There is very little nitrogen in mineral form, but the atmosphere contains up to 78 million kg of gaseous nitrogen, N2 over every hectare of land. It is very stable and represents 79 % of the atmosphere. Even though it is essential for protein synthesis, nitrogen in its gaseous form cannot be assimilated by most living beings.
Bacteria Azotobacter, Clostridium and blue algae transforming N2 into ammonia (NH3) usable by plants and other bacteria (arrow pointing from NH3 to a plant). Thanks to the food chain, nitrogen then becomes a constituent of animals. Excrement, carcasses, and dead plants are waste products that contain organic nitrogen. These wastes are transformed into ammonia (NH3) by bacteria that participate in the decomposition process. Nitrification is the process by which ammonia is transformed by nitrifying bacteria (e.g. Nitrosomonas into nitrite (NO2-) and then into nitrate (NO3-) by Nitrobacters. These nitrates can then be transformed into N2, which is returned into the atmosphere through a denitrification process carried out by many bacterial species such as Paracoccus denitrificans. In this way, nitrogen circulates in the biosphere and gets transformed from one organism to the next. Nitrates (NO3-) also fertilize plants, which absorb them through their roots and use them to synthesize amino acids and then proteins.
The atmosphere over one hectare of land contains fifty tons of carbon dioxide (CO2), and living organisms return fifty tons to the atmosphere each year. Carbon dioxide can be transformed into organic compounds by green plants, algae, cyanobacteria, purple and green phototrophic bacteria, and chemo-autotrophic bacteria.
These organisms use carbohydrates, which they produce by fixation of CO2, to build complex organic compounds such as cellulose. Bacteria and fungi use two enzymes to degrade cellulose, which is present in dead plants, into glucose. This glucose can then be used by many types of microorganisms. Complete oxidation of glucose produces H2O and CO2. Carbon dioxide is not produced solely by carbohydrate decomposition. It is also generated by decomposition of amino acids from proteolysis and decomposition of fatty acids from the decomposition of lipids.
In its elementary form, sulphur cannot be used by either plants or animals. Yet some bacteria can oxidize sulphur into sulphate which is easily used by all forms of life. Plants, for example, get the sulphur they need from sulphates in order to synthesize some amino acids (cystine, cysteine, methionine) that are in turn essential for protein synthesis.
Bacteria Thiobacillus thiooxidans oxidize sulphur into sulphate (SO42-). This is an aerobic chemo-autotrophic process during which acid is produced, resulting in a lowering of the pH of alkaline soils. Dead plants and soil microorganisms degrade sulphur-containing proteins into amino acids. These are degraded by an enzyme called desulfurase found in anaerobic bacteria of the genus Desulfotomaculum. Sulphur is released as hydrogen sulphide (H2S). Some species of green and purple phototrophic bacteria can oxidize hydrogen sulphide produced by sulphate reduction and amino acid decomposition. This oxidation process produces elemental sulphur (S).