The Cyanobacteria, blue-green algae, is a distinctive group of photosynthetic bacteria possessing a dual photosystem allowing the use of H2O as a photoreductant leading to the release of oxygen. Chlorophyll a is primarily used as the reaction centre pigment, though chlorophyll b is also present in some species (Castenholz 2001). In the majority of Cyanobacteria, comprising the clade known as the Phycobacteria, the photosynthetic pigments are embedded in membrane compartments, called thylakoids; these are absent in the basal Gloeobacterales (Cavalier-Smith 2002). Through endosymbiosis, at least one lineage of the Phycobacteria gave rise to the chloroplasts.

Molecular analyses have indicated that the majority of Phycobacteria can be placed in lineages that have been labelled the AC and E clades, with the latter further divided between E1 and E2 subclades (the latter corresponding to what is here labelled the Hormogoneae). Members of the AC clade are mostly unicellular picoplankton; those multicellular taxa in this clade are all simple with undifferentiated cells (Schirrmeister et al. 2013).

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