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Magnetotactic bacteria comprise several aquatic species that orient and migrate along geomagnetic field lines. This behavior is based on the presence of intracellular ferrimagnetic grains of the minerals magnetite ([Fe.sub.3][O.sub.4]) or greigite ([Fe.sub.3][S.sub.4]). Whereas the structural and magnetic properties of magnetite magnetosomes have been studied extensively, the properties of greigite magnetosomes are less well known. Here we present a study of the magnetic microstructures, chemical compositions, and three-dimensional morphologies and positions of Fe-sulfide crystals in air-dried cells of magnetotactic bacteria. Data were obtained using several transmission electron microscopy techniques that include electron holography, energy-filtered imaging, electron tomography, selected-area electron diffraction, and high-resolution imaging. The studied rod-shaped cells typically contain multiple chains of greigite magnetosomes that have random shapes and orientations. Many of the greigite crystals appear to be only weakly magnetic, because the direction of their magnetic induction is almost parallel to the electron beam. Nevertheless, the magnetosomes collectively comprise a permanent magnetic dipole moment that is sufficient for magnetotaxis. One of the cells, which is imaged at the point of dividing, contains multiple chains of both equidimensional Fe-sulfide and elongated Fe-oxide crystals. The equidimensional and elongated crystals have magnetic properties that are consistent with those of greigite and magnetite, respectively. These results can be useful for obtaining a better understanding of the function of magnetotaxis in sulfide-producing cells, and they have implications for the interpretation of the paleomagnetic signals of greigite-bearing sedimentary rocks. Keywords: Greigite, magnetotactic bacteria, transmission electron microscopy, electron holography, electron tomography, magnetic properties, biomineralization

The crystal habits of magnetosomes in two coccoid morphotypes of magnetotactic bacteria are studied by using the application of electron holography and high-resolution transmission electron microscopy. It is confirmed that the crystallographic positioning of successive crystals in the magnetsome chain appear to be under strict biological control.

We studied the sizes and shapes of magnetite nanocrystals produced by several types of uncultured magnetotactic bacteria to understand whether their size distributions can be used for identifying the biogenic origin of magnetite crystals in geological samples. The two-dimensional projections of the crystals were measured on transmission electron microscope (TEM) images, and features of crystalsize and shape-factor distributions (CSD and SFD, respectively) were analyzed. In agreement with previous results, most magnetite CSD curves are asymmetric and negatively skewed: however, one magnetotactic strain produced particles that have a normal size distribution. A statistical analysis of CSDs and SFDs (both from this and previous studies) reveals similarities among magnetite from magnetotactic strains from various locations. In particular, crystals in a cultured marine strain (MC-2) were indistinguishable from magnetite from a freshwater strain. We tested whether CSDs of distinct magnetosome types can be recovered from the shape and size data of all particles combined in samples that contain several types of magnetosomes; such samples can be used as models for rocks that contain magnetite nanocrystals of unknown and, presumably, various origins. If the SFDs of the distinct magnetosome types occurring in the same sample differ, the CSDs of individual magnetosome types can be retrieved from bulk data. In such cases the characteristic shape of the size distribution can be used for identifying magnetite as originating from magnetotactic bacteria.

A method for forming a redox active phosphate surface on reconstituted horse spleen ferritin (HoSF) cores is presented. The reconstituted HoSF cores derived from the method behaves similarly to that found in native HoSF cores. An investigation of the reactivity of the phosphate mineral surface and the relationship of the core size to the amount of reconstituted phosphate suggests that the phosphate layer floats on the developing iron mineral core after the layer was formed.

Research was conducted to examine the redox reactivities of air-oxidized apo rat liver ferritin and apo horse spleen ferritin using reductive optical titrations and microcoulometry. Two distinct types of redox activity were observed through microcoulometry on various independent lots of commercial HoSF while microcoulometric reduction stoichiometry was revealed by reductive optical titrations. Results indicate that the apoheteropolymers were redox-active like their native apoheteropolymer counterpaprts.

Optical and Mossbauer spectroscopy are used to characterize the diiron complex ((Fe2(C2OH36N8)(CH3CN)4)(ClO4)4). The study reveals that the binucleating ligand tetraiminoethylane dimacrocyle is a good pi acceptor. Electrochemical studies reveal the extensive mixing of metal and ligand orbitals. Ligand reduction potential in shifted to 780 mV more positive when nickel is substituted for iron complex.

The function and location of phosphate in Fe2+ binding to horse spleen holoferritin (HoSF) were reported. Native HoSF with decreased core phosphate levels and reconstituted HoSF having no phosphate present in the core were prepared. Fe2+ binding to native, phosphate altered, and reconstituted HoSF in the absence of phosphate showed that Fe2+ binding to the mineral core depends on the presence of core-bound phosphate.

A study was conducted on the interaction of Fe2+ and phosphate in bacterial ferritin from Azotobacter vinelandii (AVBF). Results show that phosphate sponsors the site of interaction of Fe2+ with the mineral core of AVBF. Further observations also suggest that the bound Fe2+ experiences a reversible electron-transfer reaction with the mineral core irons which results in the formation of 57Fe3+.

Bis-macrocyclic FeII/FeII complex is electrochemically oxidized to form a mixed valence FeII/FeII compound. Electronic interaction is facilitated by a delocalized Pi-system of binucleating ligand. Mossbauer spectrum of the mixed valence complex consists of a single, quadrupole-split doublet, showing the equivalence of two iron atoms.