Magnetosome formation in prokaryotes

Key Points Magnetotactic bacteria are motile, generally aquatic microorganisms that can move along geomagnetic field lines. This group of fastidious prokaryotes is heterogeneous and all magnetotactic bacteria identified so far are Gram-negative. Magnetotactic bacteria behave essentially like miniature magnetic compass needles and can respond to the Earth's geomagnetic field because they synthesize intracellular structures, known as magnetosomes, which contain magnetic mineral crystals. Iron oxide magnetosomes contain magnetite (Fe 3 O 4 ) crystals and iron sulphide magnetosomes contain greigite (Fe 3 S 4 ) crystals. In their aquatic habitats, most magnetotactic bacteria are found at the oxic–anoxic interface. Magnetotaxis is believed to function in conjunction with aerotaxis to guide magnetotactic bacteria to areas of correct oxygen tension. There are two different mechanisms of magneto-aerotaxis: polar magneto-aerotaxis, in which the magnetic field provides an axis and a direction for motility, and axial-magneto-aerotaxis, in which the magnetic field only provides an axis for motility. The formation of magnetosomes — which are surrounded by a lipid bilayer approximately 3–4 nm thick — is a complex process, involving several discrete steps. At present, there is no evidence to indicate that magnetotactic bacteria use unique iron-uptake systems; evidence for the presence of siderophores in some magnetotactic species, including Magnetospirillum magnetotacticum and Magnetospirillum magneticum , indicates that Fe( III ) can be taken up as well as Fe( II ). As yet it is unclear whether the magnetosome membrane vesicle is produced before the magnetic crystals have been formed, or whether the cytoplasmic membrane invaginates around the developing crystal. For many years, the inability of researchers to manipulate magnetotactic bacteria genetically hindered progress in elucidating the pathways that are involved in magnetosome synthesis. However, genetic systems for some magnetotactic bacteria have now been established. Additionally, genome sequencing of several species of magnetotactic bacteria is underway. Magnetotactic bacteria were discovered almost 30 years ago, and for many years and many different reasons, the number of researchers working in this field was few and progress was slow. Recently, however, thanks to the isolation of new strains and the development of new techniques for manipulating these strains, researchers from several laboratories have made si