green alga

An unidentified green alga found growing on the surface of soil.

Some General Characteristics

The protoctists encompass a wide assortment of single-celled and simple multicelled eukaryotes. They are defined here as including all eukaryotes other than those classified as animals, vascular plants, bryophytes, or fungi. This grouping is not monophyletic— it does not constitute a clade but rather includes taxa from all lineages of eukaryotes. These taxa are grouped together as a matter of convenience.

Groups of Protoctists

The protoctists include algae (such as phytoplankton, seaweeds, and other simple photosynthetic organisms); slime molds; water molds; and various heterotrophic flagellates, ciliates, amoeba-like organisms, and parasites.

Eukaryotic organisms may be classified into broad groups known as "supergroups". Because protoctists are so diverse, they are found in all of the eukaryotic supergroups. Five supergroups of eukaryotes (from Burki 2014) are listed below, along with some examples of common or well known protoctists belonging to each group. Due to taxonomic uncertainties, some eukaryotic organisms have not been assigned to a supergroup. Two examples of common protoctists with uncertain taxonomic status are included below.

Supergroups of Eukaryotes and Representative Protoctist Taxa

  • Opisthokonta (this group includes animals and fungi)
    • choanoflagellates (these are free-living flagellates that closely resemble the collar cells in sponges)
  • Excavata
    • Giardia (anaerobic intestinal parasites that causes watery diarrhea)
    • Trichomonas (anaerobic parasites, including one species that is sexually transmitted in humans)
    • Euglena (a photosynthetic flagellate with a secondary plastid derived from a green alga)
  • Amoebozoa
  • Archaeplastida (this group includes vascular plants and bryophytes; they have primary plastids derived from a cyanobacterium)
  • SAR (Stramenopiles, Alveolates, and Rhizaria)
    • Stramenopiles (photosynthetic stramenopiles have secondary plastids derived from a red alga)
      • water molds (heterotrophic, formerly grouped under fungi; include parasitic/pathogenic and saprophytic taxa. Many are pathogens of plants; some parasitize animals. They are the causative agents for the Irish potato famine and "sudden oak death" disease.)
      • diatoms
      • brown algae
      • golden algae
    • Alveolates
      • dinoflagellates (the ancestor to dinoflagellates had a secondary plastid derived from a red alga; this has been lost in some dinoflagellates; in some cases it has replaced by a tertiary plastid derived from a cryptophyte, haptophyte or diatom; or by a secondary plastid derived from a green alga [Keeling 2010])
      • Apicomplexa (a group of parasites including ones that cause malaria, babesiosis, and toxoplasmosis; they have secondary plastids derived from a red alga)
      • ciliates
    • Rhizaria
      • chlorarachniophytes (have secondary plastids derived from a green alga)
      • foraminiferans
      • radiolarians
  • Other Groups of Eukaryotes (not yet placed in a supergroup)
    • cryptomonads/cryptophytes (have secondary plastids derived from a red alga)
    • haptophytes (have secondary plastids derived from a red alga)


Burki, F. 2014. The eukaryotic tree of life from a global phylogenomic perspective. Cold Spring Harb Perspect Biol 2014;6:a016147.

Kamoun, S. 2003. Molecular genetics of pathogenic oomycetes. Eukaryot Cell. 2(2):191-199.

Keeling, P.J. 2010. The endosymbiotic origin, diversification and fate of plastids. Philos Trans R Soc Lond B Biol Sci. 365(1541):729-748.

Lévesque, C.A. 2011. Fifty years of oomycetes—from consolidation to evolutionary and genomic exploration. Fungal Diversity. September 2011. 50:35-46.

Margulis, L. and K.V. Schwartz. 1998.Five Kingdoms. An Illustrated Guide to the Phyla of Life on Earth. Third Edition. W.H. Freeman and Company, New York. 520 pages.

Last edited: 21 Jan. 2018