Investigating Ptychography of a Bacterium’s Inner Compass

Magnetovibrio blakemorei
(Left) Schematic of the pathway of magnetosome biomineralization for Magnetovibrio blakemorei strain MV-1. (Center) Ptychography absorption image of a single MV-1 cell (average of 76 images from 700 to 732 eV). Four spatial regions are identified: (A) gap in the magnetosome chain, (B) precursor region, (C) immature magnetosome, and (D) mature magnetosome. (Right) Fe L3 spectra from regions A through D. [Left image courtesy Xiaohui Zhu; Middle and right images from Zhu, X., et al. “Measuring Spectroscopy and Magnetism of Extracted and Intracellular Magnetosomes Using Soft X-Ray Ptychography,” PNAS 113, E8219 (2016). DOI:10.1073/pnas.1610260114.]
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 optimum environments. Some studies investigating how these magnetosomes form suggest that hematite or amorphous ferrihydrite act as precursors, while others posit 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. Researchers obtained ptychographic absorption and phase spectra of magnetosomes from a marine MTB (Magnetovibrio blakemorei strain MV-1). The data, obtained from mature magnetosomes, immature magnetosomes, precursor regions, and the gaps between magnetosome chains, indicated that different iron species can coexist in a single cell. Based on the results, the researchers proposed a model in which soluble Fe(II) is taken up from the environment and partially oxidized to Fe(III), which in turn is then oxidized and transformed into hematite and ultimately into magnetite.

Researchers also used ptychography to measure the X-ray magnetic circular dichroism (XMCD) spectra of both extracellular and intracellular magnetosomes. XMCD probes the magnetism of a crystal in a site-specific manner, showing that absorption and XMCD ptychography signals provide complementary information. 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.

Zhu, X., et al. “Measuring Spectroscopy and Magnetism of Extracted and Intracellular Magnetosomes Using Soft X-Ray Ptychography,” PNAS 113, E8219 (2016). [DOI:10.1073/pnas.1610260114].

Instruments and Facilities Used: Scanning transmission X-ray microscope (STXM) on beamline 10ID1 of Canadian Light Source at University of Saskatchewan and the Advanced Light Source Beamlines 5.3.2.1 and 11.0.2 at Lawrence Berkeley National Laboratory.