Closed eco-systems

I would like to share your idea about the following topic...

Concerning, the closed ecosystem of a habitation, the information I
found is usually based on the idea that the percentage of nitrogen is
decreased and oxygen is increased (up to 70%), so it will weight three
times less than normal air and be cost-effective...

According to my knowledge, The stabilization of the basic ecosystem will
require redundancy in its diverse systems. For example, often
interaction between populations of organisms rank more important for the
stability of an ecosystem than the activity of single organisms. On
Earth, an enzyme (nitrogenase) is responsible for taking atmospheric
nitrogen and changing it into high-energy ammonia (NH3). The ammonia is
necessary for protein production. The most important point here is that
oxygen must be kept away from away from nitrogenase. If we have too much
oxygen, there will be no production of ammonia and protein. It is hard
enough on Earth to keep Nitrogenase active without upping the amount in
the atmosphere to 80%. The nitrogen-fixing bacteria and trace elements
like molybdenum or crucial elements like phosphorus (creating the
universal energy of all living organisms (ATP)) are necessary elements
for a sustainable closed ecosystem.

so does anybody have some information or ideas about sustainable closed
ecosystems??

anilir