Circulation through nature of carbon in the form of the simple element and its compounds.
The source of carbon in living things is carbon dioxide (CO 2 ) from air or dissolved in water. Algae and green plant s (producers) use CO 2 in photosynthesis to make carbohydrate s, which in turn are used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them. The carbon may pass through several levels of herbivore s and carnivore s (consumers). Animals and, at night, plants return the CO 2 to the atmosphere as a by-product of respiration . The carbon in animal wastes and in the bodies of organisms is released as CO 2 in a series of steps by decay organisms (decomposers), chiefly bacteria and fungi (see fungus ). Some organic carbon (the remains of organisms) has accumulated in Earth's crust in fossil fuel s, limestone , and coral . The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO 2 via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonate s and some staying in the atmosphere (see greenhouse effect ).