Cyanophyceae (Blue-green algae)

Cyanobacteria /saɪˌænoʊbækˈtɪəriə/, also known as Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis, and are the only photosynthetic prokaryotes able to produce oxygen. The name “cyanobacteria” comes from the color of the bacteria (Greek: κυανός (kyanós) = blue). Cyanobacteria (which are prokaryotes) used to be called “blue-green algae”. They have been renamed ‘cyanobacteria’ in order to avoid the term “algae”, which in modern usage is restricted to eukaryotes.

— Wikipedia