Tanya Prozorov

A Cultured Greigite-Producing Magnetotactic Bacterium in a Novel Group of Sulfate-Reducing Bacteria

The crystal structure of biomineralized magnetic nanocrystals depends on environmental and genetic factors. Magnetotactic bacteria contain magnetosomes—intracellular, membrane-bounded, magnetic nanocrystals of magnetite (Fe3O4) or greigite (Fe3S4)—that cause the bacteria to swim along geomagnetic field lines. We isolated a greigite-producing magnetotactic bacterium from a brackish spring in Death Valley National Park, California, USA, strain BW-1, that is able to biomineralize greigite and magnetite depending on culture conditions. A phylogenetic comparison of BW-1 and similar uncultured greigite- and/or magnetite-producing magnetotactic bacteria from freshwater to hypersaline habitats shows that these organisms represent a previously unknown group of sulfate-reducing bacteria in the Deltaproteobacteria. Genomic analysis of BW-1 reveals the presence of two different magnetosome gene clusters, suggesting that one may be responsible for greigite biomineralization and the other for magnetite.

Magnetic irreversibility and the Verwey transition in nanocrystalline bacterial magnetite

The magnetic properties of biologically-produced magnetite nanocrystals biomineralized by four different magnetotactic bacteria were compared to those of synthetic magnetite nanocrystals and large, high quality single crystals. The magnetic feature at the Verwey temperature, TV , was clearly seen in all nanocrystals, although its sharpness depended on the shape of individual nanoparticles and whether or not the particles were arranged in magnetosome chains. The transition was broader in the individual superparamagnetic nanoparticles for which TB TV and the Verwey transition is sharply defined. No correlation between particle size and TV was found. Furthermore, measurements of M (H, T, time) suggest that magnetosome chains behave as long magnetic dipoles where the local magnetic field is directed along the chain. This result confirms that time-logarithmic magnetic relaxation is due to the collective (dipolar) nature of the barrier for magnetic moment reorientation.

Self-assembly and biphasic iron-binding characteristics of Mms6, a bacterial protein that promotes the formation of superparamagnetic magnetite nanoparticles of uniform size and shape

Highly ordered mineralized structures created by living organisms are often hierarchical in structure with fundamental structural elements at nanometer scales. Proteins have been found responsible for forming many of these structures, but the mechanisms by which these biomineralization proteins function are generally poorly understood. To better understand its role in biomineralization, the magnetotactic bacterial protein, Mms6, which promotes the formation in vitro of superparamagnetic magnetite nanoparticles of uniform size and shape, was studied for its structure and function. Mms6 is shown to have two phases of iron binding: one high affinity and stoichiometric and the other low affinity, high capacity, and cooperative with respect to iron. The protein is amphipathic with a hydrophobic N-terminal domain and hydrophilic C-terminal domain. It self-assembles to form a micelle, with most particles consisting of 20-40 monomers, with the hydrophilic C-termini exposed on the outside. Studies of proteins with mutated C-terminal domains show that the C-terminal domain contributes to the stability of this multisubunit particle and binds iron by a mechanism that is sensitive to the arrangement of carboxyl/hydroxyl groups in this domain.

Cobalt Ferrite Nanocrystals: Out-Performing Magnetotactic Bacteria

Magnetotactic bacteria produce exquisitely ordered chains of uniform magnetite (Fe(3)O(4)) nanocrystals, and the use of the bacterial mms6 protein allows for the shape-selective synthesis of Fe(3)O(4) nanocrystals. Cobalt ferrite (CoFe(2)O(4)) nanoparticles, on the other hand, are not known to occur in living organisms. Here we report on the use of the recombinant mms6 protein in a templated synthesis of CoFe(2)O(4) nanocrystals in vitro. We have covalently attached the full-length mms6 protein and a synthetic C-terminal domain of mms6 protein to self-assembling polymers in order to template hierarchical CoFe(2)O(4) nanostructures. This new synthesis pathway enables facile room-temperature shape-specific synthesis of complex magnetic crystalline nanomaterials with particle sizes in the range of 40-100 nm that are difficult to produce using conventional techniques.

Magnetic nanoparticles as efficient bulk pinning centers in type-II superconductors

Enhancement of vortex pinning by magnetic nanoparticles embedded into the bulk of type–II superconductor is studied both theoretically and experimentally. Magnetic part of the pinning force associated with the interaction between a finite-size spherical magnetic inclusion and an Abrikosov vortex was calculated in London approximation. Calculations are supported by the experimental results obtained on sonochemically modified MgB2 superconductor with embedded magnetic Fe2O3 nanoparticles and compared to MgB2 with nonmagnetic Mo2O5 pinning centers of similar concentration and particle size distribution. It is shown that ferromagnetic nanoparticles result in a considerable enhancement of vortex pinning in large-κ type-II superconductors.

Isolation of obligately alkaliphilic magnetotactic bacteria from extremely alkaline environments.

