In oxygen-poor river sediments near Libreville in Gabon, a team led by William Orsi of LMU's Department of Earth and Environmental Sciences discovered a previously unknown ciliate, Tropidoatractus magnetotacticus, that orients itself along Earth's magnetic field. Magnetotaxis is well documented in bacteria, but this is a eukaryote, a cell with a nucleus, and its compass turns out not to be its own.
Electron micrographs revealed chains of magnetite particles, strung together like pearls, produced by bacterial endosymbionts living inside the ciliate. Genetic analyses uncovered a third partner: methane-producing archaea that break down the ciliate's metabolic products, sustaining the energy metabolism of all three organisms in the anoxic sediment. Magnetic alignment lets the ciliate swim more directly downwards into the low-oxygen zones where this consortium thrives.
The study, published in PNAS under the title describing magnetotaxis via tripartite syntrophy, suggests that a magnetic sense can arise through long-term symbiosis between different microbes rather than evolving within a single organism. The authors expect similar partnerships to surface in other anoxic environments, hinting that cooperative magnetoreception may be far more widespread in the microbial world than anyone had suspected.