At least two basic approaches have been suggested, one in which the action is caused by magnetic torque of the magnetosomes on other cellular structures (like mechanosensitive transmembrane ion channels Kirschvink 1992), and an indirect one in which the strong magnetic field surrounding the magnetosome(s) alters magnetochemical reactions (e.g. Fundamentally, the rotation or translation of a magnetic particle must be able somehow to affect the electrical field across a sensory nerve membrane and thereby influence the production of action potentials that are the common currency of all neural activity. It is therefore not surprising that various physiological arrangements of single-domain biogenic magnetite have been suggested as a basis for geomagnetic field sensitivity in animals ( Gould et al. In contrast, the densely interacting crystals of biogenic magnetite in chiton teeth were found to be too weakly and randomly magnetized to serve as a compass ( Kirschvink & Lowenstam 1979), as are the large, detrital grains of titanomagnetite sometimes found in the vestibular organs of elasmobranch fish ( Vilches-Troya et al.
#FREEDOM PLANET TORQUE HERE WE GO FREE#
Clean-laboratory-based extraction studies, aided by superconducting magnetometry, have identified strings of single-domain magnetite crystals similar to those in the magnetotactic bacteria in the frontal tissues of migratory fish, which are the most free of inorganic contamination (Walker et al. Despite advances in rock magnetic theory and electron microscopy, this basic observation has stood the test of time very well over the past 30 years ( Kopp & Kirschvink 2008). 2006), allowing their vector magnetic moments to sum linearly, increasing the cellular magnetic moment ( Frankel et al. The typical bacterial geometry is to string the membrane-bound crystals (termed magnetosomes) together into linear chains ( Blakemore 1975), supported from magnetostatic collapse by intracellular cytoskeletal filaments ( Kobayashi et al. It quickly became apparent that natural selection for size, shape, chemistry, crystallographic orientation and several other properties of biological magnetite had converged on the same solutions for producing single-domain crystals in magnetotactic bacteria ( Kirschvink & Lowenstam 1979), confirming both the geophysical work and the role of the ferromagnetic materials in their magnetic response. Parallel but totally separate developments in the geophysical field of rock and mineral magnetism during the 1960s and 1970s gradually led to the understanding of the size, shape and chemical properties for magnetite that would produce the uniformly magnetized crystals (termed ‘single-domain’ particles) that were responsible for holding much of the stable, ancient magnetization in rocks ( Evans & McElhinny 1969 Butler & Banerjee 1975), providing the basis for determining the deep-time history of the geomagnetic field (the science of palaeomagnetism). 1979) and even protists ( Bazylinski et al. In the nearly 50 years since Lowenstam (1962) reported the presence of the ferromagnetic mineral magnetite (Fe 3O 4) as a hardening agent in the radular teeth of the Polyplacophoran molluscs (the chitons), and suggested it might possibly be used as a magnetic field sensor, it has been found as a matrix-mediated biological precipitate in a plethora of living organisms, including insects ( Gould et al.
We find that the common assertion that a magnetoreceptor based on single-domain magnetite could not form the basis for an inclination compass does not always hold. Also, we provide a generic classification scheme of torque transducers in terms of axial or polar output, within which we discuss the results from behavioural experiments conducted under altered field conditions or with pulsed fields. On the other hand, torque-detector models that are based on magnetic multi-domain particles in the vestibular organs turn out to be ineffective. Models based on biogenic single-domain magnetite prove both effective and efficient, irrespective of whether the magnetic structure is coupled to mechanosensitive ion channels or to an indirect transduction pathway that exploits the strayfield produced by the magnetic structure at different field orientations. We analyse from first principles the conditions under which they are viable. Several models of such biological magnetic-torque transducers on the basis of magnetite have been proposed in the literature. Provided that magnetic particles have remanence or anisotropic magnetic susceptibility, an external magnetic field will exert a torque and may physically twist them. Although ferrimagnetic material appears suitable as a basis of magnetic field perception in animals, it is not known by which mechanism magnetic particles may transduce the magnetic field into a nerve signal.