Cyanobacteria, also referred to as blue-green algae, are a type of bacterium. They contain photosynthetic pigments.   They have an off-green color.   Aeons ago, their photosynthetic activities changed the atmosphere – adding oxygen to it.  Some cyanobacteria invaded other cells, as with Paulinella and became resident as chloroplasts.  Cyanobacteria can occur in high numbers, some form mats covering the ground  – as in the case of the stream in Yellowstone National Park.  Some cyanobacteria produce toxins.

Some, like Prochlorococcus,  are tiny independent spheres (about 1 µm or 1/1000 millimetre in diameter). Others form filaments made of disc-shaped cells stuck end to end, or, in  the case of Anabaena, more rounded cells.

Anabaena contains three types of cells: the majority are normal photosynthetic cells, others form are thick-walled heterocysts that can fix nitrogen (that is, they incorporate elemental nitrogen from the environment – something only bacteria can do), and akinetes that are ‘resting’ stages that can persist when other cells die. Cylindrospermum also has three types of cells, the heterocyst is at the tip of the filament, and often there is a large akinete behind it. Nostoc filaments also include heterocysts.

Cyanobacteria may occur as isolated cells (such as Prochlorococcus), or may occur in larger assembles. Chroococcus usually occurs as pairs of cells; while Gomphospheria forms small colonies of a hundred or so single and paired cells, which when squashed reveals the mucus that binds them together.  Nostoc, one of the toxin-producing cyanobacteria, form mucus-bound colonies of filaments.   Many Cyanobacteria can move – they glide; examples being Oscillatoria and Spirulina. Often they will intertwine – forming aggregates.