Biological sensing of small field differences by magnetically sensitive chemical reactions

There is evidence that animals can detect small changes in the Earth's magn etic field by two distinct mechanisms, one using the mineral magnetite as t he primary sensor and one using magnetically sensitive chemical reactions(1 -14). Magnetite responds by physically twisting(2,15), or even reorienting the whole organism in the case of some bacteria(16), but the magnetic dipol es of individual molecules are too small to respond in the same way. Here w e assess whether reactions whose rates are affected by the orientation of r eactants in magnetic fields could form the basis of a biological compass. W e use a general model, incorporating biological components and design crite ria, to calculate realistic constraints for such a compass. This model comp ares a chemical signal produced owing to magnetic field effects with stocha stic noise and with changes due to physiological temperature variation(17). Our analysis shows that a chemically based biological compass is feasible with its size, for any given detection limit, being dependent on the magnet ic sensitivity of the rate constant of the chemical reaction.