Magnetic microstructure of bacterial magnetite by electron holography

A brackish lagoon at Itaipu, Brazil, contains magnetotactic bacteria with unusually large magnetite magnetosomes (lengths 100-200 nm). The micromagnetic structures of the magnetosomes from two different coccoid organisms from the lagoon have been determinedby electron holography.The results are consistentwith single-magnetic-domain structure in the elongated magnetosomes from one organism and metastable, singlemagnetic-domainstructure in the larger,more equi-axed,magnetosomesfrom the other organism. The results are consistent with theoretical predictionsof the transition dimension between stable and metastable single-domain structure in magnetite. Key-words: magnetite, magnetotactic bacteria, electron holography, magnetic microstructure, single-magnetic domain. Introduction twelve symmetry-related faces of the respective forms expected for the face-centered (Fd3m) spinel Magnetotactic bacteria orient and migrate along structure are not equally developed (Devouard et geomagnetic field lines. Each cell contains a peral., 1998). The elongated magnetite crystals are manent magnetic dipole comprising membrane-entypically 40 to 100 nm long, within the stable, sinclosed magnetite (Fe3O4) or greigite (Fe3S4) nanocgle-magnetic-domain (SD) size range. rystals known as magnetosomes (Gorby et al., At least four magnetite-producing, coccoid, 1988). The magnetosomes are typically organized bacterial morphotypes, Itaipu 1, 2, 3 and 4 (Spring in chain structures and have specific crystal moret al., 1998) and a rarely observed rod-shaped bacphologies within each cell type (Bazylinski & terium (Lins & Farina, 1998), occur in a brackish Frankel, 2000). In some bacterial strains, magnetite lagoon at Itaipu, located on the coast of Brazil, crystals in magnetosomes are cuboctahedra comnorth of Rio de Janeiro. Morphotype Itaipu 1, the prising [100] and [111] forms. In other bacterial largest organism, is a diplococcus with two chains strains, the magnetite crystals are elongated along a of magnetosomes; the magnetosome crystals have [111] axis parallel to the chain direction and have roughly square projections and lengths up to 200 idealized habits comprising [100], [111], and nm (Farina et al., 1994; Spring et al., 1998). These [110] forms. In these habits, the six, eight, and are the largest magnetosome crystals yet reported.