X-ray Magnetic Circular Dichroism of Individual Magnetosomes in Magnetotactic Bacteria by Scanning Transmission X-ray Microscopy

Abstract

<p>Magnetotactic bacteria (MTB) produce single-domain nano-crystals (typically 30-60 nm in size) of magnetite (Fe₃O₄) or greigite (Fe₃S₄) enclosed in a biomembrane. Known as magnetosomes, these chains of ferrimagnetic nanocrystals, each comprising a single magnetic dipole, allow MTB to align themselves with respect to the earth's magnetic field, and then migrate to their preferred habitat at the oxic-anoxic transition zone of aquatic environments. Recently, magnetotactic bacteria and magnetosomes have received much attention for their potential in various medical, environmental, biotechnological and technological applications as nano-magnetic material. Using the soft X-ray scanning transmission X-ray microscopes (STXM) at the Canadian Light Source (Saskatoon) and Advanced Light Source (Berkeley, CA), the magnetic properties and biochemistry of the magnetosomes were studied at high spatial resolution (30 nm). In order to probe the permanent magnetic moment and magnetic orientation of individual magnetosomes, X-ray magnetic circular dichroism (XMCD) was used.</p> <p>Although this technique has previously been applied to ensembles of magnetotactic bacteria as reported in the literature, the work presented in this thesis marks the first time that Fe L_2,3 XMCD signal was measured from single 30-nm diameter Fe₃O₄magnetosomes in individual MTB, in particular, those of a marine vibrio, strain MV-1 [1]. Furthennore, recent major improvements to the STXM data acquisition process have allowed measurement of much higher quality XMCD data, by using XMCD-stacks in which the left and right circularly polarized signals are measured alternately at each photon energy. As well, the associated biochemistry was explored by speciation and quantitative biochemical mapping at the C1s and O1 s absorption edges. Studies on magnetotactic bacteria will lead to a deeper understanding of the genetic and environmental factors and mechanisms involved in magnetosome chain formation, which may be a precursor to understanding biomineralization processes present in higher-level organisms, such as birds and bees.</p> <p>[1] KP Lam <em>et al.</em> (2010), <em>Chem. Geol</em>. 270, 110.</p>