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 that share certain features in their plastids, the organelles responsible for photosynthesis. Specifically, the kingdom covers all eukaryotes whose plastids contain a form of chlorophyll called chlorophyll c and are enclosed by four membranes.

If the common ancestor of this group already had chloroplasts acquired through endosymbiosis from red algae, then any non-photosynthetic members of Chromista must have lost the ability to photosynthesize at a later point. It is also possible that the various members of Chromista arose independently from the last eukaryotic common ancestor, rather than from a single shared ancestor.

The British biologist Thomas Cavalier-Smith created the taxon Chromista in 1981 to separate the stramenopiles, haptophytes, and cryptophytes. Initially, the kingdom was made up mostly of photosynthetic algae, but Cavalier-Smith later added many heterotrophic protozoa to the group. By 2022, Chromista was nearly as diverse as the kingdoms Plantae and Animalia, containing nine phyla. Notable members include marine algae, the organism that causes 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 is thought to have happened repeatedly, so that red plastids spread from one group to another. As a result, the plastids themselves have a different evolutionary history from the hosts that carry them. The plastids likely originated in the Rhodophytina, were passed to the Cryptophytina, then 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 the following features: plastids that contain chlorophyll c and lie within an extra membrane (the periplastid membrane) in the lumen of the rough endoplasmic reticulum (often inside the perinuclear cisterna); and cilia that have rigid, tubular hairs that are either tripartite or bipartite.

The kingdom includes a wide range of organisms, from algae to malarial parasites like *Plasmodium*. Molecular evidence suggests that the plastids in chromists came from red algae through a process called secondary symbiogenesis, which happened in a single event. In contrast, plants acquired their plastids directly from cyanobacteria through primary symbiogenesis. Because of this secondary event, chromist plastids are enclosed in two extra cell membranes, giving them a four-membrane envelope. This arrangement required the acquisition of many additional membrane proteins to transport molecules in and out of the organelles. The diversity seen in chromists is thought to have arisen from the degeneration, loss, or replacement of plastids in some lineages. Additional symbiogenesis involving green algae provided genes that are still present 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**

Several classification schemes have been proposed for these groups, which overlap but are not identical.

The Chromophycées (Chadefaud, 1950), later renamed Chromophycota (Chadefaud, 1960), included what are now the Ochrophyta (autotrophic Stramenopiles), Haptophyta (once placed within Chrysophyceae until Christensen, 1962), Cryptophyta, Dinophyta, Euglenophyceae, and Choanoflagellida (once placed within Chrysophyceae until Hibberd, 1975).

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

The Chromophyta (Bourrelly, 1968) included the Ochrophyta, Haptophyta, and Choanoflagellida. The Cryptophyceae and Dinophyceae were placed in the Pyrrhophyta (which is the same as Dinophyta).

Cavalier-Smith introduced the name Chromista in 1981. In this scheme, Chromophyta (or Chromobiota) refers to non-cryptophyte chromists. It includes all protists whose plastids contain chlorophyll c and is made up of three groups: Heterokonts (or Stramenopiles: brown algae, diatoms, water moulds, etc.), Haptophytes, and Cryptomonads.

By 1994, Cavalier-Smith and colleagues suggested that Chromista is probably polyphyletic—meaning its members arose independently and share no more than descent from the common ancestor of all eukaryotes. They noted that the four phyla containing chromophyte algae (those with chlorophyll c—Cryptista, Heterokonta, Haptophyta, and Dinozoa) are only distantly related to each other and to Chlorarachniophyta. The photosynthetic taxa within each of these four phyla radiated from each other well after the radiation of the four phyla themselves. This suggests that the common ancestor of these four phyla was not photosynthetic, and that chloroplasts were implanted separately into each group much later. If this polyphyly is confirmed, they argued, it would be better to treat Cryptista, Heterokonta, and Haptophyta as separate kingdoms rather than lump them together as Chromista.

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