  The interaction between weak magnetic fields and rotation can lead to
instabilities that transport angular momentum (AM) and chemical elements
affecting the evolution of massive stars. We explore the effects of the
magneto-rotational instability (MRI) in massive stars to determine its impact
on stellar evolution. We use the GENEC code to simulate the evolution of a 15
$M_\odot$ model at solar metallicity up to the end of oxygen burning. The MRI
is computed with different trigger conditions, (depending on the weighting of
chemical gradients through an arbitrary but commonly used factor), and with
different treatments of meridional circulation as either advective or
diffusive. We also compare the MRI with the Tayler-Spruit (TS) dynamo, in
models that included both instabilities interacting linearly. The MRI triggers
throughout stellar evolution. Its activation is highly sensitive to the
treatment of meridional circulation and the existence of chemical gradients.
The MRI is very efficient at transporting both matter and AM, leading to
noticeable differences in rotation rates and chemical structure, which may be
observable in young main sequence stars. While the TS dynamo is the dominant
mechanism for transferring AM, the MRI remains relevant in models where both
instabilities are included. Extrapolation of our results suggests that models
including the MRI tend to develop more compact cores, which likely produce
failed explosions and black holes, than models where only the TS dynamo is
included (where explosions an neutron stars may be more frequent. The MRI is an
important factor in massive star evolution but is very sensitive to the
implementation ofother processes in the model. The transport of AM and chemical
elements due to the MRI alters the rotation rates and the chemical make-up of
the star from the core to the surface, and may change the explodability
properties of massive stars.
