The right answer is B) atmospheric nitrogen→bacteria→plants→animals.
The nitrogen cycle is as follows:
1. Fixation:
Nitrogen gas is transformed into mineral compounds by Cyanophyceae (Anaebaena, Nestoc, Phormidium) or bacteria (Azotobacter, Clostridium, Desulfovibrio). This phenomenon only affects primitive organisms.
2. Assimilation:
The assimilation of nitrogen in the cycle consists in transforming mineral compounds into organic matter (by autotrophic organisms, ie plants) or organic matter into other organic matter (by heterotrophs, it is to say the animals). This process is what is called protein synthesis. Thus, a vegetarian will turn vegetable proteins into animal proteins, which carnivores will feed on to make other proteins. Only plants (which include bacteria) are able to synthesize proteins from mineral elements, which is why plants can be grown with a nutrient solution containing only mineral salts. it would be futile to undertake with an animal.
3. Excretion:
It is the emission of non-assimilated organic matter, because it is not digestible, and of smaller molecules, resulting from the more or less complete degradation of the organic matter, such as ammonia (amphibians of larval form, bacteria, aquatic invertebrates and vertebrates), urea (adult amphibians and terrestrial vertebrates), or in its acid form uric acid (terrestrial reptiles and birds).
4. Ammonification:
The molecules resulting from metabolism and present in the medium after excretion are decomposed into ammonia (a mineral form of nitrogen) by bacteria (Achromobacter, Bacillus, Clostridium, Corynebacterium, Flavobacterium, Nitrococcus, Proteus, Pseudomonas, Sarratia).
5. Nitrification:
It is the succession of two processes: nitritation, which leads ammonium ions to oxidize to nitrites by bacteria (Nitrosomonas, Nitrosococcus, Nitrospira) and nitration, which converts nitrite to nitrate by bacteria also (Nitrobacter) .
6. Denitrification:
Reverse process of the previous one, leading to the reduction of nitrates to nitrites and eventually to dinitrogen or ammonia and carried out thanks to denitrifying bacteria (Azotobacter, Arthrobacter, Bacillus, Pseudomonas, Thiobacillus)
