The observation of large magnetite (Fe₃O₄) crystals from magnetotactic bacteria by electron and atomic force microscopy

Abstract

SUMMARY Magnetite crystals inside coccoid magnetotactic bacteria found in lagoons near Rio de Janeiro city were examined by electron microscopy (EM) and atomic force microscopy (AFM). For AFM, ultrathin sections of bacteria embedded in Epon resin were etched with an ethanolic NaOH solution and observed both in the height and in the force modes. Comparative electron microscope images were useful for identifying crystalline reliefs in the etched sections. Different situations representing particular arrangements of crystal chains were observed by AFM. The majority of the bacteria examined presented unusually large magnetite crystals which remained strongly attached in linear chains even after the laboratory procedures for their isolation. This behaviour is different from all other biogenic magnetite crystals isolated so far. It is suggested that this attachment is due to the strong field between individual crystals as well as to the contact areas, which are the largest observed until now. The correct identification of a particular topography by AFM as a crystal relief may be critical when crystals are not aligned in chains; in these cases the linear dimensions and the presence of well‐defined edges and faces are important features to be taken into account. Characterization of the crystal faces is important for the study of magnetotactic micro‐organisms since the crystalline habits seem to be species‐specific. Observation of etched sections proved to be a helpful approach for crystal relief observation, especially when small amounts of bacteria were available.

Off-axis electron holography of magnetotactic bacteria: magnetic microstructure of strains MV-1 and MS-1

“…Significantly, both ap proaches give moments per unit length that are the same for both chains to within experimental error, suggesting that the different crystal morphologies in MV-1 and MS-1 do not arise for magnetic rea sons. However, this inference may not apply to magnetosome chains that contain considerably larger crystals, such as those reported by Farina et al (1994), which would lie in the multi-domain ré gime if they were isolated.…”

The identification and biogeochemical interpretation of fossil magnetotactic bacteria

“…The dashed upper boundary of the stable SD field and the solid upper boundary of the metastable SD field are taken from the micromagnetic models of Witt et al (2005) for characteristic magnetosome crystal shapes. Shaded regions mark size and shape of crystals from magnetotactic bacteria (Arató et al, 2005;Bazylinski et al, 1995;Devouard et al, 1998;Farina et al, 1994;Meldrum et al, 1993a;Meldrum et al, 1993b;Moench, 1988;Sakaguchi et al, 1993;Thornhill et al, 1994;Vali and Kirschvink, 1991). Komeili et al, 2006;Komeili et al, 2004;Matsunaga and Okamura, 2003;Scheffel et al, 2006).…”

Biological Permanent Magnets

“…Examples include large (up to 200 nm) magnetosomes found in coccoid cells from Lagoa de Itaipu, near Rio de Janeiro, Brazil [23,24].…”