Earth Planets Space, Vol. 61 (No. 1), pp. 143-150, 2009
Claire Carvallo1, Stanislawa Hickey1, Damien Faivre2,3, and Nicolas Menguy1
1Institut de Minéralogie et de Physique des Milieux Condensés, Campus Boucicaut, 140 rue de Lourmel, 75015 Paris, France
2Max Planck Institute for Marine Microbiology, Celsisusstr. 1, 28359 Bremen, Germany
3Max Planck Institute of Colloids and Interfaces, Department of Biomaterials, Science Park Golm, 14424 Potsdam, Germany
(Received November 19, 2007; Revised March 11, 2008; Accepted March 20, 2008; Online published January 23, 2009)
In order to study the formation of magnetite in magnetotactic bacteria, FORC diagrams were measured on a set of cultured Magnetospirillum gryphiswaldense, following an assay in which the iron uptake is used only for magnetite formation and not for cell growth. This enabled us to follow the magnetite formation independently of growth. The FORC diagrams showed a clear evolution from a size-distribution with a majority of superparamagnetic grains, to a distribution dominated by stable, single-domain grains, but still containing some superparamagnetic particles. TEM observations confirm this evolution. According to the saturation isothermal remanent magnetization cooling and warming curves, the Verwey transition can only be seen in the most mature samples, and slightly below 120 K. This suggests that the samples may have suffered from some partial oxidation.
Key words: Magnetotactic bacteria, FORC diagrams, magnetite.