On the magnetostatic control of crystal orientation and iron accumulation in magnetosomes

Abstract

Crystals of biogenic magnetite found in a variety of organisms exhibit consistent crystallographic orientations from particle to particle along magnetosome chains. typically with the [Ill] directions parallel to the chain length. In contrast to most biomineral systems where preferred crystal alignments are produced by organic matrix control, biophysical calculations presented here suggest that the common [Ill] alignment may arise spontaneously from magnetostatic interactions. A magnetosome chain produces relatively strong magnetic fields and field gradients in the surrounding space. particularly at the end where new crystals form. In this envtronment, the magnetocrystalline anisotropy of magnetite is enough to produce preferential crystal orientation for growing particles as small as 8.3 nm in size. Hence, the exceptional magnetic bacteria which do not produce such alignment must use some other form of biological control over crystal orientation. The unique magnetic environment surrounding magnetosome chains is not sufficient to cause the spontaneous accumulation of paramagnetic iron, but docs result in the exposure of most of a cell's biochemical machinery to unusually strong fields and field gradients. As this exposure has been present for much of the past 2 billion years since the evolution of the cukaryotic cell. high fields and gradients per se are unlikely to have deleterius biochemical effects.