A closed cycle for carbon and maybe also for other resources. The smelter, the incinerator and also workers would produce CO2. (Workers would need O2 in addition to rations.) The (hydroponic) farm needs the CO2 to produce plants and O2. Since some of the carbon would get trapped in cotton or plastic, some planets could have CO2 in the atmosphere or C in the ground, that enters the cycle.
1 Like