NITROGEN CYCLE

Nitrogen Cycle

Nitrogen is one of the primary nutrients critical for the survival of all living organisms. It is a necessary component of many biomolecules, including proteins, DNA, and chlorophyll.  Although nitrogen is very abundant in the atmosphere as dinitrogen 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. 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 major transformations of nitrogen are nitrogen fixation, nitrification, denitrification, anammox, 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 nitrogen cycle represents one of the most important nutrient cycles found in terrestrial ecosystems. The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms.

FIGURE: NITROGEN CYCLE

This transformation can be carried out through

1. Biological processes and
2. Physical processes

Process of Nitrogen Cycle:

The basic steps of Nitrogen Cycle include:

1. Nitrogen Fixation
2. Nitrification
3. Denitrification and Anammox
4. Ammonification
5. Immobilization/ Assimilation

1. Nitrogen Fixation: For nitrogen to be available to make proteins, DNA, and other biologically important compounds, it must first be converted into a different chemical form. The process of converting N₂ into biologically available nitrogen is called nitrogen fixation.

N₂ + 8H⁺ +8 e^-            →        2NH₃ +H₂

In nitrogen fixation, Nitrogenase Enzyme, Bacterial activity and Microorganisms are involved.

Ways to fix atmospheric nitrogen are:

1. Artificial/ Industrial Nitrogen Fixation
2. Natural Nitrogen Fixation

1. Biological fixation,
2. Combustion and
3. Lightening

2. Nitrification: Nitrification is the process that converts ammonia to nitrite and then to nitrate and is another important step in the global nitrogen cycle. Most nitrification occurs aerobically and is carried out exclusively by prokaryotes.

There are two distinct steps of nitrification that are carried out by distinct types of microorganisms.

1. The first step is the oxidation of ammonia to nitrite, which is carried out by microbes known as ammonia-oxidizers.

NH₃ + O₂ + 2e^-            →        NH₂OH + H₂O

NH₂OH + H₂O           →        NO^-₂ + 5H⁺ + 4e^-

Nitrosomonas, Nitrosospira, and Nitrosococcus carried out this process.

2. The second step in nitrification is the oxidation of nitrite (NO2-) to nitrate (NO3-). This step is carried out by a completely separate group of prokaryotes, known as nitrite-oxidizing Bacteria. Some of the genera involved in nitrite oxidation include Nitrospira, Nitrobacter, Nitrococcus, and Nitrospina.

NO^-₂ + ½ O₂    →        NO^-₃

Ammonia-oxidizers and nitrite-oxidizers are ubiquitous in aerobic environments. They have been extensively studied in natural environments such as soils, estuaries, lakes, and open-ocean environments. However, ammonia- and nitrite-oxidizers also play a very important role in wastewater treatment facilities by removing potentially harmful levels of ammonium that could lead to the pollution of the receiving waters.

3. Denitrification and Anammox: Denitrification is the process that converts nitrate to nitrogen gas, thus removing bioavailable nitrogen and returning it to the atmosphere. Dinitrogen gas (N₂) is the ultimate end product of denitrification, but other intermediate gaseous forms of nitrogen exist.

NO^-₃    →        NO^-₂    →        NO + N₂O       →       N₂

2NO^-₃ + 10e^- + 12H⁺   →       N₂ + 6H₂O

Unlike nitrification, denitrification is an anaerobic process, occurring mostly in soils and sediments and anoxic zones in lakes and oceans. Similar to nitrogen fixation, denitrification is carried out by a diverse group of prokaryotes, and there is recent evidence that some eukaryotes are also capable of denitrification. Some denitrifying bacteria include species in the genera Bacillus, Paracoccus, and Pseudomonas. Denitrification is important in that it removes fixed nitrogen (i.e., nitrate) from the ecosystem and returns it to the atmosphere in a biologically inert form (N₂).

In the ocean, another reaction occurs to cycle nitrogen back to nitrogen gas in the atmosphere. The reaction, called the anammox reaction, is enabled by certain bacteria in the water. In the reaction, ammonium and nitrite ions combine to form water and nitrogen gas. This is shown by the equation:

NH₄⁺ + NO₂^-   →        N₂ + 2H₂O

The anammox reaction may contribute up to half of the nitrogen gas released into the atmosphere by the ocean. The reaction may also significantly limit production in ocean ecosystems by removing nitrogen compounds that are needed by aquatic producers and other organisms.

4. Ammonification: When an organism excretes waste or dies, the nitrogen in its tissues is in the form of organic nitrogen (e.g. amino acids, DNA). Various fungi and prokaryotes then decompose the tissue and release inorganic nitrogen back into the ecosystem as ammonia in the process known as ammonification. The ammonia then becomes available for uptake by plants and other microorganisms for growth.

5. Immobilization/ Assimilation: It is the reverse of mineralization. All living things require N; therefore microorganisms in the soil compete with crops for N. Immobilization refers to the process in which nitrate and ammonium are taken up by soil organisms and therefore become unavailable to crops.

Questions:

1. How do bacteria convert nitrogen gas to a form that producers can use?
2. Explain why growing a crop of legumes can improve the ability of the soil to support the growth of other plants.
3. How has human activity contributed to the nitrogen cycle?
4. What is the anammox reaction and where does it occur?