We report on the application of off-axis electron holography and high-resolution TEM to study the crystal habits of magnetosomes and magnetic microstructure in two coccoid morphotypes of magnetotactic bacteria collected from a brackish lagoon at Itaipu, Brazil. Itaipu-1, the larger coccoid organism, contains two separated chains of unusually large magnetosomes; the magnetosome crystals have roughly square projections, lengths up to 250 nm and are slightly elongated along  (width/length ratio of about 0.9). Itaipu-3 magnetosome crystals have lengths up to 120 nm, greater elongation along  (width/ length ∼0.6), and prominent corner facets. The results show that Itaipu-1 and Itaipu-3 magnetosome crystal habits are related, differing only in the relative sizes of their crystal facets. In both cases, the crystals are aligned with their  elongation axes parallel to the chain direction. In Itaipu-1, but not Itaipu-3, crystallographic positioning perpendicular to  of successive crystals in the magnetosome chain appears to be under biological control. Whereas the large magnetosomes in Itaipu-1 are metastable, single-magnetic domains, magnetosomes in Itaipu-3 are permanent, single-magnetic domains, as in most magnetotactic bacteria.
- High-resolution transmission electron microscopy
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