Protoctists

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. The classification of eukaryotes into supergroups is still a work in progress and the groupings described here are likely to change as more and better data become available. Ten supergroups of eukaryotes are listed below (nine from Burki et al. 2020, and one from Tikhonenkov et al. 2022). Due to taxonomic uncertainties, some eukaryotic organisms have not yet been assigned to a supergroup.

Supergroups of Eukaryotes and Representative Protoctist Taxa

  • Amorphea (this group includes animals, fungi, most slime molds, amoebas, and related taxa)
    • Obazoa (a clade consisting of the apusomonads, breviates, and opisthokonts)
      • Apusomonada (heterotrophic flagellates)
      • Breviates (heterotrophic flagellates)
      • Opisthokonta (animals, fungi, and related unicellular taxa) Examples include:
        • choanoflagellates: free-living flagellates that closely resemble the collar cells in sponges
        • Fonticula: a cellular slime mold that forms a multicellular, stalked, volcano-shaped fruiting body
    • Amoebozoa (amoebas, some flagellates, and most slime molds [the mycetozoan slime molds]) Examples include:
      • Amoeba proteus, (a common study organism)
      • Fuligo septica (the "dog vomit slime mold")
  • Archaeplastida (this group includes vascular plants, bryophytes, green algae, red algae, and glaucophytes; all lineages have primary plastids derived from a cyanobacterium)
    • Chloroplastida (green algae, bryophytes, and vascular plants)
    • Rhodophyta (red algae)
    • Glaucophyta (glaucophyte algae; the glaucophta have blue-green-colored plastids with a vestigial peptidoglycan cell wall and carbyoxysome-like bodies (Jackson et al. 2015))
  • TSAR (Telonemids, Stramenopiles, Alveolates, and Rhizaria; includes amoebas, ciliates and flagellates; some are photosynthetic, some are heterotrophic but contain symbiotic algae, some are predatory, some are parasites, and some are mixotrophic [feed both autotrophically and heterotrophically])
    • Telonemia (free-living flagellates)
    • SAR (a clade containing Stramenopiles, Alveolates, and Rhizaria)
      • Stramenopila (most are photosynthetic and have secondary plastids derived from a red alga; but the group does contain several heterotrophic lineages that lack plastids) and many mixotrophs. Examples of stramenopiles include:
        • Diatoms
        • Brown Algae
        • Golden Algae (chrysophytes)
        • Yellow-green Algae (xanthophytes)
        • Silicoflagellates
        • Oomycetes, commonly called 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.)
        • Labyrinthulids (slime nets: include saprophytes, endosymbionts, and parasites/pathogens. They produce simple colonies with cells that glide along strands (known as slimeways or slimetracks) within a membrane-bound network. This group includes Labyrinthula zosterae, the agent associated with eelgrass wasting disease)
      • Alveolata (mostly unicellular; includes autotrophs, heterotrophs, and mixotrophs. Examples include:
        • Dinoflagellates (approximately half of known taxa are photosynthetic, others are predators or parasites; 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; many have nonphotosynthetic plastids, they are secondary plastids derived from a red alga)
        • Ciliates
      • Rhizaria (Most are heterotrophic and free-living; some are amoeba-like organisms that contain a test [skeleton/shell]) Examples include:
        • Chlorarachniophytes (mixotrophic; have secondary plastids derived from a green alga; the remnant nucleus of the endosymbiont is still present between the membranes of the plastid)
        • Foraminiferans (marine, heterotrophic, amoeba-like organisms; mostly benthic, some planktonic; with tests made of calcite or other materials; some with algal symbionts; fossils are examined by the oil industry to identify sites likely to contain petroleum [based on presence of certain indicator species])
        • Radiolarians (Polycystinea and Acantharia; generally predatory; marine planktonic amoeba-like organisms often with elaborate mineral tests made of silica (Polycystinea) or strontium sulfate (Acantharia); many species contain photosymbionts)
        • Phytomyxids (Phytomyxea) (are obligate pathogens of plants and stramenopiles; some are vectors of viruses. Includes Plasmodiophora brassicae, which causes clubroot disease in mustards, and Spongospora subterranea, which causes powdery scab disease in potatoes)
        • Paulinella chromatophora (photosynthetic; a testate amoeba with primary plastids derived from a cyanobacterium)
  • Haptista (includes haptophyte algae and centrohelids)
    • Haptophyta (flagellated marine algae; have secondary plastids derived from a red alga) Examples include:
      • Coccolithophorids (unicellular organisms that are are covered by scales of calcium carbonate and are the primary formers of chalk deposits)
      • Phaeocystis (a primary contributor to the production of dimethyl sulfide in the world's oceans)
    • Centrohelida (free-living predators: they have projections called axopodia radiating out from their spherical bodies)
  • Cryptista (includes cryptophyte algae (cryptomonads) and related flagellates)
    • Cryptophyta (have two flagella and asymmetrical cells; most are photosynthetic; they have secondary plastids derived from a red alga)
    • Katablepharida (heterotrophic flagellates)
    • Palpitomonas (heterotrophic flagellates)
  • CRuMs (an acronym for collodictyonids (diphylleids), Rigifilida, and Mantamonas; all free-living protoctists)
    • Diphylleida (swimming flagellates)
    • Rigifilida (filose amoeboid cells)
    • Mantamonas (marine heterotrophic gliding flagellates)
  • Hemimastigophora (free-living heterotrophs with two rows of flagella; considered to be an ancient lineage of eukaryotes)
  • Discoba (includes euglenophyte algae, parasites, and free-living amoebas and flagellates)
    • Euglenozoa (euglenophyte algae and relatives) Examples include:
      • Euglena (a photosynthetic flagellate with a secondary plastid derived from a green alga)
      • Trypanosomatid parasites (Trypanosomatidae; transmitted by insect vectors; include parasites that cause African sleeping sickness (trypanosomiasis), Chagas disease, and leishmaniases)
    • Heterolobosea (heterotrophic, mostly free-living amoebas and flagellates) Examples include:
      • Naegleria fowleri (the "brain-eating amoeba")
      • Acrasid (Acrasida) cellular slime molds
    • Jakobida (heterotrophic flagellates; representatives from this group contain the most gene-rich, bacteria-like mitochondria known)
    • Tsukubamonas (heterotrophic flagellates)
  • Metamonada (consists of anaerobic protoctists, including free-living taxa, intestinal symbionts, and parasites) Examples include:
    • Giardia (a parasite that reproduces in the small intestines of humans and other vertebrates, often causing diarrhea)
    • Trichomonas (parasites that infect humans and birds)
    • Trichonympha (a multi-flagellated lignocellulose-digesting symbiont found in the hindgut of lower termites and wood roaches)
  • Provora (predatory flagellates, believed to be an ancient group of eukaryotes [Tikhonenkov et al. 2022]) Two clades have been identified:
    • Nibbleridia (appear to feed by biting prey with tooth-like structures)
    • Nebulidia (consume prey via phagocytosis)

Sources

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Last edited: 12 February 2023