Background

Magnetotactic bacteria (MTB) synthesize magnetite nanocrystals within their cells, aligning themselves with the Earth's magnetic field. These membrane-bound magnetic crystals, called magnetosomes, form chains, creating a magnetic moment akin to a compass needle. This arrangement helps the bacteria narrow their search range in sediments with layered oxygen and sulfide concentrations, enabling them to move to suitable habitats.

Magnetoreceptive behavior has also been reported in nucleated, single-celled eukaryotes, but how they acquire this magnetic sense remains unclear. It has been difficult to determine whether eukaryotes create magnetic minerals using their own genes, temporarily utilize magnetosomes from ingested MTB, or rely on symbiotic microorganisms as sensory organs. In particular, in nearly anoxic environments, it is challenging to culture both the host and symbionts to verify their respective functions.

Key Findings

An international research team discovered a new anaerobic ciliate in anoxic river sediments near Libreville, Gabon, and named it Tropidoatractus magnetotacticus. This organism swam in alignment with an external magnetic field, with the magnetic field direction assisting its movement towards the anoxic sediment layer.

Examination of the cell's interior using electron microscopy revealed chains of magnetite (Fe3O4) nanoparticles arranged in parallel, resembling an oval 'necklace'. However, the producer of these crystals was not the ciliate itself. Rod-shaped bacteria, abundant within the host, contained the magnetosome chains. Analysis of the metatranscriptome and gene expression of magnetotactic ciliate cells, selected using a magnetic field, identified a Thermodesulfobacteriota-affiliated MTB. This bacterium expressed a magnetosome gene cluster that directs the biomineralization of magnetite and exhibited a genetic composition close to the known external symbiont, Candidatus Desulfarculum epimagnetica.

Two other symbionts were also present within the cells. One was a hydrogenotrophic methanogenic archaeon of the Methanoregula lineage. The metatranscriptome revealed the expression of genes for a hydrogenosome-like iron-hydrogenase and the electron transport chain of a mitochondrion-derived organelle that produces hydrogen. In the metabolic model proposed by the researchers, the ciliate produces anaerobic fermentation products, which are consumed by the MTB and methanogenic archaeon. In return, the MTB provides magnetosomes that align the entire host with the magnetic field. Thus, three domains of life—eukaryotes, bacteria, and archaea—collaborate within a single cell to complete movement and energy metabolism.

Significance and Outlook

This study demonstrates that magnetotaxis does not necessarily need to evolve independently within the genome of a single organism. The host has incorporated the physical properties of symbiotic bacteria into its own behavioral traits without directly possessing the genes for magnetic nanocrystal synthesis. The metabolic syntrophy involving the methanogenic archaeon is interpreted as the basis for maintaining the symbiotic relationship necessary for magnetic particle production in an anoxic environment.

However, the metabolic network constructed from gene expression and microscopic observations is not the result of directly measuring the carbon and hydrogen transfer rates between symbionts. Separating and culturing the three components or verifying the movement of metabolites using stable isotope tracing is necessary. The effect on the host's magnetotaxis and survival when the symbionts are removed also remains to be investigated. If similar tripartite associations are found in other anoxic sediments, it could provide evidence that symbiosis is a more widespread driver of sensory and behavioral evolution than currently known. The research results were published in the international journal PNAS on July 2026.