[adastra22]

For the most part, partial pressure is what matters, not absolute pressures. The only contribution inert nitrogen has to life is raising the boiling point of water. But on a frigid planet like Mars that’s not going to be an issue.

Being in a pure oxygen environment does not have a long term impact on animals… if the pure O2 environment is 0.21 atm oxygen. There’s a long history of biological experimentation done by space agencies on this, and the Apollo missions operated on a 0.2 atm pressure for the duration of their missions. Apollo 1 famously had trouble not because it was pure O2, but because it was 1.2atm O2.

There is no reason you need backfill inert gasses to have 1.0 atmospheres.