Magnetotactic Bacteria

Magnetoreception is one of the most fascinating sensory modalities in living organisms. Most of the work has been done in homing pigeons, migrating birds and salmon. More recently, work has been done in mammals and fruitflies. But this sense is not limited only to the most complex organisms - it is found in a number of bacterial species:

Researchers Reveal Mystery Of Bacterial Magnetism:

Scientists at the Naval Research Laboratory (NRL) and Purdue University have shed light on one of microbiology's most fascinating mysteries--why some bacteria are naturally magnetic. Their description of how being magnetic "helps" the bacteria is reported in the August 2006 issue of the Biophysical Journal.

Magnetotactic bacteria were discovered by Blakemore in 1975. You can see some cool photomicrographs of different species of magnetotactic bacteria here.

The idea is that these bacteria, all of which prefer environments low in oxygen, use the Earth's magnetic field in order to orient and swim down. Down is where the debris is decaying at the bottom of the lake and the oxygen concentration is likely to be much lower than up, at the surface of the lake.

Interestingly, bacteria caught in the Southern hemisphere have the polarity of the string of magnetite crystals directed in the opposite direction from the Northern hemisphere bacteria. Having a Northern arrangement in a Southern lake would produce the opposite effect - swimming up.

Some recent research on mutants (bacteria that do not produce magnetite or produce it but do not arrange them in strings), including this paper, suggest that magnetic sense (magnetotaxis) works together with teh chemical sense (aerotaxis) which tells the bacterium about the changes in oxygen concentrations so it can swim down the oxygen gradient (from high 02 towards the low 02 concentration):

NRL researcher Dr. Paul Sheehan adds, "by mathematically modeling their motion, we determined that being magnetic actually makes the bacteria much more sensitive to oxygen when in a magnetic field, so that they swim away from oxygen at much lower concentrations." It is as if the climber gets tired and turns around sooner when heading up the mountain, keeping her from heading too far in the wrong direction. And the stronger the magnetic field, the bigger the effect. The scientists do not yet know how the magnetic field has this affect on the bacteria, and are currently conducting additional experiments to help answer that question.

Update: Brad wonders if this could have been the very first sensory modality. I doubt it. The assembly of the magnetite chain is quite a complex process. Some simple form of chemoreception probably came first - swimming towards or away from food and/or toxins in the water.