  Magnetospirillum gryphiswaldense is a microorganism with the ability to
biomineralize magnetite nanoparticles, called magnetosomes, and arrange them
into a chain that behaves like a magnetic compass. Rather than straight lines,
magnetosome chains are slightly bent, as evidenced by electron cryotomography.
Our experimental and theoretical results suggest that due to the competition
between the magnetocrystalline and shape anisotropies, the effective magnetic
moment of individual magnetosomes is tilted out of the [111] crystallographic
easy axis of magnetite. This tilt does not affect the direction of the chain
net magnetic moment, which remains along the [111] axis, but explains the
arrangement of magnetosomes in helical-like shaped chains. Indeed, we
demonstrate that the chain shape can be reproduced by considering an interplay
between the magnetic dipolar interactions between magnetosomes, ruled by the
orientation of the magnetosome magnetic moment, and a lipid/protein-based
mechanism, modeled as an elastic recovery force exerted on the magnetosomes.
