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MAGNETOTACTIC BACTERIA Mihály Pósfai University of Pannonia, Veszprém, Hungary

Definition Magnetotactic bacteria (MTB) are a diverse group of Gram-negative, motile prokaryotes that align and migrate along the geomagnetic field. This magnetotactic behavior is based on intracellular organelles called magnetosomes, which are inorganic nanocrystals of either magnetite (Fe3O4) or greigite (Fe3S4), surrounded by a lipid bilayer membrane. The magnetosomes are typically arranged in one or more chains, conveying a magnetic dipole moment to the cell (Figure 1). Magnetotaxis is thought to be beneficial for the cell for finding its optimum position in vertical concentration gradients in aquatic habitats. History Bacteria that responded to magnetic fields were discovered independently by Bellini (1963) and Blakemore (1975), who coined the term magnetotaxis and noted the presence of crystals in the cells. Intracellular magnetite was identified and the physics of magnetotaxis was described by Frankel et al. (1979). Iron sulfide-producing MTB were found in 1990 (Mann et al., 1990; Farina et al., 1990). The ecology of MTB and the structures and morphologies of the crystalline components of magnetosomes have been widely studied since the discovery of MTB. In the past decade, significant progress has been made in elucidating the molecular, biochemical, and genetic bases of magnetosome formation (Schüler, 2007). Ecophysiology MTB are ubiquitous in both marine and freshwater aquatic habitats, where suboxic or anoxic conditions prevail, and

dissolved iron is available. Because of their fastidious nature, only a few strains are available in pure culture. With one exception, all strains in pure culture have a respiratory form of metabolism. All known MTB are microaerophiles (they require a certain amount of oxygen for growth, but exhibit a negative tactic and/or growth response to atmospheric concentrations of oxygen) or anaerobes or facultatively anaerobic microaerophiles (Bazylinski and Frankel, 2004). The physiology of MTB appears to dictate their ecology: since MTB derive energy from the proximity of oxidants and reductants, magnetite producing organisms occur in largest numbers (up to 105 cells/ml) in layers at or near the oxic–anoxic interfaces (OAI) of chemically stratified habitats. Iron sulfideproducing bacteria live below the OAI under anaerobic conditions (Simmons et al., 2004). Whereas magnetiteproducing bacteria occur virtually in all types of water bodies, sulfide-producing organisms have been found in marine habitats only. Considering the large amount of iron concentrated in magnetosomes (10
15 to 10
13 g Fe per cell) and the estimated population density of MTB at and below the OAI, MTB are likely to contribute significantly to the flux of reduced Fe to sediments in chemically stratified marine basins.

Magneto-aerotaxis Magnetotaxis means active motion guided by the magnetic field. MTB passively orient along the geomagnetic field and actively swim by means of flagella. A key element of magnetotaxis is that the cell shoul