So like. Just wondering, by how the fuck did Rocky know humans metabolize oxygen? Our air is like 78% nitrogen. By all accounts the logical conclusion for Rocky to come to is that aliens breathe nitrogen.
Not questioning how he was able to create a breathable atmosphere—it's only good sense for him to replicate the full composition—but why leave a representation of an oxygen atom, the element that makes up only 20% of our air, to communicate he's done it?
One possible explanation for the initial infection (immediately post-sampling) would be that the Taumoeba somehow infected the fuel lines from the outside, which there is some canon support for. The outside of the ship was damaged enough that the Astrophage inside wanted to evacuate to Adrian, so if there was any surviving Taumoeba on the hull while they were still inside Adrian's atmosphere it's plausible that it could have entered through the ruptured fuel tanks and gotten into the fuel lines before they were jettisoned.
I was going to suggest the structural leak could have been internal on Rocky's side of the ship, since it's stated that Taumoeba don't react to nitrogen compounds like ammonia, but I'm less confident on that. They consider it safe to pass one of Grace's Astrophage generators into Rocky's space, but I can't recall whether or not Rocky needed to sterilise the area first.
I have no clue about the second leak, though. It has to be from inside the ship, they're in deep space at that point. iirc there was some mention during the breeding tank setup about inconsistencies in atmosphere - that is, the idea that some parts of the environment would have more or less of certain gases than other parts, even though the tank itself has all of the correct ratios. That's the same reason Grace near-completely vents the ship's air during his sterilisation, so there aren't any bubbles with less nitrogen in them. So perhaps the Taumoeba got lucky and rode a 'thermal' of low-nitrogen air into the Astrophage storage bin?
Oooh, good point. There was a breach in the fuel system at the same time live taumeba would have been in contact with the exterior of the ship. Could be that's what happened!
As for the second infestation, I just remembered that the Mary is kept at a lower atmospheric pressure than earth normal, and would have a higher percentage of oxygen to compensate. I very much doubt Stratt would have risked 100% oxygen (I'd say we know she didn't, since they survived poor Rocky acting as an ignition source), but it's entirely probable the air is 30-40% oxygen. May well be low enough nitrogen percentage for some of the supertaumeba they bred for threeworld to survive.
And tying back to my original question, Rocky's air sample may have been an even higher concentration O2 due to some forgotten bends-resistance feature built into the air lock. If there's one place pure oxygen might be worth the risk, it's the tiny room by the front door.
Mmh, or it could be internal - as others in the reblogs are pointing out, there's enough evidence to assume a pure oxygen atmosphere inside the Hail Mary (the one that cinches it for me is Grace needing Dubois' nitrogen for the Taumoeba experiments, implying that either he can't access the ship's life-support nitrogen or there isn't any).
That would explain how Taumoeba could survive inside the Hail Mary, and also solve the question of Rocky's incredible leap of logic - no need to guess at which element is important if oxygen is the only thing present in the atmosphere at all.
