Magnetic properties of uncultivated magnetotactic bacteria and their contribution to a stratified estuary iron cycle.

Of the two nanocrystal (magnetosome) compositions biosynthesized by magnetotactic bacteria (MTB), the magnetic properties of magnetite magnetosomes have been extensively studied using widely available cultures, while those of greigite magnetosomes remain poorly known. Here we have collected uncultivated magnetite- and greigite-producing MTB to determine their magnetic coercivity distribution and ferromagnetic resonance (FMR) spectra and to assess the MTB-associated iron flux. We find that compared with magnetite-producing MTB cultures, FMR spectra of uncultivated MTB are characterized by a wider empirical parameter range, thus complicating the use of FMR for fossilized magnetosome (magnetofossil) detection. Furthermore, in stark contrast to putative Neogene greigite magnetofossil records, the coercivity distributions for greigite-producing MTB are fundamentally left-skewed with a lower median. Lastly, a comparison between the MTB-associated iron flux in the investigated estuary and the pyritic-Fe flux in the Black Sea suggests MTB play an important, but heretofore overlooked role in euxinic marine system iron cycle.

Characterization of extracellular minerals produced during dissimilatory Fe(III) and U(VI) reduction at 100 degrees C by Pyrobaculum islandicum.

In order to gain insight into the significance of biotic metal reduction and mineral formation in hyperthermophilic environments, metal mineralization as a result of the dissimilatory reduction of poorly crystalline Fe(III) oxide, and U(VI) reduction at 100 degrees C by Pyrobaculum islandicum was investigated. When P. islandicum was grown in a medium with poorly crystalline Fe(III) oxide as an electron acceptor and hydrogen as an electron donor, the Fe(III) oxide was reduced to an extracellular, ultrafine-grained magnetite with characteristics similar to that found in some hot environments and that was previously thought to be of abiotic origin. Furthermore, cell suspensions of P. islandicum rapidly reduced the soluble and oxidized form of uranium, U(VI), to extracellular precipitates of the highly insoluble U(IV) mineral, uraninite (UO(2)). The reduction of U(VI) was dependent on the presence of hydrogen as the electron donor. These findings suggest that microbes may play a key role in metal deposition in hyperthermophilic environments and provide a plausible explanation for such phenomena as magnetite accumulation and formation of uranium deposits at ca. 100 degrees C.

Magnetodendrimers allow endosomal magnetic labeling and in vivo tracking of stem cells.

Magnetic resonance (MR) tracking of magnetically labeled stem and progenitor cells is an emerging technology, leading to an urgent need for magnetic probes that can make cells highly magnetic during their normal expansion in culture. We have developed magnetodendrimers as a versatile class of magnetic tags that can efficiently label mammalian cells, including human neural stem cells (NSCs) and mesenchymal stem cells (MSCs), through a nonspecific membrane adsorption process with subsequent intracellular (non-nuclear) localization in endosomes. The superparamagnetic iron oxide nanocomposites have been optimized to exhibit superior magnetic properties and to induce sufficient MR cell contrast at incubated doses as low as 1 microg iron/ml culture medium. When containing between 9 and 14 pg iron/cell, labeled cells exhibit an ex vivo nuclear magnetic resonance (NMR) relaxation rate (1/T2) as high as 24-39 s-1/mM iron. Labeled cells are unaffected in their viability and proliferating capacity, and labeled human NSCs differentiate normally into neurons. Furthermore, we show here that NSC-derived (and LacZ-transfected), magnetically labeled oligodendroglial progenitors can be readily detected in vivo at least as long as six weeks after transplantation, with an excellent correlation between the obtained MR contrast and staining for beta-galactosidase expression. The availability of magnetodendrimers opens up the possibility of MR tracking of a wide variety of (stem) cell transplants.

Relaxometry and magnetometry of the MR contrast agent MION-46L.

