3D shapes of magnetite nanocrystals from magnetotactic bacteria: clues to crystal growth processes

Cells of magnetotactic bacteria (MTB) typically contain single crystals of magnetite that have strain-specific and distinct morphologies. Crystal habits deviate from the equilibrium form, the octahedron; categories including ‘cubooctahedral’, ‘prismatic’ and ‘bullet-shaped’ are widely used in the MTB literature [1]. These morphologies have been used as biomarkers, can affect the magnetic properties of the crystals and cells [2], and may offer insight into the genetic background of control over crystal growth. However, the exact morphologies of the magnetite nanocrystals have not been systematically determined.

We performed electron tomography (ET) by obtaining tilt series of high-angle annular dark-field (HAADF) images in a scanning transmission electron microscope (STEM), since the HAADF image contrast is not affected by diffraction and is directly proportional to specimen thickness. Magnetite shapes were characterized in various MTB strains containing octahedral, ‘prismatic’, ‘ladyfinger’- and ‘bullet-shaped’ particles. While the crystals of the first two types were bound by well-defined, indexable facets, the latter two types had irregular surfaces consisting of short segments of low-index faces. A common feature of all magnetite types was a pervasive presence of voids inside the nanocystals, suggesting a growth process dominated by particle attachment. ‘Non-classical’ crystal growth by the attachment and recrystallization of precursor nanoparticles is well known in several mineral systems; however, it is surprising that euhedral, single magnetite crystals form in this way. Provided that our interpretation of the growth process is correct, the current view of the genetic control over magnetite formation in MTB might need to be revised [3].

References

[1] Pósfai, M. et al., Phylogenetic significance of composition and crystal morphology of magnetosome minerals, Frontiers in Microbiology, 4 (2013) 67524.

[2] Kovács, A. et al., Influence of crystal shape and orientation on the magnetic microstructure of bullet-shaped magnetosomes synthesized by magnetotactic bacteria, Geo-Bio Interfyaces, 1 (2024) p.e1.

[3] We acknowledge support from the National Research, Development and Innovation Office (Hungary) under grant RRF-2.3.1-21-2022-00014.