  Since the emergence of oxygenic photosynthesis, living systems have developed
protective mechanisms against reactive oxygen species. During charge separation
in photosynthetic reaction centres, triplet states can react with molecular
oxygen generating destructive singlet oxygen. The triplet product yield in
bacteria is observed to be reduced by weak magnetic fields. Reaction centres
from plants' photosystem II share many features with bacterial reaction
centres, including a high-spin iron whose function has remained obscure. To
explain observations that the magnetic field effect is reduced by the iron, we
propose that its fast-relaxing spin plays a protective role in photosynthesis
by generating an effective magnetic field. We consider a simple model of the
system, derive an analytical expression for the effective magnetic field and
analyse the resulting triplet yield reduction. The protective mechanism is
robust for realistic parameter ranges, constituting a clear example of a
quantum effect playing a macroscopic role vital for life.
