Small-angle neutron scattering from bacterial magnetite

Small-angle neutron scattering was used to study the magnetic properties of intracellular magnetite crystals (magnetosomes) in the bacterium, Aquaspirillum magnetotacticum, grown in pure culture. An average of 20 single-domain magnetite (Fe3O4) particles of diameter 400–500 Å are arranged in a chain that longitudinally traverses each cell. The net magnetic dipole moment carried by each chain is sufficient to orient the bacterium in the geomagnetic field. A contrast variation series, performed on a nonmagnetic variant, established that a 30% D2O/H2O ratio in the suspension buffer resulted in the minimization of the scattering from the bacterium itself, thereby enhancing that from the magnetosomes. Measurements of A. magnetotacticum in 30% D2O and an applied field of 25 G confirmed that the scattering from the magnetite particles in bacteria grown under iron-rich conditions is of ferromagnetic character, while that from magnetosomes in bacteria grown under iron-starved conditions is characteristic of superparamagnetic particles. In both cases, the wave-vector dependence of the scattering can be represented by a sum of Lorentzian plus Lorentzian squared terms, from which longitudinal as well as transverse spin correlation lengths were extracted. The results describe dynamic spin correlation lengths on the order of 50 Å, which are superposed on the static magnetosome cluster.