Chromista is a proposed but polyphyletic[1][2][3] biological kingdom, refined from the Chromalveolata, consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids).

If the ancestor already possessed chloroplasts derived by endosymbiosis from red algae, all non-photosynthetic Chromista have secondarily lost the ability to photosynthesise.

[5] According to Cavalier-Smith, the kingdom originally consisted mostly of photosynthetic eukaryotes (algae), but he later brought many heterotrophs (protozoa) into the proposed group.

[3] Members of Chromista are single-celled and multicellular eukaryotes having basically either or both features:[5] The kingdom includes diverse organisms from algae to malarial parasites (Plasmodium).

The Chromophyta (Christensen 1962, 2008), defined as algae with chlorophyll c, included the current Ochrophyta (autotrophic Stramenopiles), Haptophyta, Cryptophyta, Dinophyta and Choanoflagellida.

It has been described as consisting of three different groups:[17] It includes all protists whose plastids contain chlorophyll c.[1] In 1994, Cavalier-Smith and colleagues indicated that the Chromista is probably a polyphyletic group whose members arose independently, sharing no more than descent from the common ancestor of all eukaryotes:[1] The four phyla that contain chromophyte algae (those with chlorophyll c--i.e., Cryptista, Heterokonta, Haptophyta, Dinozoa) are distantly related to each other and to Chlorarachniophyta on our trees.

[7] Patron et al. (2004) considered the presence of a unique class of FBA (fructose-1,6-biophosphate-aldolase) enzyme not similar to that found in plants as evidence of chromist monophyly.

[18] Fast et al. (2001) supported a single origin for the myzozoan (dinoflagellate + apicomplexan), heterokont and cryptophyte plastids based on their comparison of GAPDH (glyceraldehyde-3-phosphate dehydrogenase) genes.

All three may share a common ancestor with the alveolates (see chromalveolates), but there is evidence that suggests the haptophytes and cryptomonads do not belong together with the heterokonts or the SAR clade, but may be associated with the Archaeplastida.

[3] A 2020 phylogeny of the eukaryotes states that "the chromalveolate hypothesis is not widely accepted" (noting Cavalier-Smith et al 2018[28] as an exception), explaining that the host lineages do not appear to be closely related in "most phylogenetic analyses".