Bullet-Shaped Magnetite Biomineralization Within a Magnetotactic Deltaproteobacterium: Implications for Magnetofossil Identification

Magnetite produced by magnetotactic bacteria (MTB) provides stable paleomagnetic signals because it occurs as natural single-domain magnetic nanocrystals. MTB can also provide useful paleoenvironmental information because their crystal morphologies are associated with particular bacterial groups and the environments in which they live. However, identification of the fossil remains of MTB (i.e., magnetofossils) from ancient sediments or rocks is challenging because of their generally small sizes and because the growth, morphology, and chain assembly of magnetite within MTB are not well understood. Nanoscale characterization is, therefore, needed to understand magnetite biomineralization and to develop magnetofossils as biogeochemical proxies for paleoenvironmental reconstructions. Using advanced transmission electron microscopy, we investigated magnetite growth and chain arrangements within magnetotactic Deltaproteobacteria strain WYHR-1, which reveals how the magnetite grows to form elongated, bullet-shaped nanocrystals. Three crystal growth stages are recognized: (i) initial isotropic growth to produce nearly round ~20 nm particles, (ii) subsequent anisotropic growth along the [001] crystallographic direction to ~75 nm lengths and ~30–40 nm widths, and (iii) unidirectional growth along the [001] direction to ~180 nm lengths, with some growing to ~280 nm. Crystal growth and habit differ from that of magnetite produced by other known MTB strains, which indicates species-specific biomineralization. These findings suggest that magnetite biomineralization might be much more diverse among MTB than previously thought. When characterized adequately at species level, magnetofossil crystallography, and apomorphic features are, therefore, likely to become useful proxies for ancient MTB taxonomic groups or species and for interpreting the environments in which they lived.