  In the string axiverse scenario, light primordial black holes may spin up due
to the Hawking emission of a large number of light (sub-MeV) axions. We show
that this may trigger superradiant instabilities associated with a heavier
axion during the black holes' evolution, and study the coupled dynamics of
superradiance and evaporation. We find, in particular, that the present black
hole mass-spin distribution should follow the superradiance threshold condition
for black hole masses below the value at which the superradiant cloud forms,
for a given heavy axion mass. Furthermore, we show that the decay of the heavy
axions within the superradiant cloud into photon pairs may lead to a
distinctive line in the black hole's emission spectrum, superimposed on its
electromagnetic Hawking emission.
