#protists

(all pics from the Keeling & Eglit linked afterward, or, failing that, Wiki)

Parabasalida. A group of protists that mostly lives inside animals. Since they have little access to oxygen, they have converted their mitochondria into hydrogenosomes (other protist groups have done the same). Some have a flagellum running under the membrane to create a sort of undulating wing, and a rigid “spine” called axostyle. Trichonympha and other genera live in the gut of termites, helping them digest wood.

Diplomonadida & Oxymonadida. Other two groups of oxygen-avoiding symbionts and parasites of animals. The mitochondria of Diplomonadida have lost all metabolic function, and those of Oxymonadida (wich also help termites with wood digestion) are gone altogether. Diplomonadida have a distinctive symmetry with two tufts of flagella and two nuclei.

Euglenozoa. Protists with an elongated body and only one large flagellum. Euglenida are photosynthetic, and have developed a little red eye from one of their chloroplasts; they have a vestigial second flagellum, though it’s tiny and almost invisible. Another group, Kinetoplastida, have brought their flagellum under the membrane forming a sort of wing with which they swim in a corkscrew motion; they include pathogens like Leishmania and Trypanosoma.

Heterolobosea. A group of protists that mostly switch between amoeboid and flagellate forms, except for one that forms slime molds with stalked fruit instead. They also include the infamous “brain-eating” Naegleria.

Jakobida. Bacterial feeders that build themselves stalked cradles, harvesting dense bacterial streams with one flagellum while the other lies along the cell body and generates a continuous water current.

Stramenopiles. A group defined by having ancestrally two flagella of which one is much larger and covered in “hairs” (mastigonemes) that increase its “pull” on water enough to allow a cell to crawl forward in its direction. It includes a vast diversity of organisms: many are photosynthetic, such as diatoms (Bacillariophyta, with their two-valved shell of silica), and the multicellular, plant-like brown algae (Phaeophyta, like kelp or bladderwrack), collectively forming the group Ochrophyta. There are also non-photosynthetic groups such as the stalked Bicosoecida, the ciliated Opalinea, and the very fungus-like Oomycota (which include the potato blight).

Apicomplexa. A group of parasites with very complex life cycles (see next part), which include Plasmodium (malaria) and Toxoplasma, as well as the enormous (>0.5 mm) Gregarinida found in insect guts. Their infectious stage is elongated, with a modified chloroplast called apicoplast at the anterior end, which is used to penetrate host cells.

Dinoflagellata. Protists with a cellulose theca and two flagella, one free for thrust, the other wrapped around like a belt, wiggling to steer. They are a major component of plankton – sometimes photosynthetic, sometimes cytoplasm-sucking predators, sometimes both – though sometimes they can bloom in toxic “red tides”. Some photosynthetic species, called zooxanthellae, live in symbiosis swith coral polyps, trading sugars for protection.

Ciliophora (♥). Also known as Ciliates, they don’t have just a few long flagella but a whole coat of short cilia beating in synchrony, though often the cilia are reduced to a few patches in strategic body areas. They have two kinds of nuclei: a macronucleus for everyday cell operations, and a micronucleus that remains inactive until it’s passed to the offspring, or exchanged with partners in the sex-like process of conjugation. This keeps a copy of the DNA safe from metabolic damage. Almost always heterotrophic, they have a huge diversity of shapes and lifestyles – see below.

Foraminifera & Radiolaria. Two famous planktonic groups, although sadly most people only ever know their shells. Admittedly the shells are pretty, extremely diverse, and important to paleontology, but it should be appreciated that they are predatory amoebae extruding thin pesudopodia through the shell pores to catch preys. The pseudopodia of Foraminifera resemble spiderwebs, while those of Radiolaria are straight needles. Some Radiolaria build their shells out of strontium sulfate to weigh themselves down and stay at a controlled depth, whereas Foraminifera use calcium carbonate, and their remains are a major component of biogenic limestone.

Cercozoa. A strange group that doesn’t share any obvious similarity in appearance apart from being amoebae of some kind. Some interesting members are Chlorarachniophyta (photosynthetic, with spiderweb-like pseudopodia), Vampyrellida (which famously pierce algal cells to suck out their content) and Euglyphida (with vase-like scaly shells).

Haptophyta & Cryptomonada. Two unrelated groups of biflagellate unicellular algae. In addition to their two flagella, Haptophyta have a flagellum-like strand called haptonema, which is used to collect food particles and spoon them into the cell mouth. Cryptomonada have a distinctive mouth-like cavity and ejectisomes that, when disturbed, shoot out coiled filaments to change direction by reaction.

“Heliozoa” used to be a group of protists with a spherical body and stiff radial pseudopodia, which turned out to be a bunch of groups that independently developed this shape. The core of the old group survives as Centrohelida.

Provora. Described only in 2022, a small group of predatory biflagellate protists with subcellular harpoons to attack their preys like microscopic whalers and a feeding groove reinforced into jaws (!).

Breviatea. Another tiny, recently described group. They lack mitochondria, and have simultaneously the usual two flagella and a shapeshifting amoeboid body.

Amoebozoa. A very diverse group of amoebae, most of which lack flagella altogether. The eponymous Amoeba with its finger-like pseudopodia (Lobosea) is here, as well as most groups of temporarily-multicellular “slime molds” and various unicellular armored species (e.g. Arcellinida).

Microsporidia. Very small spore-forming parasites that invade the cells of animals, or larger protists, by shooting them with a tube and pouring themselves through it. Their mitochondria no longer produce energy, and in fact they are forced to steal ATP from their host cells. They are close relatives of fungi, or sometimes classified as fungi themselves.

Choanoflagellata. The next of kin of animals, and in fact they are bizarrely similar to the flagellate feeding cells of sponges (choanocytes). Their one flagellum, beating to generate a water current, is surrounded by a funnel of adhesive microvilli, to which food particles get stuck.

The term “algae” is almost as vague. It can refer to any photosynthetic protist, or indeed any photosynthetic organism, whether unicellular or multicellular, that isn’t a proper plant (secondarily aquatic plants such as seagrasses aren’t considered “algae” in the same way that seals and whales aren’t considered “fish”). The taxonomic mess that is behind “fish” and “reptile” and “tree” and “worm” shall be left for another day.

By the way, Patrick Keeling and Yana Eglit (2023) have made an awesome and freely available collection of illustrations of major protists groups with a uniform color scheme, which I've been sampling above:

(Keeling & Eglit 2023 fig. 4, with names added by myself; download file S2 for full HD collection. Try to find the ones I mentioned! It's not quite the same list.)

In addition to the major and mid-sized groups, a host of tiny lineages, many consisting of just one genus or even one species, have been discovered since 2000 or so as sampling techniques improve: see Malawimonas (1999),Pseudophyllomitus (2002), Tsukubamonas (2011), Picomonas (2013), Provora (2022), Loeffela (2023)… These lineages do not belong to any of the major groups, though they can be useful to reconstruct the pattern of relations.

(j/k, these clades are actually unique and really interesting, and the people who described them are doing fantastic work)

here are all the resources they provided 🔗:

🌈 slime education! 🦠 🧫