Nitrogen Cycle

Nitrogen Cycle

The element Nitrogen (N) is a key constituent in microbial cell. Nitrogen is needed for the formation of proteins, amino acids, nucleotides and is present in a number of oxidation states inside the cell. Nitrogen is cycled between atmosphere, organic compounds in living things, soil and sediments.

The processes that are involved in Nitrogen cycle are

1.     Nitrogen fixation

2.     Nitrification

3.     Ammonification

4.     Denitrification

Nitrogen fixation

Nitrogen is present as N₂ (N  N) in air (78% N₂). The triple bonded state of nitrogen makes it very stable and nitrogen in its gaseous state cannot be assimilated by plants or animals for their metabolism. Only few groups of prokaryotes are capable of breaking the triple bond and use it for building up their proteins and aminoacids. The process of reduction of gaseous nitrogen (N₂) to ammonia (NH₄) is called Nitrogen fixation. This process is carried out by a group of prokaryotes called diazotrophs.

Cyanobacteria, Rhizobium and Frankia are some of the examples of diazotrophs that can fix atmospheric nitrogen. The fixed ammonia gets incorporated into proteins and amino acids, thus building up organic nitrogen.

Ammonification

The production of ammonia during the decomposition of organic nitrogen compounds, by micro organisms after the death of plants and animals is called ammonification (Figure 11.2). Much of the ammonia released by aerobic decomposition in soil is taken up rapidly by plants and micro organisms and is converted to amino acids.

Bacteria like Bacillus, Clostridium, Pseudomonas and fungi like Aspergillus, Mucor and Penicillium are few examples of micro organisms that can ammonify.

Nitrification

The oxidation of ammonia (NH₃) to nitrate (NO₃) is called nitrification. It is carried out by nitrifying bacteria. It is a two step process where ammonia is first converted to nitrite (NO₂) and then to nitrate (NO₃).

2NH₃ + 3O₂ → 2HNO₂ + 2H₂O

The above given oxidation reaction is the first step that produces nitrite. This reaction provides energy and is carried out by Nitrosomonas and Nitrosococcus.

In the second step, the nitrite is oxidized to nitrate

2HNO₂ + O₂ → HNO₃

This reaction is carried out by Nitrobacter.

Nitrates are readily assimilated by plants but are very water soluble and rapidly leached from soil.

Denitrification

The reduction of NO₃^- from soils by denitrifying bacteria to gaseous nitrogen is called denitrification. In this process, carried out by bacteria like Pseudomonas,

Thiobacillus denitrificans, organic compounds serve as hydrogen donors and nitrates serve as electron acceptor.

NO₃ → NO₂ → N₂O → N₂

Biological Nitrogen Fixation

One of the most significant biological process taking places on the Earth is biological nitrogen fixation (BNF). This fixation of atmospheric nitrogen carried out by few prokaryotes is cost efficient because industrial production of ammonia by Haber’s Bosch process is very expensive.

Organisms carry out BNF in a free living state in soil or they can establish symbiotic association with other plants or micro organisms and fix N₂.

Organisms capable of BNF are

BNF by Rhizobium in leguminous plants

Leguminous plants belong to the family Leguminaceae and bear seeds in pods. Example: Black gram, Green peas, Soyabean, Subabul. The bacteria belonging to the genus Rhizobium which can exist in free living state in soil but can enter into symbiosis with legume plants and carry out nitrogen fixation.

Process of BNF

It consists of the following steps

1. Infection of legume roots by Rhizobium

2. Formation of root nodules

3. Reduction of N  N to NH₄ in root nodules

1. Infection of legume roots by Rhizobium

Rhizobium recognises and attaches to the root hairs of legume plant. It invades the root hairs and secretion of certain nod factors result in root hair curling typically called Shepherd’s crook symptom which leads to the formation of infection thread. Infection thread is a cellulosic tube like structure through which Rhizobium moves into the cortex from root hairs.

2. Formation of root nodules

The invaded plant cells are stimulated to divide repeatedly thus forming a mass of tissue on the roots which are called root nodules (Figure 11.3). Root nodules are fleshy light pink colored globose structures seen on the roots. The bacteria inside the root nodules transform into swollen, mishappened forms. These are called bacteroids. The bacteroids are capable of nitrogen fixation (Figure 11.4)

3. Reduction of N ≡ N to ammonia in root nodules

This biochemical process is catalysed by an enzyme called Nitrogenase present in bacteroids and happens under diminished O₂ levels. The O₂ levels in the nodules are controlled by an oxygen binding protein called leghemoglobin. This is a red, iron containing protein which can keep the nodule environment free of oxygen.

Nitrogenase

The enzyme nitrogenase is a complex enzyme consisting of 2 enzymes, dinitrogenase reductase and dinitrogenase. Electrons from organic compounds like pyruvate are passed on to dinitrogenase reductase first and then to dinitrogenase which in turn passes them to N  N thus reducing it to NH₄. This reduction needs 16 ATP, ferredoxin and cytochromes.