What is the gas released by cyanobacteria

cyanobacteria produce oxygen as a by-product. Photosynthesis is the only major source of free oxygen gas in the atmosphere.

Cyanobacteria / saɪˌænoʊbækˈtɪəriə /, also known as Cyanophyta, are a phylum of bacteria that obtain their energy through photosynthesis,[4] and are the only photosynthetic prokaryotes able to produce oxygen.[5] The name "cyanobacteria" comes from the color of the bacteria (Greek: κυανός, translit. kyanós, lit. 'blue').[6][7] 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. [8]