Cyanobacterial Heterocysts

Cyanobacteria are phototrophic bacteria carrying out oxygen-producing photosynthesis. Indeed, cyanobacteria were the inventors of oxygenic photosynthesis carried out by eukaryotic algae and plants. Besides showing the capability of building their cellular carbon from carbon dioxide, available in the atmosphere, several strains of cyanobacteria have also acquired the ability to fix molecular dinitrogen (N2). As the enzyme responsible for nitrogen fixation (nitrogenase) is highly sensitive towards oxygen, nitrogen fixation and oxygenic photosynthesis cannot take place simultaneously in cyanobacterial cells. To solve this problem, some filamentous strains are able to restrict N2 fixation to a special cell type, the heterocyst. Heterocysts are specialised, morphologically distinct, terminally differentiated cells that develop, in theabsenceofalternative sourcesof combined nitrogen, mostly in a semiregular pattern along the filament. Thus, a filament containing heterocysts provides division of labour betweenphotosynthetic carbon dioxide fixation (in vegetative cells) and anaerobic N2 fixation (in heterocysts). These cyanobacteria represent true multicellular organisms with profound morphological cell differentiation and sophisticated intercellular communication systems. Introduction