Biosphere II is a huge dome-covered self-contained structure located outside Tucson, Arizona. Biosphere II was built to try to duplicate Earth's biosphere and ecosystems in miniature and to use the information gained from the experiment to help design similar habitats on the Moon and Mars. The habitat contained small versions of many of Earth's ecosystems, including an ocean with a coral reef, mangrove wetlands, a grassland, a desert, an agricultural system, and a human habitat. Eight men and women were sealed inside the habitat for a proposed two-year stay. The air, water, and food for the "biospherians" all came from inside the sealed habitat, and their only contact with the outside was through computers and telephones.
Before the two years were up, however, the experiment failed. Supplemental oxygen was pumped into the facility and food supplies were provided through the airlocks. What caused the failure of the experiment? It was traced to nutrient pollution. The soil in some of the biosphere ecosystems was unusually rich in nutrients. These excess nutrients caused a huge population explosion in decomposer bacteria, which led to oxygen depletion.
How could a population explosion of bacteria lead to oxygen depletion?