Cyanobacteria (or blue-green algae) are photosynthetic organisms occur in terrestrial and aquatic habitats, sometimes forming planktons on the surface of freshwater and as endophytes. They may be unicellular, colonial or filamentous (trichome). Cyanobacterial cell is typically prokaryotic with a mucilage sheath (or slime layer or capsule) surrounding the cell wall which is made of peptidoglycan.
Cytoplasm is differentiated into outer coloured chromoplasm and central colourless centroplasm. Chromoplasm contains pigments, oil droplets, lipid, globules, cyanophycean starch and protein; carboxysomes rich in Rubisco enzyme for photosynthesis, 70S ribosomes and contractile vacuoles. Pigments are located on thylakoid membranes present in chromoplasm. Centroplasm contains the naked DNA (nucleoid). Some members possess special elongated cells called heterocysts economic importance. Spirulina is an important source of protein, Oscillatoria is used for sewage disposal by oxidizing organic wastes and Lyngbia is a source antibiotics.