ECOSYSTEMS • LIFE SUPPORT • OCTOBER 26, 2026
Biosphere 2’s Missing Oxygen
Soil, microbes and building materials became an unexpected lesson in designing a living world.
Eight people, two years, one enclosed ecosystem
Biosphere 2’s first mission ran from September 26, 1991, to September 26, 1993, with eight crew members. Its design combined living ecosystems with engineered systems, making it an unusually ambitious test of ecological life support. [1]
What happened to the oxygen?
A later research account describes oxygen falling from about 20.9% to 14.4% by mid-January 1993, roughly 475 days into the mission. Popular accounts often round the low point to 14.5%. Oxygen was subsequently supplied from outside. The two-year residence therefore should not be described as two years of completely unaided atmospheric self-sufficiency. [2]
Microbes consumed oxygen; concrete absorbed carbon dioxide
The 1994 investigation attributed the oxygen loss to microbial respiration of excessive organic matter in the soils. As microbes broke that material down, they consumed oxygen and produced carbon dioxide. Much of that carbon dioxide then reacted with exposed concrete to form calcium carbonate. [3]
That second process matters: oxygen could decline without an equivalent buildup of carbon dioxide in the air. Saying “concrete used up the oxygen” skips the biological consumption step and obscures the coupled chemical process.
Why this matters for future habitats
MKP interpretation: a habitat’s walls, soil and crops belong in the same design calculation. A room full of plants does not, by itself, demonstrate a balanced atmosphere. Testing should account for respiration, photosynthesis, material reactions, changing conditions and the need for backup supplies.
The Earth connection is equally useful: sustainable growing systems require attention to soil biology and resource flows, not just crop output. The space-habitat connection makes those dependencies especially visible because replacement resources are difficult to obtain.
Questions worth testing
- How would different soil organic-matter levels affect oxygen demand?
- How could engineers measure gas exchange with building materials before a crew arrives?
- Which biological functions need mechanical backups, and how should their capacity be tested?
Research sources
- Overview and Design Biospherics and Biosphere 2, mission one (1991–1993), Ecological Engineering.
- Tightly closed ecological systems reveal atmospheric subtleties – experience from Biosphere 2, Advances in Space Research. See also the 1996 GSA Today discussion of oxygen supplementation and carbon budgets.
- Severinghaus and colleagues (1994): Oxygen loss in Biosphere 2, Eos. Read the indexed abstract.