The Cyanophyceae or blue-green algae are, today, usually referred to as the cyanobacteria (blue-green bacteria). The term cyanobacteria acknowledges that these prokaryotic algae are more closely related to the prokaryotic bacteria than to eukaryotic algae.

It has been hypothesized that the cyanobacteria evolved in freshwater at some time before 2.50 billion years ago (bya) (Blank, 2013) and that they spread into the marine environment at about the time of the Great Oxidation Event (GOE) (about 2.35 bya) where, through photosynthesis, the cyanobacteria raised oxygen levels in the atmosphere, enabling the evolution of aerobic life and dramatically changing life on the planet (Schirrmeister et al., 2014). The argument for a freshwater evolution of the cyanobacteria (Blank, 2013) involves sucrose synthesis, which originated in the cyanobacteria. Sucrose synthesis is strongly associated with a low-salinity environment.

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. At one time, the occurrence of chlorophyll b in cyanobacteria was used as a criterion to place the organisms in a separate group, the Prochlorophyta. Modern nucleic-acid sequencing, however, has shown that chlorophyll b evolved a number of times within the cyanobacteria and the term Prochlorophyta has been discarded (Palenik and Haselkorn, 1992 ; Urback et al., 1992).

Morphology

The simplest morphology in the cyanobacteria is that of unicells, free-living (Figs. 2.14(c)) or enclosed within a mucilaginous envelope (Figs. 2.38, 2.43(a), (b)). Subsequent evolution resulted in the formation of a row of cells called a trichome (Fig. 2.12). When the trichome is surrounded by a sheath, it is called a filament (Fig. 2.12). It is possible to have more than one trichome in a filament (Figs. 2.43(e), 2.45(b)). The most complex thallus is the branched filament (Fig. 2.45(a)). Such a branched filament can be uniseriate (composed of a single row of cells) or multiseriate (composed of one or more rows of cells).