In Andy Weir's Project Hail Mary, nitrogen in the atmosphere is lethal for the astrophage predator taumoeba. In part of the story they are trying to breed taumeba that can live in a few percent nitrogen. This taumoeba escapes and kills all the astrophage, but why didn't the 70% nitrogen in the normal human atmosphere on the Hail Mary kill it?
There is a reference that the atmosphere aboard the Hail Mary is 40% sea-level which would suggest that the nitrogen level is certainly reduced from normal (so that the partial pressure of oxygen remains high enough) but I'm not sure if 40% means that it must be entirely oxygen.
Apparently, the entire Hail Mary is at that 40 percent pressure
Hmm. Although later on there is this part:
"Just a centimeter of transparent material separates my one-fifth atmosphere of oxygen pressure from Rocky's 29 atmospheres of ammonia."
Which suggests pure oxygen at 20% normal pressure.
And, finally, there is this reference to
one of the crews selected method of suicide (by nitrogen asphyxiation) which means that they have to provide a tank of it specifically: "'We'll make sure there's a tank with plenty of nitrogen, and a backup tank as well in case the first one leaks'"
So, even though there seems to be an inconsistency about the exact pressure, it does seem to be very strongly implied that the Hail Mary atmosphere is pure Oxygen.
Here is a link to a study on the flammability of substances based on both concentration, partial pressures and total pressure.
Flammability characteristics (per NASA-STD-6001 Test 1 MOC self-extinguishment thresholds, material ignition, and burn rates) show a strong dependence on oxygen concentration with little relation to total pressures above 41 kPa (6 psia). Below 41 kPa (6 psia), MOCs and required oxygen partial pressures show increased dependence on total pressure. Power equation models fit trends very precisely across pressure ranges spanning 2.8–119.3 kPa (0.4–17.3 psia) for both MOC and partial pressure plots against total pressures. A notable finding was that required partial pressure of oxygen necessary to sustain propagation decreases with decreased total pressures. This directly implies an increased flammability risk at lower total pressure conditions.
Although the total pressure was bellow 40 kPa the 100% oxygen content still seems worthy of discussion in the book, given that last sentence. Especially given Strats obsession with “tried and tested technology” and it’s possible effect on high temperature equipment.
It is possible that the Taumoeba could already survive the amount of nitrogen on board the ship, and just the amount that he added to it would make it so that they could not survive it. I mean if Rocky accidentally screwed up when making the breeder tanks and made a leak then the nitrogen in the Hail Mary would have gotten in.