Heterocyst in algae

cyanophyceae

1. They are usually larger than vegetative cells, develop thick tegumentary layers and intracellular hyaline buttons at the points of attachment to the vegetative cells, displaying a pale coloration and reduced  autofluorescence . Heterocysts are highly specialized in the fixation of dinitrogen under aerobic conditions. They represent a successful solution to the nontrivial problem of avoiding nitrogenase inactivation by free oxygen in oxygen-evolving organisms.

2. Glutamate metabolites Oxoglutarate maltose G6P 6PGLUC RIB5P ferrodoxin N 2 NH 4 + PS1 NADH NADH Glutamine Synthetase Glutamin Glutamte H ATP GOGAT PSII N 2 O 2 CO 2 Diagrammatic representation of carbon and nitrogen exchange between the heterocyst and vegetative cells. Carbon compounds are transported from the vegetative cells into the nitrogen fixing heterocyst in order to supply the necessary NADPH via the oxidative pentose pathway. The ammonia produced by nitrogen fixation is assimilated by glutamine synthetase and the majority of glutamine transported to the vegetative cell where it is converted into glutamate by glutamate synthase (GOGAT NITROGENASE

3. Heterocysts are the only cells that express  nif  (nitrogen fixation) genes and synthesize nitrogenase in heterocyst-forming cyanobacteria.  The adjacent vegetative cells, which in turn obtain fixed nitrogen from the heterocyst in the form of amino acids (mostly glutamine). The heterocysts protect their nitrogenase from oxygen inactivation by maintaining reduced internal partial pressures of oxygen,  The autoregulated gene  hetR , which is activated by the deficiency in combined nitrogen, seems to play a crucial role in the initiation of heterocyst development. The  heterocysts  also lack Rubisco and the  Calvin cycle  enzymes so that CO 2  fixation is limited to the  vegetative cells .   carbohydrate has to be transported into the heterocysts, probably in the form of  maltose Glucose is oxidized via the  pentose  phosphate pathway to yield NADPH in the reaction catalyzed by  glucose-6-phosphate dehydrogenase