Nitrogen cycle - Nitrogen fixation

Nitrogen is essential to life because it is a key component of many biomolecules, including proteins, DNA, and chlorophyll. Although nitrogen is abundant (78%) in the atmosphere as diatomic nitrogen gas (N₂), it is largely inaccessible in this form to most organisms, making nitrogen a scarce resource and often limiting primary productivity in many ecosystems. Only when nitrogen is converted from dinitrogen gas into ammonia (NH₃) does it become available to primary producers, such as plants. 

In addition to N₂ and NH₃, nitrogen exists in many different forms, including both inorganic (e.g., ammonia, nitrate) and organic (e.g., amino and nucleic acids) forms. Thus, nitrogen undergoes many different transformations in the ecosystem, changing from one form to another as organisms use it for growth and, in some cases, energy.

The cycling of nitrogen among its many forms is a complex process that involves numerous types of bacteria and environmental conditions. The major transformations of nitrogen are nitrogen fixation, nitrification, denitrification and ammonification. The transformation of nitrogen into its many oxidation states is key to productivity in the biosphere and is highly dependent on the activities of a diverse assemblage of microorganisms, such as bacteria, archaea, and fungi.

The biologically available forms NO^3- and NH₃ are limited; however, human activities, such as making fertilizers and burning fossil fuels, have significantly increased the availability of nitrogen to living organisms. It is predicted that by 2030, the amount of nitrogen fixed by human activities will exceed that fixed by microbial processes. Increases in available nitrogen can alter ecosystems by increasing primary productivity and impacting carbon storage. 

Because of the importance of nitrogen in all ecosystems and the significant impact from human activities, nitrogen and its transformations have received a great deal of attention from ecologists.

In general, the nitrogen cycle has five steps:

1.    Nitrogen fixation (N₂ to NH₃/ NH⁴⁺ or NO^3-)

2.    Nitrification (NH₃ to NO^3-)

3.    Assimilation (Incorporation of NH₃ and NO^3- into biological tissues)

4.    Ammonification (organic nitrogen compounds to NH₃)

5.    Denitrification (NO^3- to N₂) 

 

 

Nitrogen Fixation

Nitrogen gas (N₂) makes up nearly 80% of the Earth's atmosphere, yet nitrogen is often the nutrient that limits primary production in many ecosystems because plants and animals are not able to use nitrogen gas in that form. The process of converting N₂ into biologically available nitrogen is called nitrogen fixation.

N₂ gas is a very stable compound due to the strength of the triple bond between the nitrogen atoms, and it requires a large amount of energy to break this bond. Only some bacteria carry out this energetically demanding process. Although most nitrogen fixation is carried out by prokaryotes, some nitrogen can be fixed abiotically by lightning or certain industrial processes, including the combustion of fossil fuels.

Nitrogen fixation is the process by which gaseous nitrogen is converted to ammonia/ammonium ions via biological fixation or nitrate through combustion, volcanic action, lightning discharges, and industrial means.

However, a greater amount of biologically available nitrogen is naturally generated via the biological conversion of N₂ to NH₃/ NH⁴⁺. A small group of bacteria and cyanobacteria are capable using the enzyme nitrogenase to break the bonds among the molecular nitrogen and combine it with hydrogen.

Some nitrogen-fixing organisms are free-living while others are symbiotic nitrogen-fixers, which require a close association with a host to carry out the process. Some of these bacteria are aerobic, others are anaerobic; some are phototrophic, others are chemotrophic (i.e., they use chemicals as their energy source instead of light). Although there is great physiological and phylogenetic diversity among the organisms that carry out nitrogen fixation, they all have a similar enzyme complex called nitrogenase that catalyzes the reduction of N₂ to NH₃ (ammonia).

One of the characteristics of nitrogenase is that the enzyme complex is very sensitive to oxygen and is deactivated in its presence. For aerobic nitrogen-fixers that are also photosynthetic this can be a problem since they actually produce oxygen.

Nitrogen-fixers have evolved different ways to protect their nitrogenase from oxygen. For example, some cyanobacteria have structures called heterocysts that provide a low-oxygen environment for the enzyme and serves as the site where all the nitrogen fixation occurs in these organisms. Other photosynthetic nitrogen-fixers fix nitrogen only at night when their photosystems are dormant and are not producing oxygen. The most important soil dwelling bacteria, Rhizobium, live in oxygen-free zones in nodules on the roots of legumes. Thus the enzyme is protected from oxygen by different methods.  

Nitrogen-fixing organisms are globally distributed and have been found in many aerobic habitats (e.g., oceans, lakes, soils) and also in habitats that may be anaerobic or microaerophilic.