Following the initiative to build and operate Biosphere 2 in New Mexico, USA and Biosphere 3 in Krasnoyarsk, Russia, modeled on the Earth (accredited as “Biosphere1”) there have been a range of attempts to create a closed system.  (Closed to flows of matter, including air and water, but not isolated from energy flows (solar energy, and heat conduction in or out).

Just wondering what would work, small and simple?  Some kind of aquarium, completely sealed, that had aquatic life and some plants to complete a cycle that could sustain… Sustain for how long – the life of some fish? Or including reproduction at a balanced rate so that it is sustainable for many generations?  What would be the threats to that continuity – assume there are no disruptions to the external environment (the placing of the 3D aquarium in some safe place not to be upset by external natural or human disasters).  Would the balance of populations be controlled within a range (Malthus ideas) or may they all die from disease or hunger or some other cause?