The life-support system began unraveling. When some oxygen disappeared mysteriously,more had to be pumped in from the outside to keep the Biosphere from suffocating.
The nitrogen and carbon cycling system also failed to function properly. Levels of nitrous oxide rose high enough to threaten the occupants with brain damage and had to be controlled by outside intervention. Carbon dioxide skyrocketed to levels that threatened to poison the humans and spurred the growth of weedy vines that choked out food crop. Nutrients leached from the soil and polluted the water system.