There are about 160 genera and 1.500 species in Cyanophyceae. In India, the class is represented by 98 genera and 833 species. The members of Cyanophycrae (Myxophyceae) are commonly known as blue-green algae as their principal pigment is a bluish-green e- phycocyanin. They also contain chlorophyll a, B-carotene and e-phycoerythrin. They differ from other algae: in the absence of well organised cell organelles, (i) the pigments are not restricted to definite chromatophores, but are distributed throughout the peripheral cytoplasm (chromoplasm), and (ii) the nucleus is of primitive type which lacks nuclear membrane and mucleolus. These character show typical prokaryotic nature of the members of this group. These algae do not have flagella and their movement is brought about by gliding action.
Studies during the last three decades have clearly shown that the cell structure of blue-green algae is entirely different from the members of other algal groups. On the basis of their prokaryotic structure, many microbiologists consider blue-green algae as bacteria. Christensen (1962), on the basis of prokaryotic cell structure of blue-green algae and bacteria, placed both of them in a common phylum Prokaryota Blue-green algae have also been named as Cyanobacteria. Although there are many similarities between bacteria and blue-green algae, yet they show differences in their evolution. Phycologists regard any organism with chlorophyll a and the thallus not differentiated into roots, stem and leaves to be an alga. Therefore, most of the
algologists include Cyanophyceae in algae, rather than in bacteria The blue-green algae are found in a wide variety of habitats. Most of the species are fresh water, a few like Dermocarpa and Trichodesmium are marine. Species of Nostoc and Oscillatoria are found in terrestrial habitats. Some species of Nostoc and Anabaena grow as endophytes in roots of Cycas leaves of Azolla and thalli of Anthoceros Species of Chroococcus Gloeocapsa, Nostoc. Seytonema and Stigomema are the main algal component (phycobiont) of lichens. The blue-green algae, growing in organically rich permanent waters, form planktons. The polluted water of lakes and ponds, exposed to intense sunlight and high temperature, provides a suitable habitat for the growth of planktonic blue-greens. They are also found on the bottom of ponds and increase fertility of the underlying soil.