Archaeplastida

Archaeplastida (pronounced /ɑːrkɪˈplæstɪdə/) 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.

Archaeplastid cells typically lack centrioles and have mitochondria with flat cristae. They usually have a cell wall that contains cellulose, and photosynthesized carbohydrates are stored intracellularly in the form of starch, but these characteristics are also shared with other eukaryotes. Different archaeplastid clades differ slightly in cellular biochemistry. Red algae are pigmented with chlorophyll a and phycobiliproteins like most cyanobacteria, and accumulate starch outside the chloroplasts. Green algae, land plants (embryophytes) and the minor basal group Prasinodermophyta – together known as Viridiplantae (Latin for "green plants") or Chloroplastida – are pigmented with both chlorophyll a and chlorophyll b, but lack phycobiliproteins, and starch is accumulated inside the chloroplasts. The glaucophytes have typical cyanobacterial pigments like those of red algae, but their plastids (called cyanelles) differ in having a peptidoglycan outer layer.

The main evidence that archaeplastids form a monophyletic group comes from genetic studies, which indicate their plastids likely had a single origin. This evidence is however disputed, and based on the evidence to date, it is not possible to definitively confirm or refute alternative evolutionary scenarios to a single primary endosymbiosis event. Photosynthetic organisms with plastids of indirect origin (such as brown algae, euglenids, diatoms and dinoflagellates) do not belong to Archaeplastida and are instead grouped into numerous other protist classifications, making the term "algae" a rather vague polyphyletic group.

Archaeplastida should not be confused with the older and obsolete name Archiplastideae, which refers to cyanobacteria and other groups of bacteria.