A closed-loop life-support system tries to keep matter circulating inside a habitat. Water is recovered and purified. Carbon dioxide is removed. Oxygen is regenerated. Wastes are processed rather than simply discarded. The closer the loop, the less mass must arrive from Earth.
The ISS is already a partial model
NASA's Environmental Control and Life Support System on the International Space Station includes water recovery, air revitalization, and oxygen generation. The station currently recovers roughly 90% of its water through a combination of urine processing, humidity condensate recovery, filtration, catalytic treatment, and quality monitoring.
Why Mars raises the stakes
Earth orbit still permits relatively frequent resupply and emergency return. Mars does not. Transit windows, distance, and communication delay make autonomy much more important. A Martian habitat therefore needs graceful degradation: when one subsystem fails, the entire habitat should not immediately fail with it.
Plants can become part of the loop
Bioregenerative systems use living organisms as functional parts of life support. Plants can contribute food, oxygen production, carbon-dioxide uptake, water cycling, and psychological benefits. They do not eliminate the need for mechanical and chemical systems, but they may reduce consumable demand and provide redundancy.
Project ALCHEMY approach
For ALCHEMY, the most plausible architecture is hybrid: physicochemical recycling for precise, high-reliability control, combined with biological systems for food, nutrient recovery, atmospheric support, and long-term resilience. The target is not a perfectly sealed ecosystem. It is a system that progressively reduces dependence on Earth.