Nature's Little Boy Scout
Magnetospirillum is a Gram-negative, aquatic bacterium with a nifty feature: an internal compass.
Internal structures called magnetsomes contain magnetite (a derivative of iron) crystals, which align with magnetic fields in the environment. Magnetosomes line up to form a chain, and the dipole formed allows the bacterium to sense the magnetic fields. It then uses the information to swim in a particular direction via flagella in a process called magnetotaxis. There is an interesting correaltion between magentosome formation/activation and low oxygen levels, a condition these bacteria prefer. The crystals are encased in a membrane-bound vesicle, a trait attractive to bio- and nano-technology since they can be harvested in large quantities and the crystals disperse evenly in a solution.
Some sweet pics
Some purified magentosomes
Magnetofossils found on the 4 billion year old Martian meteorite ALH84001 have a striking similarity to magnetosomes. This allows for the possibility of some sort of prokaryotic life on Mars way, way back in the day.
Pics from Google
Here's a great resource for all things microbial: microbewiki.kenyon.edu/index.php/Microbe
The Martian paper: Thomas-Keprta et al. 2002. Magnetofossils from Ancient Mars: a robust biosignature in the Martian meteorite ALH84001. Applied and Environmental Microbiology. 68:3663-3672.