Figure 21.3 The carbon cycle. In the carbon cycle, producers take up CO₂ from the atmosphere and the water. They transfer assimilated carbon to consumers, detritivores, scavengers, and decomposers. These organisms return CO₂ to the atmosphere and oceans through respiration. Throughout an ecosystem, CO₂ is exchanged between the atmosphere and the ocean and between the ocean and sediments. Carbon that has been stored underground for long periods turns into fossil fuels, which can be extracted. CO₂ is returned to the atmosphere through combustion of fossil fuels, burning in terrestrial ecosystems, and volcanic activity.