What does cyanobacterium mean?
A cyanobacterium is any member of Cyanobacteria, a phylum of microscopic bacteria that obtain energy through oxygenic photosynthesis — the same light-driven process that plants use. Once grouped with algae under the misleading name 'blue-green algae' because of their colour and aquatic habits, they are now recognised as true prokaryotes: simple cells without a nucleus. Individual cyanobacteria may live as single cells, colonies, or filaments, and they occur almost everywhere there is light and moisture, from oceans and lakes to damp soil and rock surfaces. Ecologically they are profound contributors: ancient cyanobacteria generated the oxygen that transformed Earth's early atmosphere. Today the word appears most often in environmental and scientific contexts, particularly reports of harmful algal blooms, where certain species release toxins affecting drinking water, fisheries, recreation, and livestock. The term carries a neutral, technical register rather than an emotional one.
nounA microscopic, single-celled (or filamentous) photosynthetic bacterium belonging to the phylum Cyanobacteria, formerly known as blue-green algae.
- Any single-celled or filamentous photosynthetic prokaryote of the phylum Cyanobacteria, capable of oxygenic photosynthesis.
"A single cyanobacterium can multiply rapidly in warm, nutrient-rich water, forming a bloom visible from space."
"The lake's summer bloom was caused by a toxic cyanobacterium flourishing in the warm, phosphate-rich water."
"Under the microscope, each cyanobacterium revealed the distinctive chlorophyll pigments that give the group its blue-green hue."
Standard plural in both British and American scientific usage, following Latin conventions for bacterium → bacteria.
"Cyanobacteria were among the first organisms to colonise land."
Cyanobacteria invented oxygen-producing photosynthesis — without them, Earth's air would be unbreathable and you wouldn't exist.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cyanobacterium
The name derives from the Greek 'kyanos' (κύανος), meaning dark blue, combined with 'bacterium', reflecting the characteristic blue-green pigment of these organisms. It entered scientific usage in the twentieth century as microbiologists reclassified the group previously called blue-green algae as bacteria. Related terms sharing the Greek root include cyanosis, the bluish discolouration of skin, and cyanotype, the blueprint photographic process.
How cyanobacterium is actually used
A formal scientific term; in everyday contexts people usually refer to 'blue-green algae' or simply to blooms. The word takes the Latin-style plural 'cyanobacteria', and the singular is used mainly in technical writing about individual organisms or species.
Easily confused with cyanobacterium
Although historically called blue-green algae, cyanobacteria are true bacteria lacking a membrane-bound nucleus, unlike algae, which are eukaryotes.