Not an expert in this area, but I don't think the ISS could be considered a closed system. I hear you on the atom angle, but there's energy to take into conside

Not an expert in this area, but I don't think the ISS could be considered a closed system. I hear you on the atom angle, but there's energy to take into consideration as well. I suppose in theory it could work like one of those self contained ecosystems in a glass ball or whatever - those still take in solar energy. The issue you'd have to tackle is that mammals are ingesting carbon based food, taking the energy out of it, and exhaling the carbon through respiration. Their bodily waste is mostly either nitrogen or bacteria, which can be converted through natural processes into forms that plants can uptake in conjunction with the aforementioned exhaled carbon. So basically it seems to me like the most straightforward way to achieve what you're suggesting would be to use the processes we already use on earth.