processed presents a challenge.

The most promising leads — cryptochromes, iron-based systems, electromagnetic induction — each have their own pros and cons. Cryptochromes have been observed in various organisms including birds and humans, but does not answer the question of night-time navigation. Iron-based systems have also been observed in birds, and if proven, could form a magnetoreceptive basis for many species including turtles. Electromagnetic induction has not been observed nor tested in non-aquatic animals. Additionally, it remains likely that two or more complementary mechanisms play a role in magnetic field detection in animals. Of course, this potential dual mechanism theory raises the question, to what degree is each method responsible for the stimulus, and how do they produce a signal in response to the low magnetic field of the Earth?

Then there is the distinction of magnetic usage. Some species may only be able to sense a magnetic compass to find north and south, while others may only be able to discern between the equator and the pole. Although the ability to sense direction is important in migratory navigation, many animals also have the ability to sense small fluctuations in earth’s magnetic field to compute coordinate maps with a resolution of a few kilometers or better. For a magnetic map, the receptor system would have to be able to discern tiny differences in the surrounding magnetic field to develop a sufficiently detailed magnetic map. This is not out of the question, as many animals have the ability to sense small fluctuations in the earth's magnetic field. This is not out of the question biologically, but physically has yet to be explained. For example, birds such as the homing pigeon are believed to use the magnetite in their beaks to detect magnetic signposts and thus, the magnetic sense they gain from this pathway is a possible map. Yet, it has also been suggested that homing pigeons and other birds use the visually mediated cryptochrome receptor as a compass.

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