Ancestrally flagellates distinguished by two 'heterokont' (unequal) flagella, one with tripartite mastigonemes. The most species-rich lineage, the ochrophytes, are algae of diverse morphologies, ranging from flagellates (like golden algae) to walled ornamented cells (like diatoms, pictured) to truly multicellular macroalgae with differentiated tissues (brown algae such as kelp). All other lineages are composed of heterotrophs: bacterivorous flagellates (e.g., bicosoecids, bigyromonads), fungus-like osmotrophs (oomycetes, hyphochytrids, and labyrinthulomycetes), heliozoan amoebae (actinophryids), and ciliate-like obligate symbionts of animals (opalinids).[1]
Ancestrally flagellated predators with cortical alveoli. The colponemids represent these ancestral characteristics.[3] The most diverse group are the ciliates (pictured), with large cells covered in rows of cilia, usually at the top of the microbial food chain.[4] The remaining alveolates belong to the clade Myzozoa and are ancestrally photosynthetic; some have retained their photosynthetic ability (chromerids and many dinoflagellates), while others have evolved into parasites of animals and algae (apicomplexans, perkinsozoans, and some dinoflagellates).[5][6][7]
Amoebae with fine pseudopodia. The most species-rich group is Retaria, home to conspicuous marine amoebae encased in hard skeletons (radiolarians) or multichambered tests (foraminifers, pictured). Secondly is Cercozoa, especially abundant in soils and exhibiting a wide range of morphologies from amoeboflagellates to aggregative slime molds, testate amoebae (e.g., euglyphids), and radiolarian-like organisms (phaeodarians); some are capable of photosynthesis (e.g., chlorarachniophytes). Lastly, Endomyxa contains both free-living predatory amoebae (e.g., vampyrellids) and obligate parasites of animals, plants, and algae (e.g., phytomyxeans and ascetosporeans).[8][5]
Free-living flagellates with a unique cytoskeleton and a combination of cell structures. Present in all marine and freshwater environments feeding on bacteria.[10]
Two groups of different free-living single-celled protists: centrohelids—predatory heliozoan amoebae, widespread in aquatic and soil environments[13]—and haptophytes—coccoid or flagellated photosynthetic algae, mostly marine (e.g., coccolithophores, pictured).[14] Both can produce an outer coat of complex mineralized scales.[5]
Free-living flagellates, except one species of heliozoan amoebae, Microheliella maris.[15] Almost all of the flagellates are distinguished by specialized ribbon-shaped extrusomes known as ejectisomes. Many are photosynthetic, known as cryptomonads (pictured), while the rest are phagotrophs, consumers of bacteria. Present in aquatic environments worldwide.[16][5]
Algae with chloroplasts derived from primary endosymbiosis with a cyanobacterium. Found in all environments. Almost entirely photosynthetic, with the exception of two small groups of phagotrophic flagellates, rhodelphids and picozoans. The two major groups, red algae and green algae (pictured), exhibit diverse morphologies, ranging from single cells—coccoid, palmelloid, sarcinoid, flagellated—to colonies, simple filaments, and macroscopic thalli with varying degrees of complexity (e.g., coralline algae, sea lettuce, stoneworts). Also included are glaucophytes, rare blue-green algae found in surface waters.[17]
Three lineages of free-living predatory flagellates with unique cytoskeletons. These are: Hemimastigophora, with two rows of flagella, present in soils and aquatic sediments; Provora, fast-swimming predators of other protists through a strong feeding apparatus resembling jaws, found in low abundance in marine environments globally;[19] and Caelestes (pictured), rare inhabitants of the marine benthos whose cells protrude arms or stalks used for movement or prey capture.[20]
Amoebae of diverse morphologies, with blunt (lobose) or fine (filose) pseudopodia, and sometimes with flagella. Most are free-living phagotrophs found across terrestrial and aquatic environments, such as the archetypal genus Amoeba itself,[21] or the testate amoebae Arcellinida, one of the most conspicuous groups of protists.[22] Several groups have independently evolved the ability to form fungus-like fruiting bodies, such as myxomycetes (pictured).[23] Some of the free-living amoebae are important vectors of pathogenic bacteria or are pathogenic themselves (e.g., Acanthamoeba).[24] Others are anaerobic intestinal symbionts (e.g., Entamoeba).[5]
Anaerobic free-living amoeboflagellates with fine pseudopodia and modified mitochondria. Present only in low-oxygen marine and brackish sediments, their growth depends on mutualistic interactions with prokaryotes.[26]
Free-living flagellates distinguished by a proboscis, a sleeve-like structure that envelops one of their two flagella.[27] Found gliding on wet soil and aquatic sediments worldwide.[28]
Flagellates distinguished by a single posterior flagellum, many with complex life cycles and varying degrees of multicellularity.[23] Some are entirely amoeboid, with fine pseudopodia (e.g., filastereans and nucleariids, including slime molds),[29][30] while others become amoeboid temporarily (e.g., choanoflagellates, pictured).[31] Most species are free-living filter-feeders or predators,[32][30][33][34] but some lineages (e.g., ichthyosporids) evolved into osmotrophic parasites of animals.[35][36]
Flagellates with very different lifestyles, present in aquatic and terrestrial environments, ranging from aerobes to anaerobes. The most diverse group, Euglenozoa, includes free-living osmotrophs, phagotrophs, phototrophs (euglenophytes, pictured), and pathogens (kinetoplastids).[37] The less diverse Heterolobosea are primarily amoeboflagellates, and include some slime molds (acrasids) and well-known opportunistic parasites (e.g., Naegleria fowleri).[38] The smallest group, Jakobida, consume bacteria by suspension feeding.[39]
Anaerobic or microaerophilic flagellates, amoebae, or amoeboflagellates,[42] with reduced or completely lost[43] mitochondria. A few are free-living, found in aquatic hypoxic sediments, but most species are obligate parasites (e.g., Giardia, pictured) or commensals in animal intestines (e.g., parabasalids). Many have a high number of flagella.[44][42][5]
Tiny free-living aquatic flagellates composed of flattened cells with an inflexible pellicle and a lateral rostrum with extrusomes. Found in most aquatic habitats.[46]
Free-living flagellates and filose amoebae with a pellicle underneath the cell membrane. Almost all flagellated members can produce filose pseudopodia. Found in aquatic environments.[47]
↑Tikhonenkov, Denis V.; Mikhailov, Kirill V.; Gawryluk, Ryan M. R.; Belyaev, Artem O.; Mathur, Varsha; etal. (22 December 2022). "Microbial predators form a new supergroup of eukaryotes". Nature. 612 (7941): 714–719. doi:10.1038/s41586-022-05511-5. PMID36477531.
