Heterocysts are differentiated cells of filamentous cyanobacteria performing nitrogen fixation . With a lack of nitrogen compounds in the medium, they appear regularly along the trichomes of vegetative cells and akinet . Cyanobacteria are phototrophs that carry out oxygenic photosynthesis , but oxygen , atmospheric and released during photosynthesis, inhibits the enzyme nitrogenase , which is necessary for nitrogen fixation, therefore, in the filamentous cyanobacteria, specialized cells for nitrogen fixation appeared in the evolutionary process.
Structural Features
Heterocysts are usually unable to divide and grow. Photosystem II is destroyed in them, respectively, photosynthesis is not taking place and internal oxygen is not formed. The heterocysts are protected from external molecular oxygen by 2 thick additional shells. The inner one consists of hydroxylated glycolipids, the outer one consists of polysaccharides . For heterocysts, a high oxidase activity was noted, neutralizing the passed oxygen.
During differentiation, the nitrogenase complex (NG) is activated and the assimilation of molecular nitrogen begins. Heterocysts are associated with neighboring cells in trichome using plasmodesmah, through which the transport of bound nitrogen from the heterocysts, and organic compounds into it.
It has been shown that in some species of Anabaena, heterocysts secrete specific peptides and carbohydrates that attract heterotrophic bacteria. Heterotrophic bacteria, possessing high respiratory activity, "destroy" all the oxygen around the heterocyst, creating anaerobic conditions. This increases the productivity of nitrogenase.
The exchange of matter between heterocyst and vegetative cells
In a heterocyst, molecular nitrogen is converted to ammonium via nitrogenase, then, with the participation of glutamine synthase (HS), ammonium and glutamate are converted to glutamine . Since glutamate synthase ( GOGAT ) is mainly present in vegetative cells, the resulting glutamine is transferred there from heterocyst, where it is converted into glutamate by GOGAT. Glutamate enters from the vegetative cell into the heterocyst and the cycle closes.
From glutamate and glutamine, the remaining amino acids are formed in vegetative cells. Associated nitrogen is stored in cyanogenic granules. Cyanofitin is a copolymer of arginine and aspartate .
During photosynthesis, absorbed atmospheric carbon in vegetative cells passes into an organic form - glucose, which can be metabolized to pyruvate , from the latter - isocitrates in the tricarboxylic acid cycle . The carbohydrate (glucose / sucrose), entering the heterocyst, is oxidized in the pentose phosphate cycle to carbon dioxide. This forms the reduction potential and hydrogen ions necessary for the synthesis of ammonium from molecular nitrogen. In the heterocysts detected invertase, decomposing sucrose into glucose and fructose . Isocitrate with isocitrate dehydrogenase (IDH) is converted to α-ketoglutarate.
Regulation of heterocyst formation
Differentiation of the trichome cell towards the formation of a heterocyst is controlled by the NtcA DNA binding protein, and the hetR gene expression is increased. Its action is modulated by the products of the hetF and patA genes. This process can be inhibited by the presence of ammonium in the medium. The hetR gene product may cause differentiation of neighboring vegetative trichome cells.