What is Nitrogen fixation? and Stages in Nodule formation?

What is Nitrogen fixation? and Stages in Nodule formation?


The Biosynthetic process of reduction of atmospheric nitrogen to ammonia is called Nitrogen fixation.

Certain prokaryotic organisms converts the Nitrogen into Ammonia and these nitrogen fixing prokaryotes are called Diazotroph. All the eukaryotic organism are unable to fix the Nitrogen into Ammonia. Nitrogen fixation is a reductive process and the reduction of N2 to NH3 takes place.

Nitrogen cycle

The biological nitrogen fixation can be represented by following Reaction-

                   N+  8H+  +  8e- + 16 ATP  =  2NH +  H2 + 16ADP  +  16 Pi


Nitrogenase complex

Nitrogenase enzyme is a enzyme complex that catalyzed the biological process of Nitrogen fixation. nitrogenase generally possess 3 different forms of Nitrogenase that vary in their requirements like

1) Vanadium

2) Iron

3) Molybdenum

as critical metallic components of the cofactor associated with the Catalytic site.

Nitrogenase generally consist two proteins -

1) Dinitrogenase reductase

2) Dinitrogenase 

1) Dinitrogenase reductase

Dinitrogenase reductase is a Dimer of identical 30 kda, Dinitrogenase provides the electron with high reducing power.

2) Dinitrogenase

Dinitrogenase use these Electrons that are provided by the Dinirogenase reductase and reduce the N2 into NH3. Dinitogenase is a tetramer with two copies of 2 different subunits and contain both Molybdenum and iron.  Dinitrogenase components is also called Molybdenum - iron protein because molybdenum is present in these cluster. Molybdenum iron protein (MOFe protein) is the site of nitrogen fixation.

Genes in Nitrogenase

1) nif genes

The genes involved collectively in the synthesis of Nitrogenase and the catalytic process of N2 fixation are called nif genes.

2) Fix genes

Accessory genes are called fix genes and are also necessary for the function and regulation of nitrogenase in microaerobic and aerobic nitrogen fixing bacteria.

Examples of free living Nitrogen fixing organisms - 

1) Archaebacteria 

Methanogens - Methanococcus volate

2) Eubacteria

Anaerobes - clostridium pasteurianum 

Facultative anaerobes - klebsiella pneumoniae, purple and                                        green bacteria

Aerobes - Azospirillum

Unicellular - Gloeothece spp.


Chemotrophic bacteria

Filamentous - oscillatoria spp., Nostoc spp

Symbiotic Nitrogen Fixation 

Source - https://aem.asm.org

Symbiotic nitrogen fixing bacteria

Symbiotic N2 fixing bacteria are those bacteria that fix nitrogen by symbiotic association with an appropriate host. the most common symbiotic association results in the formation of the multicellular structure called Nodules.

Example of Symbiotic nitrogen fixing bacteria

Rhizobium and Frankia live freely in the soil but fix the Dinitrogen only when in symbiotic association with an appropriate host plant. rhizobium species possess the specific adhension protein called Rhicadhesin on their surface, this protein is a calcium binding protein and play a important major role in bacterium - plant attachment.

Stages in Nodule formation

  1. Recognition of the plant by bacterium and attachment of bacterium to root hairs.
  2. Excretion of nod factors by bacterium nod factor promotes root hair curling.
  3. Bacterial invasion of the root hair of plant.
  4. Formation of infection thread and invasion into the cortical cells.
  5. Formation of modified bacterial cells, Bacteroids, within the plant cell.
  6. Plant cell Division and formation of the mature Root nodule.

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