Magnetite Crystal Orientation in Magnetosome Chains

Abstract

One-dimensional magnetic nanostructures have magnetic properties superior to non-organized materials due to strong uniaxial shape anisotropy. Magnetosome chains in magnetotactic bacteria represent a biological paradigm of such magnet, where magnetite crystals synthesized in organelles called magnetosomes are arranged into linear chains. Two-dimensional synchrotron X-ray diffraction (XRD) is applied to cells of magnetotactic bacteria that are pre-aligned with a magnetic field to determine the crystallographic orientation of magnetosomes relative to the chain axis. The obtained pole figure patterns reveal a [111] fiber texture along the chain direction for magnetospirilla strains MSR-1 and AMB-1, whereas a [100] fiber texture is measured for Desulfovibrio magneticus strain RS-1. The [100] axis appears energetically unfavorable because it represents a magnetic hard axis in magnetite, but can be turned into an effective easy axis by particle elongation along [100] for aspect ratios higher than 1.25, consistent with aspect ratios in RS-1 magnetosomes determined earlier. The pronounced fiber textures can be explained either by a strain-specific biological control on crystal orientation at the chain level or by physical alignment effects due to intra-chain magnetic interactions. In this case, biological control of the axis of elongation would be sufficient to influence the crystallographic texture of the magnetosome chain.

Keywords: X-ray diffraction; magnetite; magnetosome chains; magnetotactic bacteria; texture patterns.

Figure 1

TEM images of wildtype (A) and ΔmamJ mutant (B) cells of MSR-1 dried in the presence of a magnetic field, scale bars represent 1 μm.

Figure 2

One dimensional diffractogram I(Q) of wildtype MSR-1 cells, obtained by azimuthal integration, compared with the reference pattern of magnetite.

Figure 3

(A) Background subtracted Cartesian plot of Q vs γ of the inner rings of the XRD pattern; (B) normalized azimuthal intensity variations I_hkl(γ) for different rings, dashed lines indicate symmetry angles, blue dashed lines are along the direction of the bacteria alignment and therefore the fiber axis.

Figure 4

AID of different Debye rings of cells of AMB-1, a ΔmamJ mutant of MSR-1, RS-1 cells and isolated magnetosomes from MSR-1. The direction of the magnetic field applied during the drying process is indicated by dashed lines. In the RS-1 graphs, the black lines indicate expected intensity maxima for a [100] fiber texture (see SI for their calculations).