12Torruella, Guifré; Galindo, Luis Javier; Moreira, David; Ciobanu, Maria; Heiss, Aaron A.; etal. (2023). "Expanding the molecular and morphological diversity of Apusomonadida, a deep‐branching group of gliding bacterivorous protists". Journal of Eukaryotic Microbiology. 70 (2) e12956. doi:10.1111/jeu.12956.
↑Richter, Daniel J.; Nitsche, Frank (2017). "Choanoflagellatea". In Archibald, John M.; Simpson, Alastair G.B.; Slamovits, Claudio H. (eds.). Handbook of the Protists. Vol.2 (2nded.). Springer. pp.1479–1496. doi:10.1007/978-3-319-28149-0_5. ISBN978-3-319-28147-6.
↑Hehenberger, Elisabeth; Tikhonenkov, Denis V.; Kolisko, Martin; del Campo, Javier; Esaulov, Anton S.; Mylnikov, Alexander P.; Keeling, Patrick J. (10 July 2017). "Novel Predators Reshape Holozoan Phylogeny and Reveal the Presence of a Two-Component Signaling System in the Ancestor of Animals". Current Biology. 27 (13): 2043–2050. Bibcode:2017CBio...27E2043H. doi:10.1016/j.cub.2017.06.006. PMID28648822.
↑Glockling, Sally L.; Marshall, Wyth L.; Gleason, Frank H. (25 April 2013). "Phylogenetic interpretations and ecological potentials of the Mesomycetozoea (Ichthyosporea)". Fungal Ecology. 6 (4): 237–247. doi:10.1016/j.funeco.2013.03.005.
↑Pánek, Tomáš; Simpson, Alastair G. B.; Brown, Matthew W.; Dyer, Betsey Dexter (2017). "Heterolobosea". In Archibald, John M.; Simpson, Alastair G.B.; Slamovits, Claudio H. (eds.). Handbook of the Protists. Vol.2 (2nded.). Springer. pp.1005–1046. doi:10.1007/978-3-319-28149-0_10. ISBN978-3-319-28147-6.
↑Simpson, Alastair G. B. (2017). "Jakobida". In Archibald, John M.; Simpson, Alastair G.B.; Slamovits, Claudio H. (eds.). Handbook of the Protists. Vol.2 (2nded.). Springer. pp.973–1004. doi:10.1007/978-3-319-28149-0_6. ISBN978-3-319-28147-6.
↑Pánek, Tomáš; Zadrobílková, Eliška; Walker, Giselle; Brown, Matthew W.; Gentekaki, Eleni; etal. (2016). "First multigene analysis of Archamoebae (Amoebozoa: Conosa) robustly reveals its phylogeny and shows that Entamoebidae represents a deep lineage of the group". Molecular Phylogenetics and Evolution. 98: 41–51. doi:10.1016/j.ympev.2016.01.011.
↑Karnkowska, Anna; Vacek, Vojtěch; Zubáčová, Zuzana; Treitli, Sebastian C.; Petrželková, Romana; etal. (2016). "A Eukaryote without a Mitochondrial Organelle". Current Biology. 26 (10): 1274–1284. doi:10.1016/j.cub.2016.03.053.
↑Yubuki, Naoji; Huang, Sam S.C.; Leander, Brian S. (2016). "Comparative Ultrastructure of Fornicate Excavates, Including a Novel Free-living Relative of Diplomonads: Aduncisulcus paluster gen. et sp. nov". Protist. 167 (6): 584–596. doi:10.1016/j.protis.2016.10.001.
12Heiss, Aaron A.; Warring, Sally D.; Lukacs, Kaleigh; Favate, John; Yang, Ashley; etal. (2021). "Description of Imasa heleensis , gen. nov., sp. nov. (Imasidae, fam. nov.), a Deep‐Branching Marine Malawimonad and Possible Key Taxon in Understanding Early Eukaryotic Evolution". Journal of Eukaryotic Microbiology. 68 (2) e12837. doi:10.1111/jeu.12837.
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