Nitrogen Fixing Systems (With Diagram)| Plant Physiology

IN this article we will discuss about the Subject-Matter and Mechanism of Nitrogen Fixing Systems.

Subject-Matter of Nitrogen Fixing Systems:

Blue-green algae share with bacteria the potentiality of fixing atmospheric nitrogen. To date, nearly 50 spp. are reported to be active nitrogen fixers. Nitrogen fixation was demonstrated initially in Nostocpunctiformae and Anabaena sp. In 1968, Fay made the suggestion that heterocysts fix the nitrogen.

In general, there are three chief groups of nitrogen fixing blue-green algae: Heterocystous species which fix nitrogen aerobically; unicellular (Gloeocapsa sp.) nitrogen fixing forms and non-heterocystous filamentous forms which fix nitrogen under microaerophilic conditions (Plectonemaboryanum).

In such species, process of nitrogen fixation occurs through enzyme conversion of di-nitrogen from the atmosphere to ammonia through nitrogenase. Nitrogenase has been localized in the heterocysts.

Further facts support the suggestion that heterocysts are the site of atmospheric nitrogen fixation; most of species with heterocysts have the capacity to fix nitrogen; combined nitrogen suppresses both the formation of heterocysts and nitrogen fixation and that nitrogen fixation is a reductive process and the enzyme nitrogenase is highly sensitive to oxygen.

Stewart and his associates in 1969 provided the first evidence for the direct fixation of nitrogen by the heterocysts who demonstrated that isolated heterocysts fix nitrogen when provided ATP and reductant (sodium dithionite).

Several bacterial types can also fix nitrogen using light energy and these include green and purple sulphur bacteria as well as non-sulphur photosynthetic bacteria. Unlike blue-green algae, these bacteria do not derive electrons from water.

On the contrary, they obtain electrons from hydrogen, sulphur, sulphide and even some organic compounds. When provided with suitable reduced electron donors these organisms appear to fix nitrogen in darkness.

However, nitrogen fixation by photosynthetic bacteria is responsive to light, though in the non-photosynthetic forms, external carbohydrates are needed to generate ATP. In general, nitrogen fixation is inhibited by oxygen, and combined nitrogen.

Mechanism of Nitrogen Fixing Systems:

The real breakthrough in the elucidation of mechanism of nitrogen fixing reactions was achieved between 1950-60 when French workers isolated nitrogen fixing enzyme.

Later on, stable isotope, ¹⁵N provided additional information. NH₃ appears to be the end product. In general, the following pathway of nitrogen fixation via organic hydrazines is recognized:

Cell free extracts from the anaerobic bacterium Closteridiumpasteurianum were capable of supporting N₂ fixation in vitro. The gross structural features of nitrogenase so far appear to be rather similar whatever the source may be. It consists of two components.

One contains molybdenum (Mo) and iron (Fe) and the other contains only Fe. The iron in each is on non-heme variety. Addition of ammonia and L-amino acids represses the biosynthesis of nitrogenase in Klebsiella. L- methionine-DL-sulfoximine an inhibitor of glutamine synthetase, which depresses nitrogenase in Rhodospirillum even in the presence of NH₄⁺, leads to light dependent excretion of ammonia into the culture-medium.

This reaction represents a utilization of solar energy for the production of ammonia from N₂. The activity of nitrogenase from Closteridium is regulated by the ratio of Mg ADP: Mg ATP. NH₄⁺, glutamine, asparagine and urea produce an immediate reversible inactivation of the light dependent reduction of C₂H₂ in Rhodopseudomonas.

The genes for nitrogen fixation (nif genes) can be transferred into other bacteria by means of conjugation or transduction. Plasmids carrying nif genes from Klebsiellapneumoniae were introduced into Salmonella typhimurium.