South-Seeking Magnetic Bacteria

Abstract

Magnetotactic bacteria, originally discovered by Blakemore (1975), are by far the most convincing and abundant example of magnetically sensitive organisms in existence. Their magnetite crystals passively align the bacteria with the earth's magnetic field like a 3-dimensional compass (Frankel et al. 1979). These microaerophilic bacteria normally live in the soupy, oxygen-poor mud/water transition zone in many freshwater and marine environments. If the mud is disturbed so that the bacteria are exposed to oxygen-rich water, the species discovered so far (all from the northern hemisphere) swim rapidly along the direction of magnetic north. Because the magnetic field dips downward in the northern hemisphere, the bacteria eventually reach the mud/water interface again and avoid poisoning themselves with oxygen. Moench & Konetzka (1978) have devised an elegant technique to purify the bacterial population based on their swimming response - the bacteria will swim towards the south magnetic pole of a bar magnet placed near their jar, purifying themselves into a characteristic little pellet containing millions of individual cells. (The north geographic pole is magnetically south, so the bacteria were still trying to go to the north and down.) Two major questions concerning the behaviour of these bacteria need to be answered, however: (1) which way do they swim in the southern hemisphere, and (2) what do they do on the magnetic equator where the field is horizontal?

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