Large numbers of magnetotactic bacteria were discovered in mud and water samples collected from a number of highly alkaline aquatic environments with pH values of ≈ 9.5. These bacteria were helical in morphology and biomineralized chains of bullet-shaped crystals of magnetite and were present in all the highly alkaline sites sampled. Three strains from different sites were isolated and cultured and grew optimally at pH 9.0-9.5 but not at 8.0 and below, demonstrating that these organisms truly require highly alkaline conditions and are not simply surviving/growing in neutral pH micro-niches in their natural habitats. All strains grew anaerobically through the reduction of sulfate as a terminal electron acceptor and phylogenetic analysis, based on 16S rRNA gene sequences, as well as some physiological features, showed that they could represent strains of Desulfonatronum thiodismutans, a known alkaliphilic bacterium that does not biomineralize magnetosomes. Our results show that some magnetotactic bacteria can be considered extremophilic and greatly extend the known ecology of magnetotactic bacteria and the conditions under which they can biomineralize magnetite. Moreover, our results show that this type of magnetotactic bacterium is common in highly alkaline environments. Our findings also greatly influence the interpretation of the presence of nanometer-sized magnetite crystals, so-called magnetofossils, in highly alkaline environments.

Nucleation of Iron Oxide Nanoparticles Mediated by Mms6 Protein in Situ

Biomineralization proteins are widely used as templating agents in biomimetic synthesis of a variety of organic-inorganic nanostructures. However, the role of the protein in controlling the nucleation and growth of biomimetic particles is not well understood, because the mechanism of the bioinspired reaction is often deduced from ex situ analysis of the resultant nanoscale mineral phase. Here we report the direct visualization of biomimetic iron oxide nanoparticle nucleation mediated by an acidic bacterial recombinant protein, Mms6, during an in situ reaction induced by the controlled addition of sodium hydroxide to solution-phase Mms6 protein micelles incubated with ferric chloride. Using in situ liquid cell scanning transmission electron microscopy we observe the liquid iron prenucleation phase and nascent amorphous nanoparticles forming preferentially on the surface of protein micelles. Our results provide insight into the early steps of protein-mediated biomimetic nucleation of iron oxide and point to the importance of an extended protein surface during nanoparticle formation.

Doping evolution of the absolute value of the London penetration depth and superfluid density in single crystals of Ba(Fe1-xCox)2As2

The zero-temperature value of the in-plane London penetration depth, λab (0) , has been measured in single crystals of Ba(Fe1-x Cox)₂ As₂ as a function of the Co concentration, x , across both the underdoped and overdoped superconducting regions of the phase diagram. For x ≳ 0.047 , λab (0) has been found to have values between 120 ± 50 and 300 ± 50 nm . A pronounced increase in λab (0) , to a value as high as 950 ± 50 nm , has been observed for x ≲ 0.047 , corresponding to the region of the phase diagram where the itinerant antiferromagnetic and superconducting phases coexist and compete. Direct determination of the doping-dependent λab (0) has allowed us to track the evolution of the temperature-dependent superfluid density, from which we infer the development of a pronounced superconducting gap anisotropy at the edges of the superconducting dome.

Sonochemical modification of the superconducting properties of MgB2

Ultrasonic irradiation of magnesium diboride slurries in decalin produces material with significant intergrain fusion. Sonication in the presence of Fe(CO)5 produces magnetic Fe2O3 nanoparticles embedded in the MgB2 bulk. The resulting superconductor–ferromagnet composite exhibits considerable enhancement of its magnetic hysteresis, which implies an increase of vortex pinning strength due to embedded magnetic nanoparticles.

Correlative electron and fluorescence microscopy of magnetotactic bacteria in liquid: toward in vivo imaging.

Magnetotactic bacteria biomineralize ordered chains of uniform, membrane-bound magnetite or greigite nanocrystals that exhibit nearly perfect crystal structures and species-specific morphologies. Transmission electron microscopy (TEM) is a critical technique for providing information regarding the organization of cellular and magnetite structures in these microorganisms. However, conventional TEM can only be used to image air-dried or vitrified bacteria removed from their natural environment. Here we present a correlative scanning TEM (STEM) and fluorescence microscopy technique for imaging viable cells of Magnetospirillum magneticum strain AMB-1 in liquid using an in situ fluid cell TEM holder. Fluorescently labeled cells were immobilized on microchip window surfaces and visualized in a fluid cell with STEM, followed by correlative fluorescence imaging to verify their membrane integrity. Notably, the post-STEM fluorescence imaging indicated that the bacterial cell wall membrane did not sustain radiation damage during STEM imaging at low electron dose conditions. We investigated the effects of radiation damage and sample preparation on the bacteria viability and found that approximately 50% of the bacterial membranes remained intact after an hour in the fluid cell, decreasing to ~30% after two hours. These results represent a first step toward in vivo studies of magnetite biomineralization in magnetotactic bacteria.