Carbon, which exists in the earth's atmosphere generally as carbon dioxide gas (CO2) needs a continuous natural cycle in order to keep their existence. Animals obtain carbon from the plant they eat and breathe out carbon dioxide as the waste product of their metabolism process. The similar thing happens to human being and aquatic animals. In other way, carbon from dissolved litter released into the air by the soil respiration process. Industrial activity can also contribute in expanding carbon dioxide amount in the air. On reverse side, assimilation and photosynthesis process takes carbon dioxide back from the atmosphere. As we know, algae and other plants need carbon dioxide for photosynthesis so that they can produce carbohydrates as the main result and oxygen (O2) as the side result, which is vital to the environment. However, some carbon removed from the circulation as the coal, fossil fuels, petroleum, and limestone. In this case, respiration and photosynthesis hold an important role to keep carbon dioxide's amount stable.
This cycle will works nonstop and continuously.