Electron microscopic studies of magnetosomes in magnetotactic bacteria

Abstract

Electron microscopic studies on magnetosomes in magnetotactic bacteria have revealed much information on their composition, structure, and even the formation of their mineral phase. The mineral phases of the magnetosomes are of two general types: iron oxides and iron sulfides. Iron oxide-type magnetosomes contain particles of the ferrimagnetic mineral magnetite (Fe3O4) while the iron sulfide-type contain ferrimagnetic greigite (Fe3S4), greigite and non-magnetic pyrite (FeS2), or possibly ferrimagnetic pyrrhotite (Fe7S8). Regardless of their composition, the crystalline particles in magnetosomes have a narrow size range: approximately 35 to 120 nm. Magnetite crystals in this size range are single-magnetic-domains and confer a permanent magnetic dipole moment to the cell. The single-domain size range for greigite is not known but is probably similar to that for magnetite. The morphology of the particles in the bacterial magnetosomes appears to be species-specific. Morphologies of magnetite crystals in different species of magnetotactic bacteria include cubo-octahedra, parallelepipedal (truncated hexahedral or octahedral prisms), and tooth- or bullet-shaped (anisotropic). Morphologies of greigite particles include cubo-octahedra and rectangular prismatic. The greigite-pyrite particles are generally pleomorphic with no consistent crystalline morphology. A membrane has been shown to surround the particles in some organisms and may be involved in the formation of the crystalline phase while also providing physical constraints on the size and the shape of the crystal. These results clearly indicate that the biomineralization process involved in the bacterial magnetosome, a good example of a self-assembled structure on a nanometer scale, is highly controlled by the organism.

Formation of magnetite by bacteria and its application

“…BacMPs, which are aligned in chains within the bacterium, are postulated to function as biological compass needles that enable the bacterium to migrate along oxygen gradients in aquatic environments, under the influence of the Earth's geomagnetic field (Blakemore 1975). BacMPs can easily disperse in aqueous solutions because they are enveloped by an organic membrane that mainly consists of phospholipids and proteins (Gorby et al 1988;Nakamura et al 1991;Grünberg et al 2004), and an individual BacMP contains a single magnetic domain or magnetite that yields superior magnetic property ( Matsunaga & Kamiya 1987;Bazylinski et al 1994). Based on the beneficial properties of BacMPs as presented above, functional BacMPs have been developed and investigated to use them for various biotechnological applications.…”

Biological Permanent Magnets

“…Although variations exist between species, almost all magnetite and greigite magnetosomes tall within a narrow size range of about 35-120 mn when measured along their long axes [11][12][13][14][15]. This size range is significant because it places these grains within the stable magnetic single domain (SD) size range for magnetite and greigite [16].…”

Synthesis, Functionalization, and Design of Magnetic Nanoparticles for Theranostic Applications

“…[266,268] The essential part of magnetosome is the magnetic mineral which yields its superior magnetic property. [271] By the help of electron microscopy, closer view of magnetosomes clearly shows these magnetic crystalline minerals enclosed by a faint [266] Copyright 2008, RSC. b) Reproduced with permission.…”