Archaeplastida

Archaeplastida (pronounced ) or archaeplastids, sometimes also regarded as the kingdom Plantae sensu lato ("in a broad sense"), are a large group of eukaryotes comprising the major clades Viridiplantae (green algae and land plants) and Rhodophyta (red algae), as well as the minor division Glaucophyta ("grey algae"). While the vast majority of archaeplastids are autotrophs, the group also includes heterotrophic lineages such as the predatorial (eukaryotrophic) flagellates Rhodelphidia and probably also the microscopic picoplankton Picozoa, both may be sister to Rhodophyta and altogether forming the larger clade Rhodaria.

With the exception of the picozoans, archaeplastids are all primary algae whose cells have pigment-bearing membrane-bound organelles called plastids, and most of their plastids contain the red/blue light-sensitive photopigment chlorophyll, which acts as the core component of oxygenic photosynthesis reaction centers. These chlorophyllic plastids, known as chloroplasts, are surrounded by two biological membranes, suggesting that they were endosymbionts evolved directly from phagocytosis of cyanobacteria, which did not result in intracellular digestion but instead achieved mutualistic symbiosis within the endomembrane system. In contrast, other photosynthetic algae, besides the amoeboid genus Paulinella (which also obtained cyanobionts directly, but independently and much later), are secondary algae with chloroplasts surrounded by three or four membranes, suggesting they were indirectly acquired via phagocytosis and subsequent endosymbiosis of archaeplastids (red or green algae) or other secondary algae, in what can be considered a form of permanent kleptoplasty. Unlike red and green algae, glaucophytes are not known to be ever involved in such secondary endosymbiosis events.