Chromista

Chromista is a proposed biological kingdom, one that remains controversial, and was refined from the earlier grouping Chromalveolata. It includes both single-celled and multicellular eukaryotic species linked by shared features in their photosynthetic organelles, or plastids. Specifically, it covers all eukaryotes whose plastids contain chlorophyll c and are enclosed by four membranes. If the common ancestor of this group already had chloroplasts obtained through endosymbiosis from red algae, then any non-photosynthetic members of Chromista must have secondarily lost the ability to photosynthesize. It is also possible that its members arose independently from the last eukaryotic common ancestor, forming separate evolutionary lines.

The kingdom was created in 1981 by British biologist Thomas Cavalier-Smith to separate the stramenopiles, haptophytes, and cryptophytes. Initially, it consisted mostly of photosynthetic algae, but Cavalier-Smith later added many heterotrophic protozoa. By 2022, the kingdom was nearly as diverse as Plantae and Animalia, containing nine phyla. Notable members include marine algae, the organism behind potato blight, dinoflagellates, *Paramecium*, the brain parasite *Toxoplasma*, and the malaria parasite *Plasmodium*.

However, many researchers reject Cavalier-Smith’s idea that chromists form a single, natural group (monophyly). Instead, they think some chromists acquired their plastids by engulfing another chromist, rather than inheriting them from a common ancestor. This process likely happened repeatedly, spreading red plastids from one group to another. As a result, the plastids—far from marking their hosts as a single clade—have a history separate from their diverse hosts. They appear to have originated in the Rhodophytina, then passed to the Cryptophytina, from there to both the Ochrophyta and the Haptophyta, and finally from the Haptophyta to the Myzozoa.

**Biology**

Members of Chromista are single-celled or multicellular eukaryotes that typically have one or both of these features: plastids containing chlorophyll c, located within an extra membrane (the periplastid membrane) in the lumen of the rough endoplasmic reticulum (often within the perinuclear cisterna); and cilia with rigid tubular hairs that are either tripartite or bipartite. The group includes a wide range of organisms, from algae to malarial parasites. Molecular evidence suggests that chromist plastids came from red algae through a single secondary symbiogenesis event—unlike plants, which got their plastids directly from cyanobacteria via primary symbiogenesis. These plastids are now wrapped in two extra cell membranes, forming a four-membrane envelope, which required the acquisition of many membrane proteins to transport molecules in and out. The diversity of chromists is thought to have arisen from the degeneration, loss, or replacement of plastids in some lineages. Additional symbiogenesis involving green algae provided genes retained in some members (such as heterokonts), and bacterial chlorophyll (indicated by the presence of the ribosomal protein L36 gene, *rpl36*) appears in haptophytes and cryptophytes.

**History and groups**

The groups involved have overlapping but not identical memberships, as shown in various classification schemes.

The Chromophycées (Chadefaud, 1950), later renamed Chromophycota (Chadefaud, 1960), included the current Ochrophyta (autotrophic Stramenopiles), Haptophyta (then part of Chrysophyceae until 1962), Cryptophyta, Dinophyta, Euglenophyceae, and Choanoflagellida (then part of Chrysophyceae until 1975).

The Chromophyta (Christensen, 1962 and 1989), defined as algae with chlorophyll c, included the current Ochrophyta, Haptophyta, Cryptophyta, Dinophyta, and Choanoflagellida, while Euglenophyceae were moved to Chlorophyta.

Another Chromophyta (Bourrelly, 1968) included the current Ochrophyta, Haptophyta, and Choanoflagellida, with Cryptophyceae and Dinophyceae placed in Pyrrhophyta (= Dinophyta).

Cavalier-Smith first introduced the name Chromista in 1981. The term Chromophyta (or Chromobiota) corresponds to non-cryptophyte chromists. It includes all protists whose plastids contain chlorophyll c and was described as having three groups: Heterokonts (or Stramenopiles: brown algae, diatoms, water moulds, etc.), Haptophytes, and Cryptomonads. In 1994, Cavalier-Smith and colleagues suggested Chromista is probably polyphyletic—its members arose independently, sharing only descent from the common ancestor of all eukaryotes. They noted that the four phyla containing chromophyte algae (Cryptista, Heterokonta, Haptophyta, Dinozoa) are distantly related, and that photosynthetic taxa within each phylum radiated much later than the phyla themselves. This suggests the common ancestor of these four phyla was not photosynthetic, and chloroplasts were implanted separately into each more recently. If confirmed, this polyphyly would make it desirable to treat Cryptista, Heterokonta, and Haptophyta as separate kingdoms rather than a single Chromista.

In 2009, Cavalier-Smith explained his reason for creating the kingdom: evidence that chromist chloroplasts were acquired secondarily by enslaving a red alga (a member of Plantae) and their unique membrane topology. Since then, Chromista has been defined in different ways at different times.