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Oxygen
Main source of oxygen are the plants which evolve oxygen as a by-product of photosynthesis. It is used chiefly by most of the living organisms in their aerobic respiration except certain anaerobics. Oxygen acts as a limiting factor in the distribution of animals in aquatic media. E.g., Black sea lacks the dissolved oxygen so is devoid of animals, and at higher altitudes of mountains, the oxygen content becomes less and so there is a marked decrease in living organisms (plants and animals).
Carbon dioxide
0.3% is used by the producers in the process of photosynthesis as a raw material. Nitrogen is a chemically inert gas (78%) but can be used to form nitrites and nitrates under electric discharges (non-biological) or by nitrogen fixing bacteria, and Nostoc (biological N2 fixation).
