Earth and Planetary Magnetism

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Magnetotactic bacteria in natural system

Our knowledge on the microbial evolution on the Earth is fragmentary and this mainly because soft-bodied microorganisms do not preserve well in sedimentary environments. Among the microbes, magnetotactic bacteria (MTB) represent a special class of organisms, because they precipitate mainly magnetite biominerals in chains, which can be preserved while the cellular matter is decomposed. Given this MTB and their fossil remains are invaluable magnetic biomarkers. A focus of our ongoing research is the use ferromagnetic resonance spectroscopy (FMR) to analyze the magnetic anisotropy properties of intact magnetite chains in cultured MTB. The FMR information is then a key to detect MTB and their fossil remains in natural samples (Figure 1).  

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Figure 1. Left: Growth series of MTB showing the formation of chains (a-f). Right: FMR spectra of the MTB exhibit the increase of the magnetite particles and the formation of a chain assembly is indicated by the evolution of an additional peak stemmed from the uniaxiality of chains (Faivre et al. Biophys. J. 99, 1268-1273.

In a current doctoral project (Barbara Lesnak) in collaboration with the Biogeoscience group of Prof. Eglinton at ETH Zurich and Prof. M. Charilaou (University of Louisiana at Lafayette, U.S.A.), we search for MTB in sediments along the coast of Namibia in order to decipher the origin and legacy of microbially-derived magnetism in marine deposits (Figs. 2-3). Preliminary FMR results show the presence of MTB in the marine sediments and from the spectral properties, it can be assumed that the MTB are better preserved in the upwelling cell.

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Figure 2. Susceptibility of the top sediments layers at the different sites costal area with upwelling cell (darker blue).
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FMR spectra of sediment sample, where the spectral simulation shows two magnetic component, the broader one (dashed red line) exhibits a high uniaxial field indicative of chain arrangement and therefore of MTB.  
  • Faivre, D., Fischer, A., Garcia-Rubio, I., Mastrogiacomo, G. & Gehring, A.U. Development of cellular magnetic dipoles in magnetotactic bacteria (2010) Biophys. J. 99, 1268-1273.
  • Charilaou, M., Winklhofer, M. & Gehring, A.U. (2011). Simulation of ferromagnetic resonance spectra of linear chains of magnetite nanocrystals. J. Appl. Phys. 109, doi:10.1063/1.3581103
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