Biological Significance Of Magnetosomes

The orientation and active swimming of MTB along the Earth's geomagnetic field lines is called magnetotaxis, which is determined both by the presence of magnetosomes and the ability to perform active movements. Dead cells containing magneto-somes also align along the geomagnetic field lines whereas alive and swimming MTB with no magnetosomes do not align. In Earth's geomagnetic field (around 0.05 mT) cells are neither attracted nor pulled towards either geomagnetic pole (Bazylinski and Frankel, 2004). It is proposed that in natural environments magnetotaxis enables the cells to locate and maintain an optimal position in water columns or in sediments, with respect to their main metabolical needs: molecular oxygen and other nutrients (Frankel et al., 1997), all in all allowing them to keep their headings as they swim in the face of the disorienting Brownian buffering by the medium (Bazylinski et al., 2007). Bazylinski and Frankel (2004) claimed that the term magnetotaxis is in fact a misnomer because in contrast with a true tactic response MTB swim neither up nor down a magnetic field gradient. They proposed the term magneto-aerotaxis to better explain the interaction between magnetotaxis and aerotactic sensory mechanisms (Frankel et al., 1997). In this respect, Smith et al. (2006) showed that magnetic, wildtype cells swimming in an applied magnetic field migrate more quickly away from the advancing oxygen than either wild-type cells in a zero field or nonmagnetic cells in any magnetic field. According to the authors, the key benefit of magnetotaxis is an enhancement of the bacterium's ability to detect oxygen, not an increase in its average speed for moving away from high oxygen concentrations. Bazylinski and Frankel (2004) developed the new concept of magnetically assisted taxis, claiming that it is possible and perhaps likely, that there are other forms of magnetically assisted taxis in response to sulfide, redox or light gradients. This is why, in dynamic water columns MTB appear to be associated with peaks in dissolved and particulate iron that were also present at oxycline. Peaks of particulate Fe(III) are often observed immediately above marine chemoclines due to the upward flux of Fe(II) into oxygenated waters (Murray et al., 1995; Zopfi et al., 2001).

Another possible function of magnetosomes is in iron homeostasis and detoxification (Mann et al., 1990). In a mutant of M. gryphiswaldense the loss of capability to form magnetosome was accompanied by a higher sensitivity against elevated concentrations of iron in the growing medium (Schübbe et al., 2003). This might be indicative of a contribution of magnetite formation to iron home-ostasis and detoxification (Schüler, 2004).

Urban (2000) favors the hypothesis that magnetosomes formation primarily affects the cell density and the cell's response to gravity and that magnetotactic behavior is only a secondary response. He showed that M. magnetotacticum placed in microgravity conditions was impaired in their ability to orient towards the magnets. It has also been speculated that magnetosomes can play a role in redox and pH control (Gorby et al., 1988; Mann et al., 1990).

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