WHAT: Spectro-ptychography X-ray absorption and X-ray Magnetic Cicular Dichroism at sub-10 nm resolution

Magnetotactic bacteria (MTB) synthesize chains of magnetic nanocrystals (magnetosomes) that interact with the Earth's magnetic field like an inner compass needle, simplifying their search for their optimum environment at the oxic-anoxic interface.
How do these magnetosomes form? Some studies suggest that hematite or amorphous ferrihydrite act as precursors, while others suggest that they are formed directly from solution-phase Fe(II) and Fe(III). Thus, identifying the chemical states of precursors would be a good way to differentiate among competing models. Ptychography is a coherent diffractive imaging technique with high resolution (7 nm in this work) and excellent chemical-state sensitivity.

At ALS beamlines 5.3.2.1 and 11.0.2, we have measured ptychographic absorption and phase spectra of magnetosomes (~40-50 nm in size) inside a single cell of a marine MTB (Magnetovibrio blakemorei strain MV-1). The spectra of mature magnetosomes, immature magnetosomes, precursor regions, and the gaps between magnetosome chains differ, indicaitng that different iron species can coexist in a single cell.

From these results we proposed a model in which soluble Fe(II) is taken up from the environment, is partially oxidized to Fe(III), which in turn is then oxidized and transformed into hematite and ultimately into magnetite. In addition, ptychography ay BL 11.0.2 was used to measure the x-ray magnetic circular dichroism (XMCD) phase and absorption spectra of both extracellular and intracellular magnetosomes. Fe 2p XMCD probes the magnetism of magnetitein a site-specific manner. These experiments demonstrate that ptychography, which combines high spatial resolution, high-sensitivity chemical speciation, and site-specific magnetic information, offers a powerful probe for biomineralization studies.

OTHER INFORMATION:

Publication: X.H. Zhu, A.P. Hitchcock, D.A. Bazylinski, P. Denes, J. Joseph, U. Lins, S. Marchesini, H.-W. Shiu, T. Tyliszczak and D.A. Shapiro, Measuring spectroscopy and magnetism of extracted and intracellular magnetosomes using soft X-ray ptychography, Proc. Nat. Acad. Sci. 113 (51) (2016) E8219-E8227 doi:10.1073/pnas.1610260114

ALS Science Brief (published 14 Mar 2017) https://als.lbl.gov/ptychography-bacteriums-inner-compass/