This icon represents all the humans in Biosphere 2 and simply requires you to input a number. The default value is 8 representing the original total of Biospherians in the 1992-1993 experiment. Oxygen was produced by photosynthesis of the plants. At the end of the second winter oxygen had decreased so much it endangered the life support of the people and more was added from a tank truck. It was the use of oxygen in the consumption of dead organic biomass in the soil that reduced its level, not its use by people.
Plants use carbon dioxide in photosynthesis to produce biomass, including food.
Microbes, animals and people produce carbon dioxide from their respiration. If the level of carbon dioxide goes down, plant production of food goes down. Carbonates are chemical compounds found in Biosphere 2 in rocks, concrete and ocean water. Carbonates can absorb or release carbon dioxide depending on its concentration. They act as a buffer to keep the carbon dioxide at moderate levels.