Protist

A protist (historically classified in the defunct kingdom Protista coined by Ernst Haeckel in 1866) is any eukaryotic organism that is neither a true animal (Metazoa), a land plant (Embryophyta), nor a true fungus. Because animals, fungi, and land plants are merely three specialized multicellular branches nested deep within a much broader radiation of microbial lineages, protists form a vast paraphyletic evolutionary grade that encompasses more than 75% of the primary phylogenetic diversity of domain Eukaryota.

Although most protists are microscopic unicellular organisms (ranging from 0.8-micrometer marine picoeukaryote Ostreococcus tauri to millimeter-scale ciliates and foraminiferans), the grade also includes colonial flagellates (choanoflagellates and Volvox), giant coenocytic (multinucleate single-celled) plasmodial slime molds and siphonous green algae (Caulerpa reaching meters in length), and complex multicellular brown macroalgae (kelps such as Macrocystis pyrifera reaching 45 to 60 meters). Ecologically, photosynthetic protists (microalgae, notably diatoms, dinoflagellates, and coccolithophores) generate roughly 45–50% of Earth's annual global net primary production, while heterotrophic protists (protozoa) govern microbial food-web grazing and cause major human parasitic diseases including malaria (Plasmodium), toxoplasmosis, sleeping sickness (Trypanosoma brucei), Chagas disease, leishmaniasis, and amoebic dysentery.

Why Kingdom "Protista" Was Replaced by Eukaryotic Supergroups

In Robert Whittaker's classic 1969 five-kingdom system (Monera, Protista, Plantae, Fungi, Animalia), all unicellular and simple multicellular eukaryotes were grouped into a single kingdom, Protista. Beginning in the 1990s and 2000s, molecular phylogenetics and phylogenomics (synthesized by Sina Adl, Alastair Simpson, Patrick Keeling, and Fabien Burki for the International Society of Protistologists) demonstrated that many "protists" are far more closely related to animals, fungi, or land plants than they are to other protists:

• Choanoflagellates (collared flagellate protists) are the direct sister group to Animals (Metazoa) within Opisthokonta, whereas nucleariid amoebae are the sister group to Fungi.
• Green algae (Chlorophyta and Charophyta) are nested inside Archaeplastida alongside land plants.
• Diatoms, giant kelps, ciliates (Paramecium), and the malaria parasite (Plasmodium) all belong to the TSAR (Telonemia–Stramenopiles–Alveolata–Rhizaria) supergroup, separated from animals and plants by over 1.5 billion years of evolution.

While all extant eukaryotes descend from a mitochondriate Last Eukaryotic Common Ancestor (LECA) that acquired an alphaproteobacterial mitochondrion via endosymbiotic theory, photosynthesis spread sideways across distantly related protist supergroups through a cascade of primary, secondary, and tertiary plastid endosymbioses:

1. Primary Endosymbiosis (2 Plastid Membranes — Archaeplastida & Paulinella): Approximately 1.5–1.6 Ga, a heterotrophic bikont protist engulfed a photosynthetic cyanobacterium and retained it as a permanent organelle (primary plastid surrounded by two membranes: the cyanobacterial inner and outer membranes), giving rise to glaucophytes (whose cyanelles still retain a bacterial peptidoglycan wall), red algae (Rhodophyta), and green algae + land plants (Viridiplantae). (A second, much younger independent primary endosymbiosis occurred ~90–140 Ma in the rhizarian amoeba Paulinella chromatophora.)
2. Secondary Endosymbiosis (3 or 4 Plastid Membranes — Green and Red Algal Lineages): Rather than engulfing a cyanobacterium directly, multiple heterotrophic protists engulfed an entire eukaryotic green or red unicellular alga and reduced it to a secondary plastid bounded by three or four membranes.

• Green-algal secondary endosymbioses occurred independently in euglenids (Euglena, 3 membranes) and chlorarachniophytes (Rhizaria, 4 membranes).
• Red-algal secondary endosymbioses gave rise to the plastids of cryptophytes (4 membranes), haptophytes (coccolithophores, 4 membranes), ochrophyte stramenopiles (diatoms and kelps, 4 membranes), and alveolates (dinoflagellates and the non-photosynthetic apicoplast of Plasmodium). Remarkably, both chlorarachniophytes and cryptophytes still retain a vestigial, miniature eukaryotic nucleus from the engulfed alga—the nucleomorph (only ~0.5–1.0 Mb across three tiny chromosomes)—trapped in the periplastidial space between the inner two and outer two plastid membranes!

1. Tertiary Endosymbiosis and Kleptoplasty: Several lineages of dinoflagellates subsequently lost their ancestral peridinin plastid and replaced it via tertiary endosymbiosis (engulfing a haptophyte, diatom, or cryptophyte), while mixotrophic ciliates (Mesodinium rubrum) and foraminiferans practice kleptoplasty—temporarily stealing functional chloroplasts from ingested algae before digesting them weeks later.