MION-46L is an ultrasmall monocrystalline superparamagnetic (SPM) iron oxide that is of current interest as an MR contrast agent. It is believed to consist primarily of small maghemite or magnetite crystals that possess a SPM moment, but the exact magnetic properties and related mechanisms of T(1) and T(2) proton relaxation enhancement are not well understood. We have obtained a comprehensive data set consisting of magnetization curves, EPR spectra, and 1/T(1) and 1/T(2) nuclear magnetic relaxation dispersion (NMRD) profiles for this contrast agent. The magnetization curves show a primary curvature consistent with a SPM moment of 10,300 Bohr magnetons (BM) per particle. In addition, there is a secondary high-field curvature that is consistent with a smaller SPM moment of 1600 BM, which may be responsible for the observed high-field increase in 1/T(2). Finally, there appear to be a considerable number of paramagnetic ions present that are needed to account for the high-field increase in magnetization, and that can provide an alternative explanation for the magnitude of the low-field T(1) plateau. This "three-phase model" appears to be successful in explaining in a self-consistent and quantitative manner the combined results of the magnetometry, relaxometry, and EPR studies. Magn Reson Med 42:379-384, 1999. Published 1999 Wiley-Liss, Inc.

Magnetic material in the human hippocampus.

Magnetic analyses of hippocampal material from deceased normal and epileptic subjects, and from the surgically removed epileptogenic zone of a living patient have been carried out. All had magnetic characteristics similar to those reported for other parts of the brain [6]. These characteristics along with low temperature analysis indicate that the magnetic material is present in a wide range of grain sizes. The low temperature analysis also revealed the presence of magnetite through manifestation of its low temperature transition. The wide range of grain sizes is similar to magnetite produced extracellularly by the GS-15 strain of bacteria and unlike that found in magnetotactic bacteria MV-1, which has a restricted grain size range. Optical microscopy of slices revealed rare 5-10 micron clusters of finer opaque particles, which were demonstrated with Magnetic Force Microscopy to be magnetic. One of these was shown with EDAX to contain AI, Ca, Fe, and K, with approximate weight percentages of 55, 19, 19, and 5, respectively.

Magnetoferritin: characterization of a novel superparamagnetic MR contrast agent.

A protein-encaged superparamagnetic iron oxide has been developed and characterized by using horse spleen apoferritin as a novel bioreactive environment. The roughly spherical magnetoferritin molecules, 120 A in diameter, are composed of a monocrystalline maghemite or magnetite core 73 A +/- 14 in diameter. Except for the additional presence of iron-rich molecules of higher molecular weight, the appearance and molecular weight (450 kd) of magnetoferritin are identical to that of natural ferritin; the molecules are externally indistinguishable from their precursor, with a pI (isoelectric point) in the range 4.3-4.6. The measured magnetic moment of the superparamagnetic cores is 13,200 Bohr magnetons per molecule, with T1 and T2 relaxivities (r1 and r2) of 8 and 175 L.mmol-1 (Fe).sec-1, respectively, at body temperature and clinical field strengths. The unusually high r2/r1 ratio of 22 is thought to arise from ideal core composition, with no evidence of crystalline paramagnetic inclusions. T2 relaxation enhancement can be well correlated to the field-dependent molecular magnetization, as given by the Langevin magnetization function, raised to a power in the range 1.4-1.6. With its nanodimensional biomimetic protein cage as a rigid, convenient matrix for complexing a plethora of bioactive substances, magnetoferritin may provide a novel template for specific targeting of selected cellular sites.

Magnetic cristobalite (?): a possible new magnetic phase produced by the thermal decomposition of nontronite.

Prolonged heat treatment (> 1 hour) of nontronite (an iron-rich smectite clay) at 900 degrees to 1000 degrees C produces a phase with some unusual magnetic properties. This new phase has a Curie temperature of 200 degrees to 220 degrees C, extremely high remanent coercivities in excess of 800 milliteslas, and a room-temperature coercivity dependent on the magnitude of the applied field during previous thermomagnetic cycling from above 220 degrees C. X-ray and magnetic analyses suggest that an iron-substituted cristobalite could be responsible, in part, for these observations. Formation of this magnetic cristobalite, however, may require topotactic growth from a smectite precursor.