But he probably isn't the only oxygen consumer in the greenhouse. There will most likely be insects, bacteria, fungi; All which also consume oxygen. On top of

But he probably isn't the only oxygen consumer in the greenhouse. There will most likely be insects, bacteria, fungi; All which also consume oxygen. On top of organic things, the Earth2 experiment famously ran into a problem where the concrete they used to build the biosphere kept curing long after construction finished, and kept sucking oxygen out of the atmosphere. And don't forget to take into account that NASA's machine can produce oxygen all day, everyday, where trees stop producing oxygen at night, and over winter (and actually start consuming oxygen)