Far from being "primitive" or "simple" single cells, unicellular protists must pack every physiological function of a multicellular animal (locomotion, osmoregulation, prey capture, sensory transduction, and defense) into a single cell, evolving some of the most intricate organelles in biology:

• Nuclear Dualism in Ciliates (Germline Micronucleus vs. Somatic Macronucleus): Ciliates (Paramecium, Tetrahymena, Oxytricha) maintain two structurally and functionally distinct nuclei within the same cytoplasm: a diploid, transcriptionally silent germline micronucleus (MIC) that undergoes meiosis and sexual exchange during conjugation, and a polyploid somatic macronucleus (MAC) (~800n in Paramecium; fragmented into >16,000 tiny single-gene "nanochromosomes" in Oxytricha) that drives vegetative mRNA transcription and is rebuilt from the micronucleus via programmed DNA elimination and RNA-guided genome unscrambling after every sexual cycle.
• The Apical Complex and Apicoplast in Apicomplexa: Obligate intracellular parasites (Plasmodium, Toxoplasma, Cryptosporidium) possess a specialized anterior apical complex composed of spirally arranged conoid microtubules and secretory organelles (rhoptries and micronemes) that fire adhesins and lipids to actively zip the parasite into a host erythrocyte or enterocyte, alongside the apicoplast—a non-photosynthetic, four-membrane relict red-algal plastid essential for parasite isoprenoid (MEP) and fatty acid (FASII) biosynthesis (making the apicoplast 70S ribosome a prime drug target for clindamycin and doxycycline in malaria and toxoplasmosis).
• Kinetoplast RNA Editing in Trypanosomes: In kinetoplastids (Trypanosoma brucei, Trypanosoma cruzi, Leishmania), the single giant mitochondrion contains the kinetoplast (kDNA)—a chainmail-like catenated network of thousands of interlocked circular DNA minicircles (0.5–2.5 kb) and maxicircles (20–40 kb). Maxicircle pre-mRNAs are transcribed as encrypted "cryptogenes" that only become translatable open reading frames after minicircle-encoded guide RNAs (gRNAs) direct the insertion or deletion of hundreds of uridine (U) nucleotides.

Why are water molds (oomycetes like Phytophthora) and slime molds classified as protists instead of fungi?

Historically, oomycetes (water molds, such as Phytophthora infestans, the agent of the 1845 Irish Potato Famine) and slime molds were studied by mycologists because they grow filamentous hyphae or produce spore-bearing fruiting bodies and feed by osmotrophy or phagotrophy. However, molecular phylogenetics and biochemistry prove they are completely unrelated to true Fungi (Opisthokonta): true fungi have cell walls made of chitin, lack flagella in almost all lineages (except basal chytrids with a single posterior flagellum), and spend most of their life cycle in a haploid or dikaryotic state. In contrast, oomycetes belong to Stramenopiles (cousins of diatoms and brown kelps), have cell walls made of cellulose (β-1,4- and β-1,3-glucans), have a diploid vegetative life cycle, and swim via biflagellate zoospores bearing a hairy anterior tinsel flagellum. Slime molds belong to Amoebozoa and creep as wall-less phagocytic amoebae before aggregating.

Do any protists completely lack mitochondria?

For decades, anaerobic gut parasites such as Giardia intestinalis (Fornicata), Trichomonas vaginalis (Parabasalia), and Entamoeba histolytica were thought to be primitive "archezoans" that diverged before the acquisition of mitochondria. Genome sequencing and cell biology overturned this hypothesis: all three possess double-membrane-bounded mitochondrion-related organelles (MROs)—either hydrogenosomes (in Trichomonas, which lack a genome and electron transport chain but ferment pyruvate to generate ATP and H₂ gas via pyruvate:ferredoxin oxidoreductase) or tiny mitosomes (in Giardia, which retain essential mitochondrial iron–sulfur [Fe–S] cluster assembly proteins). To date, the only known eukaryote to have completely lost all traces of a mitochondrial organelle is the oxymonad gut flagellate Monocercomonoides exilis (Karnkowska et al., 2016), which survived losing its mitochondrion only after horizontally acquiring a cytosolic bacterial sulfur-mobilization (SUF) system to synthesize Fe–S clusters.

How do protist shells (foraminiferans, coccolithophores, and diatoms) regulate global climate and build geological rock strata?

Marine planktonic protists drive the ocean's biological carbon pump and carbonate counter-pump:

1. Diatoms (Stramenopiles) polymerize dissolved orthosilicic acid (Si(OH)₄) into ornate hydrated amorphous silica (opal, SiO₂·*n*H₂O) cell walls (frustules) whose high specific gravity ballasts dead cells so they sink rapidly to the abyssal seafloor, sequestering gigatons of organic carbon and accumulating as siliceous ooze and diatomite.
2. Coccolithophores (Haptophyta) and foraminiferans (Rhizaria) precipitate calcium carbonate (CaCO₃) tests that sink across the carbonate compensation depth (CCD) to build vast deposits of calcareous ooze, chalk (such as the Late Cretaceous White Cliffs of Dover), and limestone, while the stable oxygen (δ¹⁸O) and carbon (δ¹³C) isotope ratios preserved in fossil benthic foraminiferan tests serve as the primary paleothermometer for reconstructing Cenozoic ice-sheet volume and ocean temperatures over the past 66 million years.

1. Adl, S. M., Bass, D., Lane, C. E., et al. (2019). "Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes." Journal of Eukaryotic Microbiology, 66(1), 4–119. https://doi.org/10.1111/jeu.12691
2. Burki, F., Roger, A. J., Brown, M. W., & Simpson, A. G. B. (2020). "The New Tree of Eukaryotes." Trends in Ecology & Evolution, 35(1), 43–55. https://doi.org/10.1016/j.tree.2019.08.008
3. Keeling, P. J. (2013). "The Number, Speed, and Impact of Plastid Endosymbioses in Eukaryotic Evolution." Annual Review of Plant Biology, 64, 583–607. https://doi.org/10.1146/annurev-arplant-050312-120144
4. Karnkowska, A., Vacek, V., Zubáčová, Z., et al. (2016). "A Eukaryote without a Mitochondrial Organelle." Current Biology, 26(10), 1274–1284. https://doi.org/10.1016/j.cub.2016.03.053