Magnetotactic bacteria (MTB) are known to produce single-domain magnetite or greigite crystals within intracellular membrane organelles and to navigate along he Earth’s magnetic field lines. MTB have been suggested as being one of the most ancient biomineralizing metabolisms on the Earth and they represent a fundamental model of intracellular biomineralization. Moreover, the determination of their specific structure and morphology is essential for paleoenvironmental studies. Yet, the mechanisms of MTB biomineralization remain poorly understood, although this process has been extensively studied in several cultured MTB strains in the Proteobacteria phylum. Here, we present a comprehensive TEM study of magnetic and structural properties down to atomic scales on bullet-shaped magnetites produced by the uncultured strain MYR-1 belonging to the Nitrospirae phylum, a deeply branching phylogenetic MTB group. HAADF-STEM imaging and XEDS elemental mapping revealed a phenotypical heterogeneity among MYR-1 cells, Phenotype III produces numerous sulfur-rich globules (fig.1a-c). Electron tomography (ET) demonstrated that the bullet-shaped magnetosomes are organized into 3–5 bundles of chains (fig.1d,e). Off-axis Electron Holography results show that each bundle of MYR-1 chains appears to behave magnetically as a large uniaxial single domain (USD) magnet that maximizes the net magnetic moment of the cell (fig.1f).

We observed a multiple-step crystal growth of MYR-1 magnetite: initial growth forming cubo-octahedral particles (fig.2), subsequent anisotropic growth and a systematic final elongation along [001] direction (fig. 3). During the crystal growth, one major {111} face is developed and preserved at the larger basal end of the crystal (fig. 4.a). The basal {111} face appears to be terminated by a tetrahedral–octahedral-mixed iron surface, suggesting dimensional advantages for binding protein(s), which may template the crystallization of magnetite  (fig. 4b,c). This study offers new insights for understanding magnetite biomineralization within the Nitrospirae phylum.

Reference : J. R. Soc. Interface 201512 20141288; DOI: 10.1098/rsif.2014.1288

Figures:

Morphology, chemical, and micromagnetic features of MYR-1. (a)-(c) HADDF-STEM image of MYR-1 cell (a) chemical maps of Fe (b) and S (c). (d) HAADF-STEM tomographic image of MYR-1 magnetosome chains and (e) of individual particle. (f) Magnetic phase contours of MYR-1 magnetosome chains overlaid onto the amplitude phase.

Multiple-step crystal growth of MYR-1 magnetite. (a) Plot of crystal length versus width of MYR-1 magnetosomes. (b)-(g) Representative HRTEM images of MYR-1 magnetosomes with crystal size lower than ~40 nm and corresponding idealized morphological models. Scale bar: 5 nm.

Typical TEM images of immature (a)-(d) and mature (e)-(h) elongated particles. (i)-(l) Morphological models for MYR-1 magnetite crystals. The red and green lines indicate {111} and {001} faces, respectively. The yellow dashed arrow line and green arrow line indicate the initial and final elongation directions, respectively.

(a) HAADF-STEM image of one particle recorded from [1-10] zone axis. (b)Atomic-resolution HAADF-STEM image of the basal end of this particle indicated by yellow rectangle in (a). (c) Comparison of the surface atomic structure of the basal end of MYR-1 magnetite with the theoretical Fe arrangement of magnetite.

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EMC Abstracts - https://emc-proceedings.com/abstract/crystal-growth-of-bullet-shaped-magnetite-in-magnetotactic-bacteria-of-the-nitrospirae-phylum/