Small angle neutron and X-ray scattering from magnetite crystals in magnetotactic bacteria

Small angel neutron and X-ray scattering were used to study the magnetic and structural properties, respectively, of intracellular magnetite crystals (magnetosomes) in the bacterium, Aquaspirillum magnetotacticum, grown in pure culture. An average of twenty magnetite particles of diameter 400–500 Å are arranged in a chain that longitudinally traverses each cell. Behaving like magnetic dipoles, the bacteria orient in the geomagnetic field. Small angle X-ray scattering measurements revealed that bacteria grown in the presence of large amounts of iron contained magnetosomes with diameters averaging approximately 400 Å. Those grown under iron-starved conditions contained particles with slightly smaller diameters. The contrast variation technique was used for the small angle neutron scattering measurements in order to establish that a 30% D 2O/H 2O ratio in the solvent resulted in the maximization of the scattering from the bacterium itself, thus enhancing that from the magnetosomes. Measurements in 30% D 2O and an applied field of 25 G confirmed that the scattering from the larger magnetic particles is of ferromagnetic character whereas that from the smaller particles is consistent with super-paramagnetic particles. The static spin correlations were found to be of relatively long range, with dynamical fluctuations corresponding to a distance scale of approximately 50 Å.