Cyanobacteria

These are also called as Blue-Green algae (Cyanin - blue-green)

They are prokaryotic,  photosynthetic, autotrophic to bacteria.

They are present in lakes, ponds, rivers, tanks, etc.

Anabaena lives as a symbiont with Azolla in paddy fields and fixes nitrogen. It acts as a biofertilizer. 

They grow well when nitrogen to phosphorus ratio will be low.

These are photosynthetic bacteria that release oxygen in photosynthesis because of the presence of Chlorophyll a  pigment. They are the most primitive organisms showing oxygenic photosynthesis.

The principal pigments are chlorophylls a (green), c-phycocyanin (blue) and c-phyco- erythrin (red).

These are aquatic or terrestrial. Cyanobacteria are present in a wide range of habitats viz. marine, freshwater, soil, biological soil crusts, snow, cryoconites, etc.

These are unicellular (Chroococcum), colonial (Microcystis) or filamentous (Spirulina, Nostoc, Anabaena).

Filament, trichome, colony or cell is covered by a mucilaginous sheath, so they are called as Myxophycea.

Protoplasm is divided into central colourless centroplasm with chromatin material and peripheral pigmented chromoplasm.

Filaments have specialized colorless cells meant for nitrogen fixation called Hetercysts (Nostoc, Anabaena).

They asexually reproduce by Hormogonia or thick walled Akinetes. Flagella are absent in life cycle. They do not reproduce sexually. 

The red color of redsea is due to a phycobilin known as Phycoerythrin in the BGA known as Trichodesmium erythrium.

The accumulation of these algae is termed as ‘algal blooms’. These blooms discolour the water and produce unpleasant taste and odour. They affect the fish population and reduces water quality. The decomposition of these blooms deplete the oxygen (BOD) and triggers the killing of fish.