(a) Describe the carbon cycle. (b) State the functions of: (i) nitrogen fixing bacteria (ii) nitrifying bacteria (iii) denitrifying bacteria; in nature.

(a) The carbon cycle

The carbon cycle is the continuous circulation of carbon between the atmosphere, living organisms and the non-living environment.

• Carbon exists in the air as carbon dioxide. Green plants take in this carbon dioxide and, by photosynthesis, fix it into organic food (carbohydrates), so carbon becomes part of the plant body.
• Animals feed on the plants (and on other animals), so the carbon passes along food chains into animal bodies (feeding/nutrition).
• Plants and animals return carbon dioxide to the air through respiration.
• When plants and animals die, decomposers (bacteria and fungi) break down their bodies, releasing carbon dioxide back into the air during decay.
• Combustion (burning) of wood and of fossil fuels (coal, petroleum, natural gas), which were formed from the buried remains of organisms, also returns carbon dioxide to the atmosphere.

In this way the amount of carbon in circulation is kept roughly in balance.

(b) Functions of bacteria in nature

• (i) Nitrogen-fixing bacteria: convert free atmospheric nitrogen into nitrogen compounds (ammonia/nitrates) that plants can absorb and use; for example Rhizobium in the root nodules of legumes and free-living Azotobacter.
• (ii) Nitrifying bacteria: convert ammonium compounds to nitrites (Nitrosomonas) and nitrites to nitrates (Nitrobacter), making nitrate available to plants (nitrification).
• (iii) Denitrifying bacteria: break down nitrates in the soil into free gaseous nitrogen, which is released back into the atmosphere (denitrification).