Definition of cyanobacterium
: any of a major group (Cyanobacteria) of photosynthetic bacteria that are single-celled but often form colonies in the form of filaments, sheets, or spheres and are found in diverse environments (such as salt and fresh water, soils, and on rocks)
Note: Like other bacteria, cyanobacteria lack a nuclear membrane and membrane-bound organelles and have cell walls containing peptidoglycan. However, cyanobacteria were more commonly grouped with algae in the past because they possess a type of chlorophyll (chlorophyll a) not found in other bacteria and because they produce oxygen as a by-product of photosynthesis.
— called also blue-green alga
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Merriam-Webster unabridged
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Time Traveler for cyanobacterium
The first known use of cyanobacterium was in 1974
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