Biosphere 2’s oxygen mystery: microbes and concrete in a glass enclosure
Eight people lived inside Biosphere 2, a glass-enclosed, three-acre Earth model in Arizona, for two years starting in 1991. Over time, the atmospheric oxygen dropped from about 21% to around 14% not due to leakages but because soil microbes consumed oxygen and produced CO2 faster than the plants could recycle it; the missing CO2 was being absorbed by the concrete as it cured, locking carbon in the walls. This explained the apparent disappearance of oxygen and highlighted how every surface—like concrete—can be an active part of a closed ecosystem. The mission faced controversy and did not fully achieve a self-sustaining closed system, but it yielded genuine insights for future life-support studies and is now used for controlled earth-science experiments by the University of Arizona.
