Magnetotaxis in Prokaryotes


Magnetotaxis refers to the behaviour of some motile, aquatic, bacteria that orient and swim along magnetic field lines. These microorganisms, called magnetotactic bacteria (MTB), contain intracellular structures known as magnetosomes, which are nano‐sized, magnetic, iron‐mineral crystals, each enveloped by a biological (phospholipid bilayer) membrane. Magnetosomes are usually arranged in chains within the cell, providing it with a permanent magnetic dipole moment that facilitates location and retention in the cell's preferred habitat at or below the oxic–anoxic interface in the water column or sediment. Although all MTB are motile by means of flagella and have a cell wall structure characteristic of Gram‐negative bacteria, their diversity is reflected by the large number of different morphotypes found in environmental samples of water or sediment, and by phylogenetic analysis of both cultured and uncultured organisms.

Key Concepts:

  • Prokaryotes (bacteria), like eukaryotes, internally compartmentalise and contain organelles.

  • The prokaryotic flagellum rotates clockwise and counter clockwise thereby propelling the cell during swimming.

  • Magnetotaxis is bacterial motility directed by a magnetic field.

  • Magnetotactic bacteria contain intracellular, nano‐scale, membrane‐enveloped, magnetic iron‐mineral crystals called magnetosomes.

  • Magnetosomes impart a permanent magnetic dipole moment to cells of magnetotactic bacteria causing them to behave as miniature, motile, compass needles.

  • Magnetotaxis works in conjunction with aerotaxis to increase energy transduction in magnetotactic bacteria.

  • Genes for magnetosome formation are clustered in a region of the genomes of magnetotactic bacteria known as a magnetosome gene or genomic island.

  • Magnetotactic bacteria are important in the cycling of a number of important elements including carbon, iron, nitrogen and sulfur.

Keywords: aerotaxis; biomineralisation; chemically stratified environments; flagellar motion; greigite; magnetite; magnetosome; magnetotactic bacteria; magnetotaxis

Figure 1.

Phylogenetic tree, based on 16S (rRNA) gene sequences, showing the phylogenetic position of known magnetotactic bacteria (bold) and their closest relatives in the Domain Bacteria. The tree is based on neighbour‐joining analyses. Bar represents 2% sequence divergence.

Figure 2.

(a) (DIC) light micrograph of North‐seeking (NS) magnetotactic bacteria, with polar magnetotaxis, aggregated at the edge of the drop of a ‘hanging drop’. (b) Magnetic enrichment of magnetotactic bacteria from an environmental sample by applying the south pole of a magnet (M) several centimeters above the water–sediment interface for 30 min. Magnetotactic bacteria that accumulate at the magnet are shown as a dark spot at the arrow.

Figure 3.

(TEM) images of magnetotactic bacteria and magnetosomes. (a) TEM of a cell of the genus Magnetospirillum species showing the chain of magnetosomes inside the cell. The magnetite crystals in the magnetosomes have cuboctahedral morphology. The magnetosome chain is fixed in the cell and the interaction between the magnetic dipole moment associated with the chain and the local magnetic field causes the cell to align along magnetic field lines. Rotation of the bipolar flagella (at arrows) causes the cell to swim along the field lines. (b) High‐magnification TEM image of bullet‐ or tooth‐shaped magnetite crystals with one pointed and one flat end or pointed at both ends; (c) High‐magnification TEM image of cuboctahedral magnetite crystals; (d) High‐magnification TEM image of elongated pseudo‐prismatic magnetite crystals; and (e) High‐magnification TEM image of greigite crystals.

Figure 4.

Schematic showing the formation of magneto‐aerotactic bands in flat capillary tubes with both ends open placed in a magnetic field B. (a) Diffusion of air into both ends results in a double oxygen concentration gradient in the tube, with the minimum oxygen concentration at the centre (c, capillary; m, meniscus). (b) Bacteria with the axial magneto‐aerotactic mechanism form bands at both ends of the tube. (c) Northern hemisphere bacteria with the polar magneto‐aerotactic mechanism form a band only at end of the tube for which the B is antiparallel to the oxygen gradient. (c) Southern hemisphere bacteria would form a band only at the other end of the tube.

Figure 5.

Schematic showing how polar magneto‐aerotaxis keeps cells at the preferred oxygen concentration in the oxic–anoxic interface (OAI) in chemically stratified water columns and sediments (NH, northern hemisphere; SH, southern hemisphere; Bgeo, geomagnetic field). In both hemispheres, cells at higher than optimal oxygen concentration in the ‘oxidised state’ swim forward by rotating their flagella counter clockwise (ccw), whereas cells at lower than optimal oxygen concentration in the ‘reduced state’ rotate their flagella clockwise (cw) and swim backward without turning around. Note that the geomagnetic field selects for cells with polarity such that ccw flagellar rotation causes cells to swim downward along the magnetic field lines in both hemispheres.



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Further Reading

Bazylinski DA and Schübbe S (2007) Controlled biomineralization by and applications of magnetotactic bacteria. Advances in Applied Microbiology 62: 21–62.

Frankel RB and Bazylinski DA (2009) Magnetosomes and magneto‐aerotaxis. Contribution in Microbiology 16: 182–193.

Moskowitz BM (1995) Biomineralization of magnetic iron minerals. Reviews of Geophysics Supplement 33: 1234–1128.

Schüler D (ed.) (2007) Microbiological Monographs Vol. 3: Magnetoreception and Magnetosomes in Bacteria, pp. 319 Berlin, Heidelberg: Springer‐Verlag.

Taylor BL, Zhulin IB and Johnson MS (1999) Aerotaxis and other energy‐sensing behavior in bacteria. Annual Reviews of Microbiology 53: 103–128.

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Lefèvre, Christopher T, Frankel, Richard B, and Bazylinski, Dennis A(Dec 2011) Magnetotaxis in Prokaryotes. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000397.pub2]