Blue-green algae, although often referred to as algae, are in fact scientifically classified as cyanobacteria. These microscopic organisms are more closely related to bacteria, because of their cellular structure and genetics, than they are to algae. Cyanobacteria contain the pigments chlorophyll a, which is the same pigment found in plants, as well as the bluish pigment called phycocyanin.

These pigments give them a blue-green colour, which is why they were given the name cyanobacteria, that is derived from the Greek word kyanó meaning blue. Yet, not all cyanobacteria are blue-green. Some species of cyanobacteria contain other pigments such as orange pigments known as carotenoids and pink pigments known as phycoerythrin.

Whatever their colour, cyanobacteria are capable of photosynthesis and use sunlight to make their own food, which is generally only typical of plants and is the reason why they were erroneously referred to as blue-green algae.