Biomineralization, crystallography and magnetic properties of bullet-shaped magnetite magnetosomes in giant rod magnetotactic bacteria

Magnetosomes produced by magnetotactic bacteria are of great interest for understanding bacterial biomineralization along with sedimentary magnetism and environmental magnetism. One of the most intriguing species, Magnetobacterium bavaricum can synthesize hundreds of bullet-shaped magnetite magnetosomes per cell, which contribute significantly to magnetic properties of sediments. However, the biomineralization mechanism and magnetic properties of such magnetosomes remain unknown. In this paper, we have conducted a comprehensive study of the crystallography and magnetic properties of bullet-shaped magnetosomes formed by uncultivated giant rod magnetotactic bacteria (referred to as MYR-1), recently discovered in Lake Miyun (Beijing, China). Transmission electron microscopy observations reveal that each MYR-1 cell contains hundreds of bullet-shaped magnetite magnetosomes, which are arranged into 3 - 5 braid-like bundles of chains. The formation of the bullet-shaped magnetosomes can be divided into two stages: initial isotropic growth (up to ∼ 20 nm) followed by elongation along the [100] direction, which is unusual compared with the expected [111] magnetic easy axis. Although the [100] orientation is the hard axis of the face-centered cubic magnetite, the shape anisotropy of bullet-shaped magnetosomes and intra-bundle magnetostatic interactions confine the magnetization direction of the chain along the long axis of the cell/bundle. Due to each bundle of magnetosome chains effectively behaving as an elongated single domain particle, the MYR-1 cells show high coercivity, weak intra-bundle magnetostatic interaction, and high δ-ratio. These results provide new insights into the biomineralization process and magnetic properties of bullet-shaped magnetite magnetosomes.