Magnetotactic bacteria are a type of bacteria that contain the magnetosome organelle which, through biomineralization, produces paramagnetic iron oxide. The iron oxide forms forms magnetic, single-domain, colloids that during formation line up with the earths magnetic field. Having these nanoparticles inside the cell leads to a normal alignment of the bacteria towards the bottom of the ocean where more anaerobic conditions exist. The flagellum of these bacteria then propel it to these conditions where the bacteria thrive. They were first discovered in the 1960s.
Because magnetotactic bacteria are so small, yet have large colloids which are single domains, they have been used to identify the polarity of very small magnetic domains, such as those found in geological rocks. Also these colloids have a very tight range of lengths, being quite monodisperse, which may lead to future applications. Finally, like in this paper, Magnetic Colloids from Magnetotactic Bacteria: Chain Formation and Colloidal Stability, there may be applications in the fact that the bacteria shield the magnetic forces of there magnetic colloids quite well.
Magnetosomes are the organelle that biomineralizes the magnetic colloids using ionic iron and oxygen to form <math> Fe_2O_3</math>. The magnetosomes also regulate the amount of iron in the cell. They have a similar function of ferritin, which is ubiquitous across most of life. Ferritin has a spherical architecture that nucleates iron as ionic iron passes through hydrophilic pores in the protein.