Nanoscale engineering of magnetic particles by magnetotactic bacteria

Magnetotactic bacteria (MB) are a group of mainly aquatic, motile, gram-negative prokaryotes that belong phylogenetically in the domain bacteria. They are a metabolically and morphologically diverse group but have in common the fact that their motility is influenced by the Earth´s geomagnetic and other external magnetic fields, a phenomenon known as magnetotaxis. Magnetotaxis is due to the presence of intracellular enveloped magnetic crystals called magnetosomes. MB biomineralize magnetosomes de novo through a process known as biologically-controlled mineralization in which they appear to control the composition, the shape, and the size of the magnetosome mineral phase. Magnetosomes contain crystals of two types: Fe oxides or Fe sulfides. The Fe oxides consist mainly of the mineral magnetite, Fe₃O₄, and the Fe sulfides, greigite, Fe₃S₄. In general, mature magnetosome crystals are stoichiometric Fe₃O₄, while the Fe₃S₄ crystals may contain impurities such as other transition metals and Fe sulfide precursors to Fe₃S₄. MB, in general, form only one compositional type of magnetosome. MB synthesize species- and/or strain-specific crystal morphologies: either cubo-octahedral, elongated parallelepipedal, or tooth- or bullet-shaped particles. Idealized magnetosome particle morphologies derived from electron microscopic images show that they are of relatively high structural perfection. Mature magnetosome crystals lie in a very narrow size range, from about 35-120 nm, which has physical significance. Particles of Fe₃O₄ and Fe₃S₄ in this size range are single magnetic domains (SMD) and have a permanent stable magnetic dipole moment at room temperature. Smaller particles are superparamagnetic and are only magnetic at room temperature in the presence of a very strong magnetic field. Domain walls would form in larger particles thereby reducing the magnetic moment of the particle. By forming SMD crystals, the MB have maximized the magnetic remanence of each individual magnetosome crystal. Most MB arrange their magnetosomes in a chain-like motif in which the total magnetic dipole moment of the cell is the sum of the moments of the individual crystals. Thus, the cell has maximized it magnetic- dipole moment by arranging its magnetosomes in chains. It is clear that the MB have found a way through evolution to construct a masterpiece of magnetic engineering. The biological advantages to the magnetosome and their use in biotechnology and as magnetofossils will also be discussed in this talk.