The concept of chromists, at its most expansive, includes the heterokonts (stramenopiles), alveolates, rhizarians, heliozoans, telonemians, haptophytes and cryptophytes. There is mounting evidence that this grouping is not valid. Even in the narrowest sense (the heterokonts), chromists include very diverse forms, exhibiting a great variety of trophic mechanisms. This great diversity in form and feeding make it difficult to identify any unifying features, but molecular phylogenetic studies have shown that this group of organisms is indeed monophyletic. The distribution of morphological characters over reconstructed trees allows for the identification of potential synapomorphic characters that have been secondarily lost or modified across the group. These include a combination of mitochondria with tubular cristae; the biflagellate heterokont condition; and, if photosynthetic, then with chlorophyll c, girdle lamellae and four membranes around the chloroplast, the outer continuous with the nuclear envelope. Heterotrophy appears to be ancestral but is also occasionally a derived state from autotrophic forms.

Key Concepts:

  • There is no consistency in the ranking of the various eukaryotic taxa, making reference to particular forms and their relationships awkward.

  • Molecular studies, particularly over the last decade, indicate an ever‐increasing delimitation of the Chromista to include very diverse forms.

  • The chromalveolate theory, at the root of the concept of Chromista, has been fiercely debated and most recent evidence points to multiple independent events involving red algal endosymbionts in diverse eukaryotic hosts.

  • Even the original grouping of Chromista (heterokonts, haptophytes and cryptophytes) is tenuous, making it more sensible to equate chromists with the heterokonts (=stramenopiles).

  • Heterokonts enjoy robust support from molecular phylogenetic analyses, but there are no universal morphological and physiological characters.

  • The most universal character is the biflagellate condition of swimming cells, with one tinsel (hairy) and one smooth flagellum. The hairy flagellum is invested with two opposite rows of tri‐partite, tubular hairs that are responsible for reversing thrust.

  • Plastids of autotrophic heterokonts have consistent features, including two additional surrounding membranes (the periplastidial membrane and the RER), girdle lamellae and thylakoids stacked in groups of three.

  • The classification within the heterokonts also has a chequered history, but multigene phylogenetic analyses are providing a clearer idea of groupings.

  • Heterotrophic forms are rooted deeply in phylogenetic trees reconstructed using molecular markers, but some are secondarily derived from autotrophic forms.

  • Heterokonts play an important role both ecologically and economically.

Keywords: bacillariophytes; Bigyra; chromalveolates; chrysophytes; Heterokonta; oomycetes; phaeophytes; phylogeny; Pseudofungi; silicoflagellates; stramenopiles

Figure 1.

A typical heterokont flagellate. Most heterokont motiles have apical or near‐apical insertion of their flagella whereas this diagram shows the lateral insertion typical of phaeophyte gametes and spores. The tinsel flagellum is anteriorly directed and the smooth flagellum trails the cell during swimming.

Figure 2.

The heterokont chloroplast has the normal plastidial double‐membrane envelope, but surrounded by a periplastidial membrane and by rough endoplasmic reticulum which may be confluent with the outer membrane of the nuclear envelope. The periplastidial membrane is considered to be the remnants of the eukaryotic endosymbiont's (a red alga) plasmalemma. The chloroplast interior is occupied by thylakoids, the outer one/s of which is/are continuous and just beneath the envelope – the girdle lamella.



Adl SM, Simpson AGB, Farmer MA et al. (2005) The higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology 52: 399–451.

Beech PL, Heimann K and Melkonian M (1991) Development of the flagellar apparatus during the cell cycle in unicellular algae. Protoplasma 164: 23–37.

Baurain D, Brinkmann H, Petersen J et al. (2010) Phylogenomic evidence for separate acquisition of plastids in cryptophytes, haptophytes, and stramenopiles. Molecular Biology and Evolution 27: 1698–1709.

Burki F, Inagaki Y, Bråte J et al. (2009) Large‐scale phylogenomic analyses reveal that two enigmatic protest lineages, Telonemia and Centroheliozoa, are related to photosynthic chromalveolates. Genome Biology and Evolution 1: 231–238.

Burki F, Shlachian‐Tabrizi K and Pawlowski J (2008) Phylogenomics reveals a new ‘megagroup’ including most photosynthetic eukaryotes. Biology Letters 4: 366–369.

Cavalier‐Smith T (1981) Eukaryote kingdoms: seven or nine? Biosystems 14: 461–481.

Cavalier‐Smith T (1998) A revised six‐kingdom system of life. Biological Reviews 73: 203–266.

Cavalier‐Smith T (1999) Principles of protein and lipid targeting in secondary symbiogenesis: euglenoid, dinoflagellate, and sporozoan plastid origins and the eukaryote family tree. Journal of Eukaryotic Microbiology 46: 347–366.

Cavalier‐Smith T (2010) Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree. Biology Letters 6: 432–345.

Cavalier‐Smith T and Chao EE (1996) 18S rRNA sequence of Heterosigma carterae (Raphidophyceae), and the phylogeny of heterokont algae (Ochrophyta). Phycologia 35: 500–510.

Cavalier‐Smith T and Chao EE‐Y (2003) Phylogeny of Choanozoa, Apusozoa, and other protozoa and early eukaryote megaevolution. Journal of Molecular Evolution 56: 540–563.

Cavalier‐Smith T and Chao EE‐Y (2006) Phylogeny and megasystematics of phagotrophic heterokonts (kingdom Chromista). Journal of Molecular Evolution 62: 388–420.

Graham LE and Wilcox LW (2000) Algae, 640pp. Upper Saddle River, NJ: Prentice Hall.

Guillou L, Chrétiennot‐Dinet M‐J, Medlin L et al. (1999) Bolidomonas: a new genus with two species belonging to a new algal class, the Bolidophyceae (Heterokonta). Journal of Phycology 35: 368–381.

