Magnetotactic bacteria biosynthesize magnetite nanoparticles of high structural and chemical purity that allow them to orientate in the geomagnetic field. In this work we have followed the process of biomineralization of these magnetite nanoparticles. We have performed a time-resolved study on magnetotactic bacteria Magnetospirillum gryphiswaldense strain MSR-1. From the combination of magnetic and structural studies by means of Fe K-edge X-ray absorption near edge structure (XANES) and high-resolution transmission electron microscopy we have identified and quantified two phases of Fe (ferrihydrite and magnetite) involved in the biomineralization process, confirming the role of ferrihydrite as the source of Fe ions for magnetite biomineralization in M. gryphiswaldense. We have distinguished two steps in the biomineralization process: the first, in which Fe is accumulated in the form of ferrihydrite, and the second, in which the magnetite is rapidly biomineralized from ferrihydrite. Finally, the XANES anal...

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

During mineralization, the hard outer magnetite-containing shell of the radular teeth of Cryptochiton stelleri undergoes four distinct stages of structural and phase transformations: (i) the formation of a crystalline α -chitin organic matrix that forms the structural framework of the non-mineralized teeth, (ii) the templated synthesis of ferrihydrite crystal aggregates along these organic fi bers, (iii) subsequent solid state phase transformation from ferrihydrite to magnetite, and (iv) progressive magnetite crystal growth to form continuous parallel rods within the mature teeth. The underlying α -chitin organic matrix appears to infl uence magnetite crystal aggregate density and the diameter and curvature of the resulting rods, both of which likely play critical roles in determining the local mechanical properties of the mature radular teeth.

Phase Transformations and Structural Developments in the Radular Teeth of Cryptochiton Stelleri

The ferrimagnetic mineral magnetite Fe3O4 is biomineralized by magnetotactic microorganisms and a diverse range of animals. Here we demonstrate that confocal Raman microscopy can be used to visualize chains of magnetite crystals in magnetotactic bacteria, even though magnetite is a poor Raman scatterer and in bacteria occurs in typical grain sizes of only 35-120 nm, well below the diffraction-limited optical resolution. When using long integration times together with low laser power (<0.25 mW) to prevent laser induced damage of magnetite, we can identify and map magnetite by its characteristic Raman spectrum (303, 535, 665 cm(-1)) against a large autofluorescence background in our natural magnetotactic bacteria samples. While greigite (cubic Fe3S4; Raman lines of 253 and 351 cm(-1)) is often found in the Deltaproteobacteria class, it is not present in our samples. In intracellular sulfur globules of Candidatus Magnetobacterium bavaricum (Nitrospirae), we identified the sole presence of cyclo-octasulfur (S8: 151, 219, 467 cm(-1)), using green (532 nm), red (638 nm) and near-infrared excitation (785 nm). The Raman-spectra of phosphorous-rich intracellular accumulations point to orthophosphate in magnetic vibrios and to polyphosphate in magnetic cocci. Under green excitation, the cell envelopes are dominated by the resonant Raman lines of the heme cofactor of the b or c-type cytochrome, which can be used as a strong marker for label-free live-cell imaging of bacterial cytoplasmic membranes, as well as an indicator for the redox state.

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

Control over magnetite (Fe 3 O 4 ) formation is diffi cult to achieve in synthetic systems without using non-aqueous media and high temperatures. In contrast, Nature employs often intrinsically disordered proteins to tightly tailor the size, shape, purity, and organization of the nanocrystals to optimize their magnetic properties. Inspired by such “fl exible polyelectrolytes,” here random copolypeptides having different amino acid compositions are used as control agents in the bioinspired coprecipitation of magnetite through a ferrihydrite precursor, following a recently developed mineralization protocol. Importantly, the copolypeptide library is designed such that the amino acid composition can be optimized to simultaneously direct the size of the nanoparticles as well as their dispersibility in aqueous media in a one-pot manner. Acidic amino acids are demonstrated to regulate the crystal size by delaying nucleation and reducing growth. Their relative content thus can be balanced to tune between the superparamagnetic and ferrimagnetic regimes, and high contents of negatively charged amino acids result in colloidal stabilization of superparamagnetic nanoparticles at high pH. Conversely, with positively charged lysine-rich copolypeptides ferrimagnetic crystals are obtained which are stabilized at neutral pH and self-organize in chains, as visualized by cryotransmission electron microscopy. Altogether, the presented fi ndings give important insights for the future development of additive-mediated nanomaterial syntheses.

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Bioinspired Magnetite Crystallization Directed by Random Copolypeptides

Iron oxide nanoparticles obtained from steel waste recycling as a green alternative for Congo red dye fast adsorption

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 major Raman signals at 662 cm � 1 and 740 cm � 1 were observed. The former was assigned to the Raman signal of magnetite and the latter, to that of ferrihydrite. The Raman signal of ferrihydrite was observed not only in the membrane fraction, but also in the cytoplasmic fraction. Based on the results, the role of ferrihydrite in magnetosome synthesis in the magnetotactic bacteria was discussed. [doi:10.2320/matertrans.MER2007333]

Laser Raman Spectroscopic Study on Magnetite Formation in Magnetotactic Bacteria

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.

Masaharu Yamanaka

Papers

Laser Raman Spectroscopic Study on Magnetite Formation in Magnetotactic Bacteria

Laser Raman Spectroscopic Study on Magnetite Formation in Magnetotactic Bacteria