noun, plural: magnetosomes
The magnetosomes are membranous cytoplasmic structures consisting of mineral crystals (about 15-20) that act together like a compass needle. They were found inside the cells of magnetic bacteria such as Magnetospirillum species. The magnetosomes within the cells of Magnetospirillum species orient them to geomagnetic fields. This response to geomagnetic field helps them to move towards their preferred microaerophilic environment. The mineral crystals within magnetosome may be crystals of magnetite (Fe3O4) or crystals of greigite (Fe3S4). The magnetosomes that contain magnetite crystals are referred to as iron oxide magnetosomes whereas those that contain greigite crystals are called iron sulfide magnetosomes. Apart from greigite crystals, though, iron sulfide magnetosomes were also found to contain other sulfide minerals such as mackinawite.
Apart from these prokaryotes, there are also certain eukaryotes that possess cytoplasmic magnetosomes, particularly the so-called magnetotactic algal species. Within their cells are magnetosomes containing thousands of magnetite crystals.
The presence of magnetosomes enables these organisms to orient themselves towards geomagnetic fields and move towards it. This response to geomagnetic field is termed magnetotaxis.