Why Cyanobacteria are known also as Blue-Green Algae???

Cyanobacteria contain chlorophyll a, a green pigment, and phycobilin, a blue pigment.

The combination of the two in many species makes them appear a brilliant cyan color, and lots of others fall somewhere along the green-blue spectrum. Hence the name “Cyanobacteria”.

However, they also contain lots of other pigments in varying, species-specific concentrations. Some species contain various carotenoids (yellowish) and phycoerythrin (red), meaning that Cyanobacteria can present as anything from green-blue to dark red, depending on the species. The most cyan-y of the Cyanobacteria tend to grow in places people don’t regularly interact with, like volcanic hot springs (such as those found at Yellowstone). The Cyanobacteria that the “average” 21st century human is likely to encounter are the kinds that tend to live in fish tanks, which more often than not are very dark green, near black, or red. If they’re the only ones you’ve seen up close, the name “Cyanobacteria” would seem like a weird mislabel.