  Weak magnetic field exposure can affect many biological processes across a
wide range of living organisms. Recently, it has been observed that weak
magnetic fields can modulate reactive oxygen species (ROS) concentration,
affecting regeneration in planaria. These effects show unusual nonlinear
dependence on magnetic field strength, including a sign change. In another
study by the same group, superoxide is identified as the particular ROS being
modulated. We propose a radical pair mechanism based on a flavin-superoxide
radical pair to explain the modulation of superoxide production and its effect
on planarian regeneration. The results of our calculations favor a triplet-born
radical pair. Our yield calculations can reproduce the observed magnetic field
dependence, including the sign change. Moreover, to explain the size of the
effect on ROS concentration, we suggest a simple amplification model inspired
by known biochemical mechanisms and lay out the conditions for such a model to
work. Further, we also make empirical predictions concerning the hypomagnetic
field effects on planarian regeneration.
