In the book it is mentioned that Grace has no way of growing his own food, there are no plants, seeds or food he can grow food from (except himself of course).
Typically us humans rely on plants to convert co2 back into oxygen, the biodome doesnt have that and as far as we know Erid doesnt have flora. It can be assumed that if Erid does have flora they may not work the same way as the flora on Earth because of the ammonia rich atmosphere.
I imagine the Eridians utilized the system from the Hail Mary that recycles co2 to combat this issue in the biodome.
Now imagine, Grace dies and Rocky is of course grieving deeply. The rest of Erid feels a sense of melancholy and respect for their savior and protector, the bravest human they know, Ryland Grace. This sentiment is felt so deeply throughout Erid that wherever Rocky wanders he cant seem to escape the melancholy that hangs in the atmosphere for far too long but somehow not long enough.
Rocky and the science Eridians that maintained the biodome continue to do so after Graces passing for science purposes. They slowly watch the co2 levels lower as the recycling system does its work. Watching this saddens Rocky. One of the last remaining evidences of Graces existence and bravery he brought to his species is slowly fading away.
Co2 is unique to humans. Its a byproduct of our existence. Rocky heard and watched Graces lungs breathe in a soothing, rhythmic way for years. He watched over Grace when he slept and must have felt comforted by this simple human function over and over. It’s almost like a soft lullaby, unique to Grace and only Grace, his human song. A comforting constant he could always hear and rely on for a reminder that he is safe. Grace is here and Rocky is ok, Rocky is protected and so is all of Erid.
Eventually the recycling system is shut off in order to preserve this last bit of Graces humanly handprint he left on Erid. Rocky will never breathe the same air Grace did but he will always know its there and be surrounded by this bittersweet reminder of his friend, his protecter, his savior, Ryland Grace.