Hackett JD, Yoon HS, Li S et al. (2007) Phylogenomic analysis supports the monophyly of cryptophytes and haptophytes and the association of Rhizaria with chromalveolates. Molecular Biology and Evolution 24: 1702–1713.

Harper JT, Waanders E and Keeling PJ (2005) On the monophyly of chromalveolates using a six‐protein phylogeny of eukaryotes. International Journal of Systematic and Evolutionary Microbiology 55: 487–496.

Hausman, K, Hülsmann, N and Radek, R (2003) Protistology, 3rd edn. Stuttgart: E. Schweizerbart'sche Verlagsbuchhandlung. 379pp. ISBN 3‐510‐65209‐6.

Inouye I (1993) Flagella and flagellar apparatuses of algae. In: Berner T (ed.) Ultrastructure of Microalgae, pp. 99–133. Boca Raton: CRC Press.

Kawai H and Inouye I (1989) Flagellar autofluorescence in forty‐four chlorophyll c‐containing algae. Phycologia 28: 222–227.

Keeling PJ (2009) Chromalveolates and the evolution of plastids by secondary endosymbiosis. Journal of Eukaryotic Microbiology 56: 1–8.

Kim E and Graham LE (2008) EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata. PLoS ONE 3: e2621.

Medlin LK and Kaczmarska I (2004) Evolution of the diatoms: V. Morphological and cytological support for the major clades and a taxonomic revision. Phycologia 43: 245–270.

Moestrup Ø (1995) Current status of chrysophyte ‘splinter groups’: synurophytes, pedinellids, silicoflagellates. In: Sandgen CD, Smol JP and Kristiansen J (eds) Chrysophyte Algae. Ecology, Phylogeny and Development, pp. 75–91. Cambridge: Cambridge University Press.

Moestrup Ø (2000) The flagellate cytoskeleton. Introduction of a general terminology for microtubular roots in protists. In: Leadbeater BSC and Green JC (eds) The Flagellates. The Systematics Association Special Volume 59, pp. 69–94. London: Taylor and Francis.

Moustafa A, Beszteri B, Maier UG et al. (2009) Genomic footprints of a cryptic plastid endosymbiosis in diatoms. Science 324: 1724–1726.

Okamoto N, Chantagsi C, Horák A, Leander BS and Keeling PJ (2009) Molecular phylogeny and description of the novel katablepharid Roombia truncate gen. et sp. nov., and establishment of the Hacrobia taxon nov. PLoS ONE 4: e7080.

Parfrey LW, Barbero E, Lasser E et al. (2006) Evaluating support for the current classification of eukaryotic diversity. PLoS Genet 2: e220.

Patterson DJ (1999) The diversity of eukaryotes. American Naturalist 154: S96–S124.

Petersen J, Teich R, Brinkman H and Cerff R (2006) A ‘green’ phosphoribulokinase in complex algae with red plastids; evidence for a single secondary endosymbiosis leading to haptophytes, cryptophytes, heterokonts, and dinoflagellates. Journal of Molecular Evolution 62: 143–157.

Riisberg I, Orr RJS, Kluge R et al. (2009) Seven gene phylogeny of heterokonts. Protist 160: 191–204.

Shalchian‐Tabrizi K, Eikrem W, Klaveness D et al. (2006) Telonemia, a new protist phylum with affinity to chromist lineages. Proceedings of the Royal Society B 273: 1833–1842.

Takashita K, Yamaguchi H, Maruyama T and Inagaki Y (2009) A hypothesis for the evolution of nuclear‐encoded, plastid‐targeted glyceraldehydes‐3‐phosphate dehydrogenase genes in ‘chromalveolate’ members. PLoS ONE 4: e4737.

Van Valkenburg SD and Norris RE (1970) The growth and morphology of the silicoflagellate Dictyocha fibula Ehrenberg in culture. Journal of Phycology 6: 48–54.

Further Reading

Anderson RA (2004) Biology and systematics of heterokont and haptophyte algae. American Journal of Botany 91: 1508–1522.

Archibald JM (2009) The puzzle of plastid evolution. Current Biology 19: R81–R88.

Battacharya D, Yoon HS, Hedges SB and Hackett JD (2009) Eukaryotes (Eukaryota). In: Hedges SB and Kumar S (eds) The Timetree of Life, pp. 116–120. Oxford: Oxford University Press.

Karpov SA, Sogin ML and Silberman JD (2001) Rootlet homology, taxonomy, and phylogeny of bicosoecids based on 18S rRNA gene sequences. Protistology 2: 34–47.

Margulis L, Corliss JO, Melkonian M and Chapman DJ (eds) (1990) Handbook of Protoctista, 914pp. Boston: Jones and Bartlett.

Moriya M, Nakayama T and Inouye I (2000) Ultrastructure and 18S rDNA sequence analysis of Wobblia lunata gen. et sp. nov., a new heterotrophic flagellate (stramenopiles, incertae sedis). Protist 151: 41–55.

Moriya M, Nakayama T and Inouye I (2002) A new class of the stramenopiles, Placididea classis nova: description of Placidia cafeteriopsis gen. et sp. nov. Protist 153: 143–156.

Nozaki H (2005) A new scenario of plastid evolution: plastid primary endosymbiosis before the divergence of the ‘Plantae,’ emended. Journal of Plant Research 118: 247–255.

Sekiguchi H, Moriya M, Nakayama T and Inouye I (2002) Vestigial chloroplasts in heterotrophic stramenopiles Pteridomonas danica and Ciliophrys infusionum (Dictyochophyceae). Protist 153: 157–167.

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Sym, Stuart D, and Maneveldt, Gavin W(Nov 2011) Chromista. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001960.pub2]