Adding greatly to their flexibility, cyanobacteria have diverse morphology and are able to differentiate several types of cells. In all prokaryotes differentiation is controlled by environmental factors however, cyanobacteria are unique in the degree to which differentiation takes place. Cyanobacteria consist of four types of cells: vegetative cells, heterocysts, akinetes and hormogonia. Vegetative cells are the site of photosynthesis and carbon fixation. Heterocysts provide a microaerobic sight for nitrogenase, the nitrogen-fixing enzyme, and depend on vegetative cells as their source of fixed carbon. In non-symbiotic forms, heterocysts are produced in response to nitrogen limitation. Akinetes are spores, the method of cyanobacterial propagation
(Carr and Whitton, 1982).
Hormogonia are structures necessary for infection of symbiotic tissues
(Rai et al., 2000).
Cyanobacteria can be unicellular, branched or filamentous. Multicellular cyanobacteria are surrounded by a bacterial sheath
(Rowell and Kirby, 1991).