Laser Raman Spectroscopic Study on Magnetite Formation in Magnetotactic Bacteria

Abstract

Magnetotactic bacteria have one or more chains of magnetosome, consisting of nano sized magnetic crystal covered with a phospholipid bilayer and use it to sense the geomagnetic fields. In order to elucidate the molecular process to make magnetosome from the iron compounds found in the bacteria, laser Raman spectroscopic measurements were performed with the magnetotactic bacterium, Magnetospirillum magnetotacticum MS 1 and the fractions separated from it. The clear Raman signals were observed at 662 cm -1 and 740 cm -1 . The former was observed in whole cell and magnetosome fraction, but not in membrane and cytoplasmic fraction and assigned to the Raman signal of magnetite. The Raman signal of the latter was observed not only in the magnetosome fraction, but also in the cytoplasmic fraction and membrane fraction. This signal was assumed to ferrihydrite. Based on the results, the pathway of the magnetosome synthesis and possible roles of ferrihydrite in the magnetotactic bacteria were discussed.

Magnetite Biomineralization in Magnetospirillum gryphiswaldense: Time-Resolved Magnetic and Structural Studies

“…On the other hand, Faivre et al, 15 using the same techniques in a time-resolved study, do not find evidence of the existence of a mineral precursor, and suggest as a possible mechanism of magnetite biomineralization a fast coprecipitation of Fe 2þ and Fe 3þ ions within the magnetosome vesicle, a hypothesis that was proposed earlier by Arakaki et al 16 According to Faivre et al, 15 these Fe 2þ and Fe 3þ ions would be converted into an intracellular ferrous high-spin species (Fe 2þ ) predominantly located in the membrane and into a membrane-associated ferritin (Fe 3þ ). The role of ferrihydrite as a source of Fe for magnetite biomineralization has also been suggested by Watanabe et al 17 by means of Raman spectroscopy. A different mechanism is proposed by Staniland et al 18 In this work, by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism on the soft Fe L 2,3 -edges, they found a shell of an Fe oxide, hematite (R-Fe 2 O 3 ), around the magnetite particles, which they suggest acts as the precursor of magnetite.…”

“…According to Faivre et al ., these Fe 2+ and Fe 3+ ions would be converted into an intracellular ferrous high-spin species (Fe 2+ ) predominantly located in the membrane and into a membrane-associated ferritin (Fe 3+ ). The role of ferrihydrite as a source of Fe for magnetite biomineralization has also been suggested by Watanabe et al . by means of Raman spectroscopy.…”

Sub-Micrometer-Scale Mapping of Magnetite Crystals and Sulfur Globules in Magnetotactic Bacteria Using Confocal Raman Micro-Spectrometry

“…In essence, the effective resolution is enhanced by first acquiring Raman spectra on a grid with mesh width 100 nm and then by filtering the spectra for the specific Raman lines of magnetite. Confocal Raman microscopy has been used to detect biomineralized magnetite in situ [25] , [26] , but to our knowledge not for mapping the intracellular distribution of magnetite. Confocal laser scanning microscopy in reflectance mode [27] or transmission mode [28] has been applied to image chains of magnetosomes, but relies on additional analytical techniques to determine the chemical composition of the material.…”