Hitchhiking bacteria might help their host navigate via magnetic fields

Deep in the mud of the Mediterranean Sea, scientists have caught microscopic protists dancing to a strange beat—the beat of Earth’s magnetic fields. Now, a new study reveals how these tiny clusters of cells orient themselves along those fields: by letting magneto-sensing bacteria hitch a ride on their outer membranes.

Researchers used microscopes to examine protist-packed sediment taken from the bottom of the Mediterranean Sea near Carry-le-Rouet, France. When they placed a magnet with its north pole facing a water droplet from the sediment, the hundreds of protists inside immediately began to swim toward the droplet’s edge. When the researchers reversed the magnet so its south pole was facing the droplet, the protists fled in the other direction (above).

The friction of the protists against the glass microscope slides eventually caused them to divide, revealing tiny hitchhikers attached to their outer membranes, the researchers report today in Nature Microbiology . Those hitchhikers turned out to be Deltaproteobacteria, a class of bacteria with organelles containing magnetic crystals that force the bacteria to align with Earth’s magnetic fields like a compass needle.

Scientists still aren’t sure why the protists and bacteria pair up. The bacteria could help their hosts navigate toward more optimal habitats, or they could collect and supply hydrogen from the surrounding atmosphere—a key element in the protists’ metabolism. The bacteria may get other nutrients out of the deal. But more research needs to be done to fully understand how this relationship evolved, and why the protists and bacteria keep hanging around each other.