• Basic constituent of all organic compounds is carbon (C). The source of organic carbon compounds in nature is carbon dioxide present in the atmosphere & is in dissolved condition in water on the earth.
• Green plants, certain bacteria & algae use CO­₂ through photosynthesis in the presence of sunlight to produce carbohydrates, simple fats, utilized & digested by animals.
• Flesh eating animals feed on herbivores & the carbon compounds are again digested & converted into other forms.
• Carbon is released in the form of carbon dioxide in respiration of plants & animals.
• Bacteria & fungi attack dead remains of plants & animals and degrade the complex organic compounds into simple substances which are the available for other cycle.
• In animals, carbon may get tied up in hard parts such as shells & hence remain in the form of inorganic carbonates for long times.
• Marine deposits of animal carbonates & inorganic precipitation of carbonate in water may result into limestone. Through erosion & dissolutions, limestone can return carbonate very slow to the living carbon cycle.
• Carbon may also get fixed into organic deposits of coal & petroleum, where it may remain for millions of years unless released by combustion. In this way, part of organic carbon becomes incorporated into the earth’s crust as coal, gas, petroleum, limestone & coral reef.

Microbes like- Trichoderma, Aspergillus, Penicillium, Marasmus, Ganoderma, Psalloitta, Coryne-bacterium, Methanosomonas & anaerobic purple sulphur bacteria helps in the decomposition of plant residues into carbon-dioxide, water, methane & sulphur-dioxide. While the animal residues are also decomposed into CO₂, H₂O, CH₄& SO₂ by above mentioned microbes except Marasmus, Ganoderma & Psalloitta microbes.