Oxygen concentrations have dropped sharply in the air of “Biosphere 2,” an enclosed experimental ecosystem located in southern Arizona. Biosphere 2 is a 3.15-acre airtight structure roofed in glass and underlain by an impermeable liner. It houses an artificial ecosystem containing soil, air, water, flora, and fauna and was built primarily as an apparatus for the experimental investigation of biogeochemical cycles, whole ecosystems, and life-support systems for space habitation [see Nelson et al., 1993].
O2 in Biosphere 2 decreased during the first 16 months of closure from the ambient 21% to 14%, enough to cause health problems in the human occupants. We present evidence that the O2 loss is caused by microbial respiration of the excessive amount of organic matter incorporated into the experiment's soils and furthermore, that the respired CO2 is reacting with the structure's concrete to form calcium carbonate.