Magnetite in the foraminifera R. bilocularis. (a) LM (light microscopy) image showing the rusty colour of the organic walled test of R. bilocularis (Gooday et al., 2008). (b) LM image of the larger chamber of R. bilocularis stained with Rose Bengal, with fresh stercomata (waste pellets) and stained cytoplasm concentrated just inside the aperture. (c) SEM image of a thin section of R. bilocularis from the Challenger Deep with numerous stercomata (b) inside the test (s). Magnetite (m) is contained in the stercomata. (d) Enlarged SEM image of the area indicated by the white rectangle in (c). Magnetite (m) is contained in the stercomata (b), within the yellow box. Raman analysis position is marked with a red dot. (e) NanoSIMS elemental mapping of R. bilocularis. Blue = Si; Green = Fe; Red = Ti. SEM-EDX elemental maps are presented in Figure S-15. (f) Raman spectral combined images obtained from a stercome containing magnetite. Combined maps of intensity of 666 cm−1, 538 cm−1, 304 cm−1 (red) and 1367 cm−1, 1582 cm−1 (blue), indicating magnetite and organic carbon in stercomata, respectively. (g, h, i) Secondary electron image of magnetite extracted from R. bilocularis showing a euhedral and porous structure. L = carbon-containing membrane. (j) An enlarged SEM image of the carbon-containing membrane enveloping the magnetite in Figure 1g. L = carbon-containing membrane, M = magnetite. (k) The elemental mapping of the area in white rectangular of Figure 1j. Green = carbon; Red = iron; other elements are shown in Figure S-16. Scale bars of a,b = 50 μm, c = 20 μm, d,e = 8 μm, f–i = 2 μm, j,k = 0.5 μm.
Unicellular magnetic microorganisms include magnetotactic bacteria and some protist species. Although magnetosome magnetite in bacteria (prokaryotes) is well studied, little is known regarding the characteristics and origin of magnetic minerals in protists (eukaryotes).
Stercomata stored within tests of the hadal foraminifera R. bilocularis from the Mariana Trench (6980-10,911 m depth), contain magnetite crystals. As a result, this species can orient in accordance with magnetic fields. The magnetite differs chemically and physically from that in the surrounding sediments.
The crystals also differ from bacterial magnetosomes in being of variable size, porous structure, not arranged in chains, and encapsulated in a lipid membrane. Putting available evidence together indicates a biological origin of the magnetite, although a sedimentary source cannot be eliminated. This is the first record of a magnetic protist from hadal depths, opening a new window for the biomagnetism in the Earth’s extreme environment.
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