cyanobacterium

/ sī′ə-nō-băk-tîr′ē-əm /

, Plural cyanobacteria

1. Any of a phylum of photosynthetic bacteria that live in water or damp soil and were once thought to be plants. Cyanobacteria have chlorophyll as well as carotenoid and phycobilin pigments, and they conduct photosynthesis in membranes known as thylakoids (which are also found in plant chloroplasts). Cyanobacteria may exist as individual cells or as filaments, and some species live in colonies. Many species secrete a mucilaginous substance that binds the cells or filaments together in colored (often bluish-green) masses. Cyanobacteria exist today in some 7,500 species, many of which are symbiotes, and have lived on Earth for 2.7 billion years. Since all species produce oxygen as a byproduct of metabolism, it is thought that much of Earth's atmospheric oxygen can be attributed to cyanobacteria. Many species can also fix nitrogen and so play an important role in the nitrogen cycle.
2. Also called blue-green alga