Biosphere ii is a self-contained ecological system that was sealed with researchers inside. unfortunately, the microbes in the soil consumed rich organic matter and produced carbon dioxide. oxygen levels plummeted and, subsequently, the mission failed. this depletion of oxygen was caused by small microorganisms carrying out the process of cellular .

The correct answer is "respiration".

Explanation:

Cellular respiration is the set of metabolic processes at which the cell obtains energy from nutrients and generates waste products. The cell needs an oxidizing agent in order to obtain energy from nutrients, the most widely used is oxygen. Cellular respiration is a vital process for most organisms, therefore, the depletion of oxygen in the self-contained ecological system most likely occurred from small microorganisms that performed this biological process.