Structure, morphology and crystal growth of bacterial magnetite

Abstract

Recent high-resolution transmission electron microscopy (HRTEM) studies of the structure and morphology of bacterial magnetite (Fe₃O₄) crystals isolated from a magnetotactic coccus¹ and from an unidentified bacterium extracted from sediment² have shown the crystals to be well ordered single-domain particles with a morphology based on a hexagonal prism of {011} faces truncated by specific low index planes. We report here a HRTEM study of intact magnetite crystals (magnetosomes) in the microaerophilic bacterium Aquaspirillum magnetotacticum, grown in pure culture^3,4. Our aim has been to investigate the structure, morphology and crystal growth of the magnetite particles in the light of a recent Mossbauer spectroscopy study of this organism⁵ which indicated, in addition to magnetite, the presence of hydrated iron(III) oxide phases together with the magnetosomes. Our results show that the mature particles are well ordered single-domain crystals of magnetite with a morphology very different from previously studied crystals and based on an octahedral prism of {111} faces truncated by {100} faces. We also show the first direct evidence for both crystalline and non-crystalline phases within individual magnetosomes. The results are important in aiding elucidation of the crystal growth mechanisms of biogenic magnetite.