How do microorganisms orient themselves to find the optimal habitat? Scientists have thoroughly investigated so-called magnetotactic bacteria, which use the Earth’s magnetic field as a biological compass.
Some eukaryotic single-celled organisms – that is, more complex organisms like ciliates, which, unlike bacteria, have a cell nucleus – also possess this capability. But how they acquire it has largely remained a puzzle.
An international team led by Professor William Orsi from the Department of Earth and Environmental Sciences at LMU has discovered a previously unknown magnetotactic ciliate that pursues an unusual strategy: partnership with two other organisms inside its cell. The discovery is published in Proceedings of the National Academy of Sciences.
River sediments
The researchers found the new ciliate Tropidoatractus magnetotacticus in oxygen-poor river sediments near Libreville in Gabon.
Electron micrographs revealed that the ciliate possesses tiny magnetite particles arranged in chains resembling strings of pearls, which help the creature orient itself in the magnetic field. The images also showed that these particles originate from living bacterial symbionts inside the single-celled organism.
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“When we first observed these cells, we immediately realized we were looking at something different. Discovering that they navigate by using these endosymbionts revealed a fascinating new way in which eukaryotes can exploit the Earth’s magnetic field,” says Leon Kaub, lead author of the study along with Mitali Chitnis.
Cooperation as adaptation to habitat
The micrographs also indicated that the ciliate has further microbial partners, a finding confirmed by genetic analyses. In addition to the magnetite-producing symbionts, the ciliate hosts methane-producing archaea, which process metabolic products of the ciliate and thus create favorable conditions for the energy metabolism of the partners in the anoxic sediments.
Together with use of the Earth’s magnetic field for orientation, which helps the ciliate swim downward and reach the low-oxygen sediments faster, this symbiosis enables all the partners to survive more efficiently in anoxic sediments.
“Our discovery opens up new perspectives on the evolution of magnetoreception,” says Chitnis. “It indicates that this capability can emerge not just through the evolution of a single organism, but also from the long-term symbiosis of various microorganisms.” Orsi adds: “Now that this type of symbiosis has been discovered, I anticipate that many more similar cooperations will be discovered in anoxic environments and that these associations are more common than previous known.”
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