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Geomagnetic Field Effects on Living Systems

Magnetotactic bacteria do not require O2 for respiration (anaerobic). Because it is dangerous for them

to have a lot of harmful light and O2 near the water surface, they must dive in the less oxygenated mud

of lakes and seabeds. Terefore, it has been reported that anaerobic magnetotactic bacteria respond to

high O2 levels by swimming downward into areas with low or no O2 toward geomagnetic north in the

Northern Hemisphere (Blakemore, 1975, 1982; Moench and Konetzka, 1978) and geomagnetic south in

the Southern Hemisphere (Blakemore et al., 1980; Kirschvink, 1980). Magnetotactic bacteria will dive

downward, but this does not mean that they can sense gravity and move in the direction indicating

“compass information” directly below. It has been found that magnetotactic bacteria aiming downwards

can swim and orient themselves along the direction of the MF lines that are inclined from the horizon­

tal direction. Magnetotactic bacteria can sense the “inclination” or “dip angle” with a higher degree of

accuracy, which is the angle between the horizontal plane (H) and the total feld vector (F) (World Data

Center for Geomagnetism, Kyoto).

In contrast, the opposite cases of the overwhelming majority of other Northern or Southern

Hemisphere magnetotactic bacteria have been found. As is the case with the bacteria in the Northern

Hemisphere that swim south along the direction of the MF lines (Simmons et al., 2006), the bacteria in

the Southern Hemisphere swim north along the direction of the MF lines (Leão et al., 2016). Tese fnd­

ings support the idea that magnetotaxis is more complex than previously thought, and may be modu­

lated by factors other than O2 concentration and redox gradients in sediments and water columns (Leão

et al., 2016).

Te discovery of magnetite had been done afer the discovery of magnetotactic bacteria. Magnetite

was the constituent element of the chiton teeth, which was discovered in 1962 by an American scien­

tist, Lowenstam (Lowenstam, 1962). One tooth of the chiton is one of the largest magnetites that living

organisms possess, and its size is 1 mm (1 million times that of in vivo magnetite). It is reported that

chitons may be responsible for natural magnetizations on the order of 10⁻¹⁰ T in marine sediments,

whereas mud magnetotactic bacteria could produce remanence near 10⁻¹² T in both marine and fresh

water sediments (Kirschvink and Lowenstam, 1979). However, so far, there are no reports that chitons

use their teeth to sense the GMF, and it is not yet clear why their teeth are magnets. In the subsequent

research, these small biomagnets were used not only for chitons and bacteria but also for nematodes,

Caenorhabditis elegans (Cranfeld et al., 2004) that have made important contributions to molecular

genetics and developmental biology. Nematodes, C. elegans, are model organisms, which refect their

evolutionary relationships with familiar animals such as pigeons, migratory birds, salmon and dol­

phins, and furthermore, humans. Moreover, in C. elegans, the physiological recordings of the magneto-

sensory amphid fnger (AFD) bilateral neuron pair showed that neuronal responses saturate at larger

than GMF intensity, ~65 μT (Wu and Dickman, 2012; Vidal-Gadea et al., 2015). C. elegans uses the

lef-right pair of AFD sensory neurons that project sensory structures to the tip of the worm’s head

(Clites and Pierce, 2017). If the origin of magnetite lies in magnetotactic bacteria, it is exactly the realiza­

tion of the “endosymbiotic theory” (anaerobic eukaryotes coexisted by swallowing aerobic bacteria and

evolved into the present eukaryotic cells), which was proposed in 1967 by an American biologist “Lynn

Margulis” (Sagan, 1967).

6.2.2 Magnetic Sense of Animals

Suppose you have a geomagnetic sensor, such as a magnetic compass, inside your body. It seems likely

that you know the direction indicating “compass information,” and more specifcally, the location indi­

cating “map information” of the latitude and longitude coordinates where you are. So where should

we go to reach our destination? To get this answer, you need something like a map that tells you the

positional relationship between your current location and your destination. By detecting the GMF, it

shows its position exactly on the “magnetic map” (frst described by Wallraf, 1999), and it is used as the

magnetic map like a car navigation system or GPS as several mobile or migratory animals that travel

long distances for their living.