Heterocyst

Heterocysts are specialized cells in some cell chains ( = filament) forming cyanobacteria ( see figure there), in which there is the enzymatic fixation of atmospheric nitrogen by the enzyme nitrogenase (biological nitrogen fixation). These cells are formed when bound nitrogen (nitrate, ammonium ) is missing in the area. Approximately every tenth vegetative cell of a filament is differentiated irreversibly in a ten to 15-hour development program. Under the microscope, these often fall somewhat larger cells by their thickened cell wall, light green to almost yellowish transparent appearance and the older heterocysts eye-catching, from the reserve polymer cyanophycin (arginine -aspartate ) copolymer existing polar body at the connection points to the vegetative cells.

At the end of the cell differentiation program of heterocysts is the expression of a cell type-specific nitrogen fixation system ( development NIF1 controlled system). In vegetative cells of some cyanobacteria may also ( after only about two hours), a private nitrogen fixation system are turned on, however, in this, the absence of fixed nitrogen AND of oxygen in the environment required ( environmentally controlled NiF2 system).

Since all nitrogenases are extremely sensitive to oxygen, the oxygen-evolving part of photosynthesis ( photosystem II) in heterocysts not active. Photosystem I works, however, and may therefore - in addition to the breathing - in the light contribute to the very high demand for energy (ATP) to ensure the nitrogenase. Against the ingress of oxygen from outside the heterocysts protects an extra set outsourced cell wall layer of glycolipid and polysaccharide layers, the layer thickness, the oxygen content of the environment adapts ( oxygen is required in the synthesis of this layer ). In the surrounding vegetative cells of the filament is through oxygen-evolving photosynthesis produces sugar (probably mainly sucrose). These are transported to the heterocysts and there oxidized to the point that on the one hand enough electrons for the reduction of atmospheric nitrogen to ammonium are available and on the other hand, the resulting partially oxidized carbon skeletons for installation of ammonium available (final product: glutamine). Of glutamine bound in nitrogen air is transported into the adjacent cells, thus supplying the entire cell string -bound nitrogen. The running in the heterocyst metabolism thus complements the running in the vegetative cells of photosynthetic CO2 fixation by a photosynthetic N2 - fixation. In this way, forming heterocysts in cyanobacteria from the " inexhaustible " precursors H2O, CO2 and N2 is produced biomass maximum use of light energy. This is the most effective known method of primary production.

Evolution of Heterocyst

It is likely that the development of the simultaneous photo- fixation of carbon dioxide and dinitrogen for primary production in an organism " in the air " very early in Earth's history: in response to the decline in other productive organic and inorganic energy, carbon and nitrogen sources, caused by the metabolic activity of competing bacteria. Signs of heterocysts at least there is already from fossils in sediments that have an age of about 2.2 billion years. During this time the atmosphere was still virtually free of oxygen. This finding supports the conclusion that the development of heterocysts first goes back to the protection of nitrogenase from oxygen, which is produced in their own vegetative cells by simultaneous photosynthesis, and can lead to high local excess concentrations of oxygen in the microenvironment of the Zellfilamente.

• Photosynthesis
• Microbiology