Richard P. Blakemore

Magnetite in Freshwater Magnetotactic Bacteria

A previously undescribed magnetotactic spirillum isolated from a freshwater swamp was mass cultured in the magnetic as well as the nonmagnetic state in chemically defined culture media. Results of Mossbauer spectroscopic analysis applied to whole cells identifies magnetite as a constituent of these magnetic bacteria.

Fe3O4 precipitation in magnetotactic bacteria

Abstract Using Mossbauer resonance spectroscopy of 57Fe, we have determined the nature and distribution of major iron compounds in the magnetotactic bacterium Aquaspirillum magnetotacticum. In addition to magnetite (Fe3O4), cells contained a low-density hydrous ferric oxide, a high-density hydrous ferric oxide (ferrihydrite), and ferrous iron. Analysis at different temperatures of whole cells harvested early and late in growth, of mutant cells unable to synthesize magnetite, and of cell fractions enriched in 57Fe indicated that Fe3O4 precipitation resulted from partial reduction of the high-density hydrous ferric oxide precursor.

Navigational Compass in Magnetic Bacteria

Abstract Magnetotactic bacteria that orient and swim in a preferred direction in the geomagnetic field contain sufficient single domain magnetite to constitute a biomagnetic compass.

Microaerobic Conditions Are Required for Magnetite Formation Within Aquaspirillum magnetotacticum

Abstract The amount of magnetite (Fe3O4) within magnetosomes of the microaerophilic bacterium Aquaspirillum magnetotacticum varies with oxygen and nitrogen supply. The development of optical methods for directly measuring cell magnetism in culture samples has enabled us to quantitate bacterial Fe3O4 yields. We measured final cell yields, average cell magnetic moments, and magnetosome yields of growing cells. Cultures were grown with NO3‐, NH4 +, or both, in sealed, unshaken vials with initial headspace Po2 values ranging from 0 (trace) to 21 kPa. More than 50% of cells had detectable magnetosomes only when grown in the range of 0.5–5.0 kPa O2. Optimum cell magnetism (and Fe3O4 formation) occurred under microaerobic conditions (initial headspace Po2 of 0.5–1 kPa) regardless of the N source. At optimal conditions for Fe3O4 formation, denitrifying cultures produced more of this mineral than those growing with O2 as the sole terminal electron acceptor. This suggests that competition for O2 exists between proc...

Structure, morphology and crystal growth of anisotropic magnetite crystals in magnetotactic bacteria

Bacterial magnetite particles of anisotropic morphology have been studied by high-resolution transmission electron microscopy. Lattice images of individual crystals are consistent with a well-ordered magnetite cubic inverse spinel structure. The idealized morphology of the biogenic crystals is based on an elongated cubo-octahedral form comprising a hexagonal prism of {111} and {100} faces capped by (1̄1̄1) and (111̄) faces with associated {111} and {100} truncations. Analysis of many particles of diverse size suggests that crystal growth takes place in two stages. The first stage is associated with the formation of well-ordered, isotropic, single-domain crystals of cubo-octahedral morphology. In this stage the crystal length and width develop concurrently up to a size of 20 nm. The second stage involves the anisotropic growth of the isotropic particles along the [112̄] direction. A crystal growth mechanism is postulated which involves the specific nucleation of the (1̄1̄1) face on a surrounding organic membrane. Unidirectional growth then occurs by selective suppression of certain crystallographic axes through spatial and chemical constraints induced by the adjacent organic boundary.

Intercellular structure in a many-celled magnetotactic prokaryote

A many-called magnetotactic prokaryote obtained from brackish water was observed to possess intercellular connections at points of contact between the outer membranes of constituent cells. Each aggregate organism consisted of 10 to 30 individual Gram-negative cells containing material with the appearance of poly-β-hydroxybutyrate and magnetosomes of unusual arrangement, structure and composition. The aggregate, which possessed prokaryotic-type flagella arranged at the outwards surfaces of each cell, showed motility indicative of co-ordination between individual component cells. These results suggest that this organism could be a multicellular prokaryote.

Ultrastructure and Characterization of Anisotropic Magnetic Inclusions in Magnetotactic Bacteria

Ovoid magnetotactic bacteria extracted from the Exeter River, New Hampshire, U. S. A., contain chains of 20–35 anisotropic magnetic inclusions running longitudinally in each of three lateral cell positions adjacent to the inner surface of the cytoplasmic membrane. The inclusions are bullet-shaped and have characteristic flattened end faces. Some particles show kinking and curvature in their morphology. In cross section the particles have a hexagonal shape. The length of the inclusions varies over a wide range (45–135 nm) with a mean value of 97.8 nm. In contrast, the width of the particles is restricted to a range of 30–45 nm with a mean value of 36.9 nm. Many particles are surrounded by an organic electrondense envelope. The crystallographic structure of the inclusions has been determined by electron diffraction and corresponds to the mineral magnetite (Fe3O4). The dimensions of the crystals fall within the magnetic single-domain range for magnetite and the magnetic moment of one cell is approximately 4 x 10–12 emu (4 fJT–1).

The Magnetic Behavior of Mud Bacteria

When separated from the substrate, Blakemore’s mud bacteria swim back to the bottom of the sea following the earth’s magnetic field lines. Their magnetotactic response appears to be due to the presence of internal ferromagnetic dipole moments of single-domain properties.

Iron reduction byAquaspirillum magnetotacticum

Iron reductase activity in cell extracts ofAquaspirillum magnetotacticum strain MS-1 (wild type) or nonmagnetotactic mutant strain NM-1A was located primarily in the periplasm. Cytoplasm contained 20%–35% and membrane fractions 3% of total iron reductase activity detected. Iron reduction was reversibly inhibited by oxygen, required β-NADH, and was enhanced by flavins. Reduced disulfide bonds and uncomplexed sulfhydryl groups were necessary for reductase activity. Respiratory inhibitors did not appreciably affect iron reductase activity. Iron complexed with quinic acid, dihydroxybenzoic acid, acetohydroxamic acid, citric acid, or deferrioxamine B was reduced by soluble iron reductases of strain MS-1.

Nitrogen fixation (acetylene reduction) inAquaspirillum magnetotacticum

Aquaspirillum magnetotacticum strain MS-1 and two nonmagnetic mutants derived from it reduced C2H2 microaerobically but not anaerobically even with NO3−. This organism apparently is not capable of NO3−-dependent nitrogen fixation. Cells ofA. magnetotacticum reduced C2H2 at rates comparable to those ofAzospirillum lipoferum grown under similar conditions, but much lower than that ofAzotobacter vinelandii grown aerobically. Cells ofA. magnetotacticum in anaerobic cultures lacking NO3− did not reduce C2H2 until O2 was introduced. Optimum rates of C2H2 reduction byA. magnetotacticum were obtained at 200 Pa O2. C2H2 reduction was inhibited by more than 1 kPa O2 or 0.2 mM NO3− or NH4+. These results suggest thatA. magnetotacticum fixes N2 only under microaerobic, N-limited conditions.