Heterocyst

Heterocysts ( Greek ἕτερος héteros "different", κύστις kýstis "bladder") are specialized cells in some cell chains (= filament) forming cyanobacteria (see figure there), in which the enzymatic fixation of atmospheric nitrogen by the enzyme nitrogenase takes place (biological nitrogen fixation ) .

These cells are formed as soon as there is a lack of bound nitrogen ( nitrate , ammonium ) in the environment. About every tenth vegetative cell of a filament is irreversibly differentiated in a ten to 15 hour development program. Under the microscope, these often somewhat larger cells are noticeable due to their thickened cell wall, their light green to almost yellowish-transparent appearance and the polar bodies at the connection points, which are conspicuous in older heterocysts and consist of the reserve polymer cyanophycin (arginine-aspartate copolymer) the vegetative cells.

At the end of the cell differentiation program of the heterocysts, there is the expression of a cell-type-specific nitrogen fixation system (development-driven Nif1 system). In the vegetative cells of some cyanobacteria, their own nitrogen fixation system can also be switched on (after about two hours), but this requires the absence of bound nitrogen AND oxygen in the environment (environmentally controlled Nif2 system).

Since all nitrogenases are extremely sensitive to oxygen , the oxygen-developing part of photosynthesis ( photosystem II ) is not active in heterocysts. Photosystem I works, however, and can therefore - in addition to breathing - contribute in the light to guarantee the very high energy requirement ( ATP ) of nitrogenase. The heterocysts are protected from outside oxygen by an extra layer of the cell wall made of glycolipid and polysaccharide layers , the thickness of which adapts to the oxygen content of the environment (oxygen is also required for the synthesis of this layer). In the surrounding vegetative cells of the filament, oxygen-evolving photosynthesis produces sugar (probably mainly sucrose ). These are transported into the heterocysts and oxidized there to such an extent that, on the one hand, enough electrons are available to reduce the atmospheric nitrogen to ammonium and, on the other hand, the partially oxidized carbon frameworks are available to incorporate the ammonium (end product: glutamine ). The atmospheric nitrogen bound in the glutamine is transported to the neighboring cells and thus supplies the entire cell thread with bound nitrogen. The metabolism taking place in the heterocysts supplements the photosynthetic CO ₂ fixation taking place in the vegetative cells by a photosynthetic N ₂ fixation. In this way, in cyanobacteria that form heterocysts, biomass is generated from the "inexhaustible" raw materials H ₂ O, CO ₂ and N ₂ with maximum utilization of light energy . It is the most effective known type of primary production .

Evolution of the heterocyst

It can be assumed that the development of the simultaneous photofixation of carbon dioxide and nitrous oxide for primary production in an organism was "in the air" very early in the history of the earth: as a reaction to the decline of other abundant organic and inorganic sources of energy, carbon and nitrogen, caused by the metabolic activity of competing bacteria. In any case, there are already signs of heterocysts from fossils in sediments that are around 2.2 billion years old. At that time the atmosphere was still practically free of oxygen. This finding supports the conclusion that the development of heterocysts can be traced back to the protection of nitrogenase from oxygen, which is produced in the own vegetative cells through simultaneous photosynthesis and which can lead to high local overconcentrations of oxygen in the microenvironment of the cell filaments.

Individual evidence

1. ^ C. Peter Wolk, Anneliese Ernst, Jeff Elhai: Heterocyst Metabolism and Development. In: The Molecular Biology of Cyanobacteria. Advances in Photosynthesis and Respiration. Volume 1, 2004, pp. 769-823. ISBN 978-0-7923-3222-0 .