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Biological Processes Involving N

N compounds are essential to all forms of life. Most important biomolecules contain N in a form similar to ammonia (-3 oxidation state). Almost all such N is called ammoniacal N - one of the hydrogen atoms combined with the N atom of ammonia being replaced by a carbon atom, e.g., C-NH2. N is a vital component of proteins, peptides, enzymes, energy-transfer molecules (ATP, ADP), and genetic material (RNA and DNA) - materials that are vital to all organisms.

Whereas the amount of N needed by animals, microorganisms, and plants varies considerably (Table 3), the amounts of N required are always great enough to make N fall into the category of being an essential macronutrient (needed in large amounts relative to other important essential nutrients such as: calcium (Ca), phosphorus (P), potassium (K), sulfur (S), and magnesium, (Mg)). In all cases, the nutritional requirements for N are exceeded only by those of carbon (C), hydrogen (H), and oxygen (O).

Table 3. Typical C:N Ratios of Some Organic Materials.

Material C:N
Marine phytoplankton
Microbial biomass
Soil humus
Legume residues
Cereal residues and straw
Forest wastes

Certain bacteria also use N compounds in respiration (energy production). In all organisms, respiration is an oxidation-reduction (redox) reaction involving an oxidant (electron acceptor) and a reductant (electron donor). Aerobes (including humans) use O as the electron acceptor. The energy available from various reactions that are mediated by organisms is given in the first seven rows of Table 4, where "CH2O" (the generic formula for carbohydrate) indicates organic matter. (The reactions shown in Table 4 are the net result of complex multi-step biochemical processes.)

Table 4. Reduction and oxidation reactions used in respiration

Free Energy
1/4O2(g)+ 1/4CH2O ®
1/4CO2(g)+ 1/4H2O
Aerobic respiration -119
1/5NO3- + 1/4CH2O + 1/5H+ ®
1/10N2 + 1/4CO2(g) + 7/20H2O
Denitrification -113
MnO2(s) + 1/4H2O + H+ ®
Mn2+ + 1/4 CO2(g) + 3/4H2O
Manganese reduction -97
1/8NO3- + 1/4H+ + 1/4 CH2O ® 1/8NH4+ + 1/4CO2(g) + 1/8H2O Nitrate reduction -76
Fe(OH)3(s) + 1/4CH2O + 2H+ ®
Fe2+ + 1/4CO2(g) + 11/4H2O
Iron reduction -47
1/8SO42- + 1/4CH2O + 1/8H+ ®
1/8HS- + 1/4CO2(g) + 1/4H2O
Sulfate reduction -21
1/4CH2O ® 1/8CO2(g)+ 1/8CH4(g) Methane fermentation -18
1/4CH2O + 1/4H2O ® 1/4CO2(g) + 1/2H2(g) Hydrogen fermentation -1
1/6NH4+ + 1/4O2(g) ® 1/6NO2- + 1/3H+ + 1/6H2O Nitrification -45
1/2NO2- + 1/4O2(g) ® 1/2NO3- Nitrification -38
Notes: Source: Morel and Hering 1993
* 25ºC, pH 7
1 M dissolved Mn or Fe

The electron donor that yields the most energy usually determines the predominant type of respiration in a particular environment. Therefore, when oxygen is present, aerobic respiration is the predominant form of respiration. Denitrification is the second most energetic reaction in Table 4. Therefore, when oxygen becomes depleted, then NO3- becomes the preferred electron acceptor, followed by manganese and iron oxides, and finally sulfate (SO42-). This sequence of redox reactions is observed in environments that are not in contact with the atmosphere, including sediments, flooded soils, and aquifer systems. The last two rows of Table 4 show N species as electron donors.