The Cyanophyceae or blue-green algae are, today, usually referred to as the cyanobacteria (bluegreen bacteria). The term cyanobacteria acknowledges that these prokaryotic algae are more closely related to the prokaryotic bacteria than to eukaryotic algae.
Cyanobacteria have chlorophyll a (some also have chlorophyll b or d), phycobiliproteins, glycogen as a storage product, and cell walls containing amino sugars and amino acids.
The simplest morphology in the cyanobacteria is that of unicells, free-living or enclosed within a mucilaginous envelope. Subsequent evolution resulted in the formation of a row of cells called a trichome. When the trichome is surrounded by a sheath, it is called a filament. It is possible to have more than one trichome in a filament. The most complex thallus is the branched filament. Such a branched filament can be uniseriate (composed of a single row of cells) or multiseriate (composed of one or more rows of cells).