Nitrogen is a key element present in many biochemical compounds (such as nucleotide phosphates, amino acids, proteins, and nucleic acids) of living cells. Only oxygen, carbon, and hydrogen are more abundant in the cell. The entry of organic nitrogen in the food chains of natural ecosystems is essentially due to the activity of photoautotrophic organisms (cyanobacteria, algae, and terrestrial plants). These primary producers take up nitrogen from the environment mainly as nitrate, reduce it to ammonia, and then assimilate ammonia into organic compounds to form amino acids. However, this process of assimilatory reduction of nitrate is not the only change that nitrogen undergoes.

In the biosphere nitrogen passes through many forms, ranging from the most reduced, NH3 (or NH^), to the most oxidized, NO-, in a biogeochemical cycle whose steps depend on both physical and biological events. The processes that involve living organisms (Figure 1) include the following:

• ammonification, carried out by saprophytic bacteria (e.g., Clostridium) and fungi that detach NH3 from organic nitrogenous compounds and release this reduced form of inorganic nitrogen into the environment;

• nitrification, carried out by chemosynthetic bacteria that draw energy from oxidation of ammonia to nitrite (e.g., Nitrosomonas) and of nitrite to nitrate (e.g., Nitrobacter); and

• assimilatory reduction of nitrate that leads to the recovery of organic nitrogen, thus closing the cycle.

Nonetheless, a biological pahtway of irreversible nitrogen loss also exists, which is carried out by facultative aerobic bacteria. These microorganisms (e.g., Alcaligenes) bring about a process of dissimilatory reduction of nitrate, called denitrification. Under anoxic conditions they activate anaerobic respiration by using nitrate instead of oxygen as the final electron acceptor of the respiratory chain. This leads to volatile nitrogen forms (N2O or N2), which are lost to the atmosphere. In natural ecosystems, the recovery of nitrogen, necessary to satisfy the nutritional demands of the inhabiting organisms, occurs through biological nitrogen fixation (Figure 1). This event is of capital importance and consists in the reduction of molecular nitrogen (N2) to ammonia (NH3), providing the Earth's ecosystems with

Loss Recovery

Recovery about 200 milion tons N per year. It has been estimated that the 80-90% of the nitrogen available to plants in natural ecosystems originates from biological nitrogen fixation.

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