Polar-opposite bacteria swim south in the north.

Some aquatic bacteria that orient themselves using Earth's magnetic field swim in the opposite direction from what researchers typically expect, calling into question a longstanding theory of what this navigational behavior accomplishes.

Such magnetotactic bacteria typically live between the high-oxygen surface and low-oxygen floor of ponds or sediments. Researchers have long proposed that the bacteria take advantage of Earth's geomagnetic field to hover in this preferred middle zone. Lab studies seemed to support this idea: In high-oxygen environments, microbes in the northern hemisphere usually swam, in high oxygen conditions, toward geomagnetic north, which took them deeper into a body of water. Those in the southern hemisphere swam toward the geomagnetic south, the down direction in that part of the world.

While researching magnetotactic bacteria's role in the environmental cycling of iron and sulfur, Sheri Simmons of the Woods Hole (Mass.) Oceanographic Institution and her colleagues serendipitously discovered bacteria with opposite navigational behavior. In the lab, the previously unknown species, isolated from salt ponds in Woods Hole, prefers to swim south.

Why these unusual bacteria go against the flow isn't clear. However, Simmons and her colleagues suggest in the Jan. 20 Science that the original explanation may be too simple to account for the microbes' behavior. "We need more controlled experiments to see what's really regulating their swimming behavior," she says.--C.B.