The Nitrogen Cycle

The nitrogen cycle is the most complex of the cycles of elements that make up biological systems. This is due to the importance and prevalence of N in cellular metabolism, the diversity of types of nitrogen metabolism, and the existence of the element in so many forms. Procaryotes are essentially involved in the biological nitrogen cycle in three unique processes.

Nitrogen Fixation: this process converts N₂ in the atmosphere into NH₃ (ammonia), which is assimilated into amino acids and proteins. Nitrogen fixation occurs in many free-living bacteria such as clostridia, azotobacters and cyanobacteria, and in symbiotic bacteria such as Rhizobium and Frankia, which associate with plant roots to form characteristic nodules. Biological nitrogen fixation is the most important way that N₂ from the air enters into biological systems.

N₂ ----------------> 2 NH₃  nitrogen fixation

Anaerobic Respiration: this relates to the use of oxidized forms of nitrogen (NO₃ and NO₂) as final electron acceptors for respiration. Anaerobic respirers such as Bacillus and Pseudomonads are common soil inhabitants that will use nitrate (NO₃) as an electron acceptor. NO₃ is reduced to NO₂ (nitrite) and then to a gaseous form of nitrogen such as N₂ or N₂O (nitrous oxide). The process is called denitrification. (A related process conducted by some Bacillus species, called dissimilatory nitrate reduction reduces NO₃ to ammonia (NH₃), but this is not considered denitrification.) Denitrifying bacteria are typically facultative microbes that respire whenever oxygen is available by aerobic respiration. If O₂ is unavailable for respiration, they will turn to the alternative anaerobic respiration which uses NO₃. Since NO₃ is a common and expensive form of fertilizer in soils, denitrification may not be so good for agriculture, and one rationale for tilling the soil is to keep it aerobic, thereby preserving nitrate fertilizer in the soil.

NO₃ ----------------> NO₂ ----------------> N₂ denitrification

The overall reactions of denitrification shown above proceed through the formation of nitrous oxide (N₂O). A recent article by Wunsch an Zumft in Journal of Bacteriology, vol. 187 (2005), sheds new light on the process of denitrification. N₂O is a bacterial metabolite in the REVERSAL of Nitrogen fixation. The anthropogenic atmospheric increase of N₂O is a cause for concern, as noted above (as a greenhouse gas, N₂O  has 300 times the heat absorbing capacity as CO₂). Denitrifying bacteria respire using N₂O  as an electron acceptor yielding N₂  and the thereby provide a sink for N₂O. This article provides new insight into this process by identifying a membrane-bound protein in denitrifying bacteria called NosR, that is necessary for the expression of N₂O reductase from the nosZ gene. The NosR protein has redox centers positioned on opposite sides of the cytoplasmic membrane, which allows it to sustain whole-cell N₂O respiration by acting on N₂O reductase.

Nitrification is a form of lithotrophic metabolism that is chemically the opposite of denitrification. Nitrifying bacteria such as Nitrosomonas utilize NH₃ as an energy source, oxidizing it to NO₂, while Nitrobacter will oxidize NO₂ to NO₃. Nitrifying bacteria generally occur in aquatic environments and their significance in soil fertility and the global nitrogen cycle is not well understood.

The Overall process of Nitrification

NH₃ ----------------> NO₂ (Nitrosomonas)

NO₂ ----------------> NO₃ (Nitrobacter)

A final important aspect of the nitrogen cycle that involves procaryotes, though not exclusively, is decomposition of nitrogen-containing compounds. Most organic nitrogen (in protein, for example) yields ammonia (NH₃) during the process of deamination. Fungi are involved in decomposition, as well.

Plants, animals and protista, as well as the procaryotes, complete the nitrogen cycle during the uptake of the element for their own nutrition. Nitrogen assimilation is usually in the form of nitrate, an amino group, or ammonia.
 

Figure 2. The Nitrogen Cycle