The amount of atmospheric CO2 decreases in the Northern Hemisphere in summer due to increased photosynthetic activity. Amounts increase in the winter when respiration exceeds photosynthesis. Atmospheric CO2 is increased by combustion of fossil fuels by humans, disturbing the balance. The nitrogen cycle Nitrogen is a key chemical in ecosystems as it is found in all amino acids which comprise the proteins of organisms. Only certain prokaryotes can fix nitrogen.
Cyanobacteria fix nitrogen in aquatic ecosystems.
Nitrogen fixing prokaryotes are fulfilling their own metabolic needs, but other organisms benefit since excess ammonia is released into the soil or water. Plants assimilate the NO 3 - released from these bacteria and convert it to organic forms, such as amino acids and proteins. Animals can only assimilate organic nitrogen which they obtain by eating plants and other animals. Denitrification occurs when bacteria obtain the oxygen necessary for their metabolism from NO 3 - rather than O 2 under anaerobic conditions. This process returns nitrogen to the atmosphere by converting NO 3 - to N 2 . 3 .