Magnetosomes are structures formed from crystals of magnetic materials such as greigite Fe3S4 or magnetite Fe3O4 that are found in magnetotactic bacteria.
They measure between 35 and 120 nm and are aligned inside these bacteria which, subjected to the Earth's magnetic field, can follow the lines of force to reach environments favourable to their growth. They are said to move by magnetotactism.
Sampling and observation of magnetotactic bacteria producing magnetite and greigite. Figure on the left: transmission electron microscope view of a bacterium producing nanocrystals of greigite and (on the right) a chain of magnetosomes of magnetite and greigite (stars). © LBC/CEA