IMPORTANCE OF HETEROCYST IN NITROGEN FIXATION

Heterocyst

• Many of the Blue Green Algae (BGA) have the power of Nitrogen fixation, and for this nature has provided them a very special enlarged cell, which is called as Heterocyst (Hetero - different; Cyst - swollen and encapsulated cell).
• Heterocyst is the site for Cyanobacterial Nitrogen Fixation which is an enlarged cell, and may be present terminally or intercalary in the Filamentous cyanobacterial cell.
• Heterocyst is very much enlarged than its other body cells (Vegetative cells).
• Heterocysts are terminally differentiated cells that form in a semi-regular pattern in filaments starved for a source of fixed nitrogen. These cells, occurring about every 10 - 15 cells in a filament are evident by differences in structure compared to Vegetative cells. 

Figure – 1: Heterocyst and Vegetative cells in BGA 

• Heterocysts typically somewhat larger than Vegetative cells with a thicker cell envelope.
• The Heterocyst is made up of three different cell wall layers - the outer fibrous and middle homogenous layers are made up of Non-cellulose polysaccharide. Whereas, the inner laminated layer is made up of Glycolipids.
• Cyanobacterial cells, including Heterocysts have a Gram negative peptidoglycan cell wall with an outer bilayer membrane.  However, the peptidoglycan layer in cyanobacteria is thicker than is typical of Gram negative bacteria and has the cross-linking and covalently linked polysaccharides that are characteristic of the walls of Gram-positive cells.
• Heterocysts have additional thick layers external to the cell wall, not present in Vegetative cells, that limit the diffusion of gases into these cells.

Factors controlling Heterocyst formation in BGA

• The production of Heterocyst increases in the conditions of low light intensity and increase in the amount of Phosphate in the medium.
• Heterocyst formation depends upon the availability of Carbon intermediaries and ATP.
• The formation of Heterocyst is inversely related to Nitrogen concentration. Heterocyst differentiation is inhibited in the presence of combined sources of Nitrogen (Nitrate and Ammonium nitrogen), but is induced in the presence of Nitrogen gas.
• Ammonium salts are among the most powerful inhibitors of Heterocysts.
• Formation of Heterocyst is also triggered by Molybdenum.
• Heterocyst formation shows a definite requirement for light. Red light favours heterocyst formation, whereas green and blue light do not. The effects of light seem to be mainly due to photosynthesis, although some effects may be morphogenetic.
• Heterocyst differentiation is Genetically controlled but its Phenotypic expression is dependent on growth conditions in the medium.

Characteristic features of Heterocyst

• Heterocysts are found in many species of filamentous blue-green algae. They are cells of slightly larger size and with a more thickened wall than the vegetative cells.
• Structural details of the heterocyst are the presence of three additional wall layers, the absence of granules, sparse thylakoid network throughout, except at the poles where a dense coiling of membranes occurs.
• Heterocysts are Photosynthetically inactive.
• Peculiarities in the pigment composition of the heterocyst include an abundance of Carotenoids and absence of Phycobilins, and a short-wave form of Chlorophyll a.
• Studies with metabolic inhibitors have revealed the involvement of photosynthesis, respiration and protein synthesis in heterocyst formation. Photosynthesis provides carbon skeletons, whereas ATP is most probably supplied by oxidative metabolism.
• Heterocyst provides an Ananerobic environment for Nitrogen fixation. The mature Heterocysts contain no functional Photosystem II and cannot produce Oxygen. Instead, they contain only Photosystem I, which enables them to carry out Cyclic Photophosphorylation and ATP regeneration. These changes provide the appropriate conditions for the functioning of the Oxygen-sensitive Nitrogenase.
• In the process of Cyanobacterial Nitrogen Fixation, Hydrogen gas (H₂) is also evolved as a by-product and 40 % of it is recycled by the hup gene (Hydrogen uptake gene) whereas remaining 60 % Hydrogen gas can be used by biotechnologists as a source of future clean fuel.